Key Takeaways
- Air duct sizing converts airflow to duct diameter: A = Q ÷ V, then D = √(4A/π).
- Industrial exhaust targets 1000–2500 fpm (5–13 m/s). Supply tops out near 3000 fpm, return near 1800 fpm; below 1000 fpm dust settles, above 2500 fpm adds noise and fan power.
- Use the CFM–diameter table: find airflow, round up to a standard size, verify velocity. The table is built at 1500 fpm; doubling the diameter cuts friction loss by roughly 1/32 (relative).
- Compare rectangular ducts via De = 1.30·(a·b)^0.625/(a+b)^0.25, and keep the aspect ratio ≤ 4:1.
- Worked example: 2000 CFM (3400 m³/h) at 1500 fpm → 16 in (φ400 mm class PP), 16×12 in rectangular equivalent. Design values are not field measurements — installed systems need balancing.
- A complete RFQ carries airflow, target velocity, diameter, duct length, fitting count, and process gas. Wall thickness varies by pressure class — confirm it with the supplier.
1. Air Duct Sizing: The Core Formulas and What They Do
1.1 What air duct sizing means and why you need the formulas
Air duct sizing is the engineering step that converts a design airflow into a duct diameter. You know how much air needs to move (the exhaust or supply flow rate from the process or equipment), and you choose how fast it should travel inside the duct (the target velocity). The formulas then tell you the minimum duct cross-section and, from that, the diameter of a round duct.
The decision this enables is straightforward: you cannot order a duct, quote a job, or check a supplier’s proposal without a diameter. Sizing by guesswork or by copying a neighbouring run’s duct size is the most common source of undersized or oversized ducts in industrial exhaust systems — both expensive in different ways. The formulas below give you a defensible number in under five minutes, and they are the same math that every duct sizing calculator and reference chart uses internally.
1.2 The two formulas that do the work: A = Q ÷ V, then D = √(4A/π)
The continuity equation for duct flow, rearranged, gives the required cross-section area:
A = Q ÷ V (ft² = CFM / fpm)
where A is the internal cross-section area in square feet, Q is the airflow in CFM, and V is the target average velocity in feet per minute. For a round duct, area and diameter are linked by the geometry of a circle: A = πD²/4. Solving for D gives:
D = √(4A / π) (feet; multiply by 12 for inches)
A quick micro example shows the numbers in action. A 600 CFM exhaust run at a target 1200 fpm: A = 600 ÷ 1200 = 0.5 ft². Then D = √(4 × 0.5 / π) = √0.637 = 0.798 ft = 9.6 in. In practice you round up to the next standard size (10 in) and re-check the velocity, which drops to about 1100 fpm. The calculation chain — area first, diameter second, standard-size rounding third, velocity re-check fourth — never changes, regardless of the airflow.
1.3 The full calculation chain in six steps
| Step | What you do | What you get |
|---|---|---|
| 1 | Define the airflow in CFM (or m³/h) | A design flow for the run |
| 2 | Choose a target velocity in fpm (or m/s) | A design velocity from the industrial window |
| 3 | A = Q ÷ V | The required cross-section area |
| 4 | D = √(4A / π) | The round-duct diameter |
| 5 | Round up to a standard size and re-check velocity | A buildable duct size that still meets the target |
| 6 | Convert to a rectangular equivalent if space demands | A layout-compatible section with a checked aspect ratio |
Decision point: after this section you know the two formulas that drive every duct sizing calculation and the six-step chain they belong to. The next section covers the two inputs you need before you can run the numbers.
2. Inputs You Need Before You Start
2.1 Airflow Q — where the number comes from
The airflow is the design flow of the fan or the process, and it is normally taken from the equipment specification or the process requirement, not guessed. In a laboratory exhaust system the airflow is set by the fume hood’s face velocity requirement and the sash opening; in an industrial process it comes from the capture velocity needed at the source and the hood or enclosure design. You do not derive Q from the duct sizing calculation — you bring it to the calculation.
The common mistake at this stage is treating the airflow as adjustable. A duct sizing calculation takes Q as a fixed input; if you change the airflow later (adding a second hood to the same run, for example), the diameter must be re-calculated. For initial sizing, use the maximum design flow for the run, not an average or a part-load value.
2.2 Target velocity V — choosing the right design speed
For industrial exhaust main ducts, a target velocity of 1000–2500 fpm (about 5–13 m/s) is the typical design window. This range is the consensus from standard duct design references such as the Engineering ToolBox air duct velocity guide, and it is the range used for the worked example and the quick-reference table later in this guide. The two related limits that matter: supply ducts are commonly designed with an upper bound near 3000 fpm, and return ducts near 1800 fpm.
The consequence of picking a velocity outside this window is not just a different diameter — it changes the system’s operating behaviour. Below roughly 1000 fpm, heavier dust and particulates in exhaust streams begin to settle in the bottom of the duct, creating a maintenance problem that a designer cannot fix by cleaning alone. Above roughly 2500 fpm, velocity adds noise, duct-wall abrasion, and pressure drop (friction loss grows with the square of velocity), so the same airflow pushed through a smaller duct costs more fan power and erodes the duct wall faster. Pick velocity first, then let the diameter follow — the reverse order is where most sizing mistakes originate.
2.3 Consistent units — the first thing to get right
The easiest way to introduce an error is to mix unit systems inside one calculation. Use CFM and fpm together, or m³/h and m/s together, but not a combination. If your airflow is in m³/h and you want to use the formula in CFM, convert the airflow first (1 CFM = 1.699 m³/h, or 1 m³/h = 0.589 CFM), then run the calculation in one system. The unit conversions are given in detail in section 5, but the principle belongs here: decide on a unit system before you start, and convert only at the boundaries.
Decision point: after this section you have the two inputs (airflow and target velocity) and you know which unit system you are working in. The next section walks through the velocity method step by step.
3. Step by Step: The Velocity Method
3.1 Step 1: Define the airflow for the duct run
The first step is to write down the design airflow Q in CFM (or m³/h) for the specific duct run you are sizing. This is the number you brought from the equipment specification or process requirement, as described in section 2.1. For a single run feeding one exhaust point, this is the flow for that point. For a branch in a larger system, it is the flow for that branch, which may be a fraction of the fan’s total airflow.
A written airflow at this stage prevents the common slip of using the fan’s total flow for every branch. If the fan moves 6000 CFM and you have three branches, each branch carries its own portion, not the full 6000 CFM. The diameter calculation is per-run, not per-system.
3.2 Step 2: Choose a target velocity from the industrial window
Select a target velocity V in fpm (or m/s) that fits your system type and the guidelines in section 2.2. For a laboratory exhaust or general industrial fume extraction, 1500 fpm is a reasonable mid-range starting point — high enough to keep particulates entrained, low enough to avoid excessive noise and pressure drop. Adjust upward for abrasive streams (where you want velocity high enough to avoid settling but not so high that erosion accelerates) or downward for noise-sensitive installations.
The choice at this step is a design decision, not a calculation. Document the chosen velocity and the reason for it, so that when the diameter is reviewed later the velocity assumption is visible.
3.3 Step 3: Calculate the area — A = Q ÷ V
Divide the airflow by the velocity to get the required cross-section area. With Q in CFM and V in fpm, the result is in square feet:
A = Q ÷ V = 2000 ÷ 1500 = 1.333 ft²
This is the area the duct must provide to move the design airflow at the chosen velocity. The number is independent of duct shape — a round duct, a rectangular duct, or any other shape must have at least this internal area. The shape determines the linear dimensions that follow.
3.4 Step 4: Convert area to round duct diameter — D = √(4A/π)
For a round duct, the diameter in feet is the square root of four times the area divided by π. Multiply by 12 to get inches:
D = √(4 × 1.333 / π) = √1.697 = 1.303 ft = 15.64 in
This is the minimum round-duct diameter that provides the required area. Because 15.64 in is not a standard duct size, the next step is to round up to the nearest standard size (16 in) and re-check the velocity. The re-check is essential: a larger diameter means lower actual velocity, and you must confirm that the new velocity still sits inside your design window.
Decision point: after this section you can calculate the diameter from any airflow and velocity using the two formulas. The next section provides a pre-computed table that does this calculation for you at a typical industrial velocity — useful for quick reference and as a cross-check on your hand calculation.
4. Air Duct Sizing: CFM to Duct Diameter Quick-Reference Table
4.1 How to read the table
The table below converts airflow to a minimum round duct diameter at a single assumed velocity of 1500 fpm — a mid-range value inside the industrial 1000–2500 fpm window. Three rules apply. First, the table is a calculation aid, not a field measurement: the diameters are computed from A = Q ÷ V at 1500 fpm, then rounded up to standard sizes. Second, each project should re-run the velocity method at its own target velocity; the table gives a fast first pass, and the “velocity at standard size” column shows what happens after rounding. Third, always round up to a standard size, never down — an undersized duct is the more expensive mistake.
4.2 CFM → minimum round duct diameter
Calculation basis: A = Q ÷ V at V = 1500 fpm (typical industrial main-duct velocity, mid-range of 1000–2500 fpm); diameters computed from D = √(4A/π), then rounded UP to the nearest standard size. Shown values are computed examples — verify against your project’s airflow and target velocity.
| Airflow (CFM) | Airflow (m³/h) | Minimum Ø (in / mm) | Standard Ø (in / mm) | Velocity at standard size (fpm) |
|---|---|---|---|---|
| 200 | 340 | 4.9 / 126 | 5 / 127 | 1467 |
| 400 | 680 | 7.0 / 178 | 7 / 178 | 1497 |
| 600 | 1019 | 8.6 / 218 | 9 / 229 | 1358 |
| 800 | 1359 | 9.9 / 251 | 10 / 254 | 1467 |
| 1000 | 1699 | 11.1 / 281 | 12 / 305 | 1273 |
| 1200 | 2039 | 12.1 / 308 | 14 / 356 | 1123 |
| 1500 | 2548 | 13.5 / 344 | 14 / 356 | 1403 |
| 2000 | 3398 | 15.6 / 397 | 16 / 406 | 1432 |
| 3000 | 5097 | 19.1 / 486 | 20 / 508 | 1375 |
| 4000 | 6796 | 22.1 / 562 | 24 / 610 | 1273 |
| 6000 | 10194 | 27.1 / 688 | 28 / 711 | 1403 |
| 8000 | 13592 | 31.3 / 794 | 32 / 813 | 1432 |
| 10000 | 16990 | 35.0 / 888 | 36 / 914 | 1415 |
To use the table: find your airflow in the first two columns, read the minimum diameter, then take the standard size from column four. For a 2000 CFM run you land on 16 in — the same result the worked example reaches by hand in section 7, confirming the two paths agree. Any duct sizing calculator runs the same logic (airflow in, diameter out, velocity assumed), so a table and a calculator should always agree when the inputs match.
4.3 Round up to a standard size — and verify the velocity
Rounding up is not just about catalogue availability; it is usually the quieter and more efficient choice. A slightly larger duct lowers velocity and friction for the same airflow, which means less noise and lower fan power draw. Because friction loss falls steeply with diameter — doubling the duct diameter reduces friction loss by roughly a factor of 32 (a relative comparison) — the margin from rounding up is cheap insurance against measurement uncertainty and future airflow growth.
The one check that must accompany rounding is the velocity re-check. When you take a larger standard size than the minimum, the actual velocity drops, and you should confirm it still sits in your design window (for industrial exhaust, above the dust-settling floor and below the noise/abrasion ceiling). The table’s last column already shows the velocity at the rounded size for the 1500 fpm basis; for your own numbers, recompute V = Q ÷ A with the actual standard size. A rounded-up duct that drops velocity below 1000 fpm in a dusty exhaust may need rethinking.
Decision point: after this section you can look up a diameter from the table in under a minute, confirm the resulting velocity, and decide whether to round up or to a different standard size. The next section covers the unit conversions you need when working with metric data.
5. Unit Conversions and Common Calculation Pitfalls
5.1 CFM ↔ m³/h and in ↔ mm
Two conversion rules cover most mixed-unit situations. To convert CFM to m³/h, multiply by 1.699 (the volumetric conversion factor at standard air conditions). To convert inches to millimetres, multiply by 25.4. In the other direction: 1 m³/h = 0.589 CFM, and 1 mm = 0.0394 in. These are the only conversions you need for the velocity method, provided you keep the rest of the calculation in one system.
A worked pair: 2000 CFM × 1.699 = 3398 ≈ 3400 m³/h. And 16 in × 25.4 = 406.4 mm, which is close to the PP standard φ400 mm (the 6 mm difference is within the tolerance of the size class assignment). For velocity, 1500 fpm × 0.00508 = 7.62 m/s, or roughly 7.6 m/s for engineering purposes.
5.2 The most expensive mistake: mixing unit systems
The error that costs real money is mixing CFM and metres inside one calculation. Using CFM in a formula expecting m³/h, or comparing a diameter in inches against a PP catalogue dimension in millimetres without converting, produces a wrong diameter that is not caught by a simple sanity check — the numbers look plausible but the duct is off by a factor.
The fix is a simple discipline: pick one unit system (CFM / fpm / inches) and run the entire calculation in it. Convert only at the beginning (your airflow may be in m³/h) and at the very end (your PP catalogue is in mm). A calculation that mixes units inside the A = Q ÷ V step is wrong, and the error propagates to the diameter, the velocity re-check, and the equipment selection. When in doubt, write the units next to every number in the calculation and confirm they cancel correctly.
5.3 Velocity pressure: p_v = (V/4005)²
The velocity pressure formula p_v = (V / 4005)² gives the dynamic pressure in inches of water column for standard air at roughly 20 °C and sea-level density. At 1500 fpm: (1500 / 4005)² ≈ 0.14 in. W.C. This is the kinetic energy of the moving air, and it is small on its own but scales with the square of velocity. A system designed at 2500 fpm has a velocity pressure of about 0.39 in. W.C. — nearly three times higher than at 1500 fpm.
The 4005 constant is not universal; it changes with air density, so at high altitude or elevated temperature the same formula requires a density correction. For most industrial exhaust applications near sea level and room temperature, the standard value is adequate for preliminary sizing. The full pressure picture, including fitting losses and the equal friction method, is covered in section 6.4.
Decision point: after this section you can decide which unit system to keep through the rest of the calculation, convert any airflow or duct dimension between common systems, and name the velocity pressure term that appears in the pressure-drop calculation. The next section connects the diameter to the equipment you actually buy.
6. How the Diameter Changes Your Equipment Selection
6.1 Round duct → PP duct size mapping
A calculated diameter maps directly to a standard polypropylene duct size class, and the mapping is not one-to-one — it depends on the manufacturer’s product line. A 16 in round duct is 406 mm, which falls into the φ400 mm class of PP duct. The typical wall thickness for this size range is in the 5 mm class, but the exact pairing of diameter and wall thickness should be confirmed against the product specification for your project, because different pressure classes and material grades use different wall thicknesses for the same nominal diameter.
The consequence of ignoring the mapping is an ordering mismatch: you specify a 16 in duct, the supplier quotes φ400 mm, and the two are close enough to work, but the wall thickness may be wrong for your pressure requirements. Always confirm the size class and wall thickness with the supplier before ordering, and carry the design airflow and velocity on the RFQ so the supplier can cross-check the sizing.
6.2 Rectangular equivalent: when space forces a flat duct
When a duct run must fit in a tight ceiling plenum or along a wall, a rectangular section is the standard solution, and the equivalent round diameter lets you compare it to the round duct you already sized. The Huebscher formula for circular equivalent (also called equivalent round diameter) is:
De = 1.30 × (a·b)^0.625 / (a+b)^0.25
where a and b are the rectangle sides in inches and De comes out in inches. For a candidate 16×12 in section: (a·b) = 192, (a+b) = 28, so De = 1.30 × 192^0.625 / 28^0.25 ≈ 1.30 × 26.7 / 2.30 ≈ 15.1 in. This rectangular duct behaves like a round duct of roughly 15 in for friction purposes — very close to the 16 in round from the worked example, which is why 16×12 in is a common rectangular stand-in for it.
The formula is the same one used by duct design references such as the Engineering ToolBox equivalent-diameter reference. Treat the result as an engineering approximation, not a precision instrument, and keep the sides in the same unit when applying the formula.
6.3 Aspect ratio and the round vs rectangular trade-off
The aspect ratio is the longer side divided by the shorter side, and industry guidance keeps it at or below 4:1. For a 16×12 in duct it is 1.33; for a 40×10 in duct it is 4.0. The reason is geometric: a long thin rectangle has a larger perimeter for the same cross-section area, which means more surface friction and more duct material per unit of airflow. Ratios up to about 5:1 are sometimes used in space-constrained work, but every step past 4:1 buys space at the price of efficiency and material cost.
For the same cross-section area, a round duct is the more efficient shape — the circle minimises perimeter, which means lower friction per unit length and less material for the same airflow. Rectangular wins where net headroom is tight: shallow ceiling spaces where a round duct’s full diameter cannot fit, or where a flat duct can follow a wall or run under beams more easily. The frequent question “how many CFM can a 20×8 in duct carry?” is answered through the same tool: De ≈ 13.5 in, then at 1500 fpm, A ≈ 0.99 ft², so Q = A × V ≈ 1490 CFM. Rectangular sizing is the same velocity method with an equivalent-diameter step inserted before the diameter is read.
6.4 Equal friction method for large branched systems
The equal friction method sizes every duct run to the same friction loss per unit length, typically 0.1 in. W.C. per 100 ft for supply and 0.08 in. W.C. per 100 ft for return ducts — commonly used starting points in duct design references. The advantage of this method is that velocities end up roughly uniform across a branched system, which distributes air more evenly and keeps noise generation consistent. The trade-off is that it is more work than a single velocity-method calculation, which is why it pays off for larger systems and is overkill for a single exhaust run.
The sizing situation table below summarises when each method is the better fit:
| Sizing situation | Better fit | Why |
|---|---|---|
| Single exhaust run, known airflow | Velocity method | One duct, one calculation — fastest and sufficient |
| Small ducted system, rough first pass | Velocity method | Quick estimate before detailed design |
| Large system with many branches | Equal friction | Keeps velocities and noise more uniform |
| Noise-sensitive installation | Equal friction (lower rate) | Uniform velocity avoids hot spots of high-speed flow |
| Full HVAC supply-and-return layout | Equal friction | Standard practice for multi-branch distribution; see our duct design guide |
The pressure picture that either method must account for includes three components: straight-duct friction (what the diameter calculation addresses), fitting losses (elbows, tees, transitions — a single sharp elbow can add as much loss as several feet of straight duct), and velocity pressure (section 5.3). A full loss calculation belongs in detailed design, but knowing the three components helps you judge whether the diameter you calculated is likely to be in the right ballpark.
Decision point: after this section you can map your calculated diameter to a PP duct size, convert between round and rectangular sections, check the aspect ratio, and choose between the velocity method and equal friction for your system. The next section walks through a complete worked example.
7. Air Duct Sizing Worked Example: A 2000 CFM (3400 m³/h) Exhaust Run
7.1 Steps 1–4: Define airflow, choose velocity, calculate diameter
A laboratory exhaust run must move 2000 CFM of air. Converting to metric: 2000 × 1.699 ≈ 3400 m³/h. The target velocity is set at 1500 fpm — inside the industrial 1000–2500 fpm window, high enough to avoid dust settling, low enough to keep noise and pressure reasonable.
Step 3 — area from continuity: A = Q ÷ V = 2000 ÷ 1500 = 1.333 ft².
Step 4 — diameter from area: D = √(4 × 1.333 / π) = √1.697 = 1.303 ft = 15.64 in.
Because 15.64 in is not a standard size, round up to 16 in. The re-check: actual velocity V = Q ÷ A = 2000 ÷ (π/4 × (16/12)²) = 2000 ÷ 1.396 = 1432 fpm, comfortably inside the design window and above the dust-settling floor. Note that the table in section 4.2 gives exactly this result (2000 CFM → 16 in → 1432 fpm), confirming the hand calculation against the pre-computed chart.
7.2 Step 5: Check rectangular equivalent and aspect ratio
If the run must be rectangular, convert first, then check the ratio. For a candidate 16×12 in section:
De = 1.30 × (16 × 12)^0.625 / (16 + 12)^0.25 ≈ 1.30 × 26.7 / 2.30 ≈ 15.1 in
This is nearly identical to the 15.6 in minimum from step 4 — the rectangular section behaves like the round duct for friction purposes. The aspect ratio is 16 ÷ 12 = 1.33, well below the 4:1 guideline. If the available space forced a flatter section such as 24×8 in (ratio 3.0), it would still be acceptable against the guideline but at a higher perimeter cost; anything past 4:1 needs a deliberate efficiency trade-off.
7.3 Step 6–7: Pressure check, PP size mapping, and next step
At the equal-friction starting point of 0.1 in. W.C./100 ft, a 50 ft main run contributes roughly 0.05 in. W.C. of straight-duct friction; fittings and velocity pressure (about 0.14 in. W.C. at 1500 fpm) add on top, so the total will be higher than the straight-run number alone. The full loss calculation, including the fan’s static pressure, belongs in detailed design.
PP size mapping: a 16 in round duct is 406 mm, which maps to the φ400 mm class of PP duct with a typical wall thickness in the 5 mm class for this size range. The exact wall thickness should be confirmed against the product specification for your project, because different pressure classes and material grades use different wall thicknesses for the same nominal diameter. For material guidance, see our guides on what PP duct is and why polypropylene is a common choice for corrosive exhausts, plus the material comparison in PP duct vs PVC vs galvanized.
The next step is procurement: with the diameter, the rectangular alternative, and the PP size class in hand, you can prepare an RFQ — the checklist in the next section covers what to include. The product page lists the size range and available wall thicknesses for round polypropylene duct.
Decision point: you have walked a complete sizing chain — 2000 CFM → 1500 fpm → 15.6 in → 16 in standard → 1432 fpm verified → 16×12 in rectangular equivalent (ratio 1.33) → φ400 mm PP with 5 mm-class wall. The next section gives a checklist for what to include when you order.
8. RFQ Checklist: What to Tell Your Supplier When Ordering PP Duct
8.1 Core sizing data: airflow, velocity, diameter, run length
A complete RFQ for PP duct starts with the four numbers that define the design: airflow (CFM or m³/h), target velocity (fpm or m/s), selected diameter (in or mm), and total run length (ft or m). These four numbers let the supplier confirm that the sizing is consistent and that the selected diameter is appropriate for the intended flow. Without them, the supplier can only quote a standard product without verifying it fits your application.
Include the standard size you selected (16 in, φ400 mm, or whatever your calculation produced) and the actual velocity at that size. If the run has multiple branches, provide the airflow and diameter for each branch separately, not just the fan total.
8.2 Process conditions: gas type, temperature, pressure class
The duct material and wall thickness depend on what is moving through the duct, not just how much air. The process gas composition (what chemical compounds are present, at what concentration), the operating temperature range, and the pressure class (negative or positive, static pressure at the fan) determine whether standard PP is adequate or a specialised grade is needed.
A corrosive fume stream, for example, may require a thicker wall than the standard 5 mm class, or a different material formulation. High-temperature exhaust may require a different material altogether. Include the process gas description and the expected continuous and peak temperatures on the RFQ, and note whether the system runs under negative pressure (fan downstream of the exhaust source) or positive pressure (fan upstream). For guidance on material selection for corrosive fumes, see our duct material selection guide.
8.3 Wall thickness: why it matters and how to confirm it
Wall thickness is not a single number for all PP ducts of a given diameter — it varies with the pressure class, the material grade, and the manufacturer’s standard. A φ400 mm PP duct may be available in 4 mm, 5 mm, 6 mm, or thicker walls depending on whether it is a standard-duty, heavy-duty, or pressure-rated product. The wall thickness determines the duct’s pressure rating, its rigidity under negative pressure, and its resistance to chemical permeation.
The way to confirm the wall thickness is to provide your design data (airflow, velocity, pressure, gas type, temperature) to the supplier and ask them to confirm the recommended wall thickness for your application — rather than assuming a standard wall thickness is correct. The supplier’s product specification should state the wall thickness for each diameter and pressure class, and you should compare it against your design requirements. For the general picture of round PP duct sizes and wall thicknesses, the PP air duct product page lists the available range.
Decision point: after this section you can prepare a complete RFQ that includes the core sizing data, process conditions, and a wall-thickness request. The final section covers what this calculation cannot replace and answers the most common questions.
9. What This Calculation Cannot Replace + FAQ
9.1 Design values ≠ field measurements
A calculated diameter is a design value, and the difference between design and field is where real systems live. Installed ducts have fittings with real losses, the fan has an actual performance curve rather than a nominal rating, and branch balancing in a multi-duct system shifts airflow in ways a single-run calculation cannot predict. The result: even a correctly sized duct should be balanced and verified after installation — measure velocity and static pressure at the fan and at the critical branches, adjust dampers, and confirm the design airflow actually arrives where it was specified.
This is why the numbers in this guide are described as computed examples and design inputs rather than guarantees. Sizing charts and formulas get you to the right starting point; they do not remove the need for commissioning, measurement, and adjustment on site.
9.2 FAQ: standard sizes, rule of thumb, undersize vs oversize
What are the standard duct sizes? Round duct sizes step through common diameters — 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 24, 28, 32, 36 in, with the φ-range of PP duct continuing up to φ600 mm. Each table or chart assumes you round up to the nearest of these. Our duct sizing guide includes the standard-size table for the range covered here.
What is the rule of thumb for duct sizing? The familiar “400 CFM per ton” is a residential HVAC rule for sizing air-conditioning airflow to equipment capacity. It does not apply to industrial air duct sizing, where airflow is set by the process or the fume capture requirement rather than by tonnage. For industrial work the correct starting point is the actual design airflow, not a per-ton rule.
Is it better to oversize or undersize ductwork? Generally, oversizing is the safer error: a larger duct lowers velocity and pressure drop, adds noise and abrasion margin, and costs only material — while undersizing raises velocity, pressure drop, and noise, and is expensive to fix after installation. The practical discipline is to round up to standard sizes (as done throughout this guide) and to re-check velocity rather than oversize so far that dust settles in a low-velocity exhaust.
When should I use a calculator instead of doing it by hand? A duct size calculator or interactive sizing tool is faster for iterating over many scenarios and is a good cross-check for a hand calculation. This page deliberately works through the formulas and table so you understand what the calculator does; use the tool for speed, and use the reasoning here to sanity-check the tool’s output — the two should agree when the inputs (airflow and target velocity) match.
9.3 When to hand off to a professional
Hand the sizing to a professional designer when the system is large or highly branched, the process gas is highly corrosive or abrasive, the system runs at high negative or positive pressure, or the exhaust is safety-critical for personnel. In those cases, treat this page as a preliminary sizing sanity-check and route the final design through an engineer who will verify the numbers, the fan selection, and the installation. Our duct material selection guide for corrosive fumes covers the material-specific considerations that go beyond diameter sizing.
Final note: a calculated diameter is a design input, not a field measurement. Verify the installed system by balancing and measuring, confirm size and wall thickness with the supplier, and route extreme, large, or safety-critical systems through professional design. If you are at the specification stage and your case fits the scope here, the contact page will route your enquiry to the PP ductwork team, who can confirm the sizing and wall thickness for your project before you commit to the design. For the general picture of PP ductwork, see the PP air duct guide.
