Ductwork Sizing Calculator
Size round and rectangular ducts using the equal friction method per ASHRAE Fundamentals Ch. 21 / SMACNA. Darcy-Weisbach friction using Swamee-Jain / Colebrook-White.
ASHRAE / SMACNA ยท SI unitsDuct Parameters
Round Duct Sizing
Engineering Reference
Equal Friction Method โ ASHRAE
The most common duct design approach (ASHRAE Fundamentals Ch. 21). Select a design friction rate (typically 0.8โ1.2 Pa/m) and size each duct segment to achieve that rate. Balancing dampers compensate for different branch resistances.
For a more balanced system, the static regain method can be used on long main runs, sizing each segment so that velocity pressure recovery compensates for friction. This is rarely needed for most building systems.
Darcy-Weisbach Pressure Drop
where: f = Darcy friction factor (Colebrook-White)
L = duct length [m], D = diameter [m]
ฯ = air density โ 1.20 kg/mยณ, v = velocity [m/s]
Friction factor f is calculated using the Swamee-Jain approximation to the Colebrook-White equation. For laminar flow (Re < 2300), f = 64/Re. Typical turbulent f for galvanised ductwork: 0.018โ0.025.
Rectangular Duct Equivalent Diameter
ASHRAE equivalent diameter gives the round duct diameter with the same friction loss per unit length. A 400ร300 mm rectangular duct has De = 374 mm. Keep aspect ratios โค4:1 (SMACNA) โ high aspect ducts have higher surface-to-area ratios, increasing friction and heat gain/loss.
Velocity Limits (SMACNA)
| Application | Max m/s | Max ft/min |
|---|---|---|
| Main supply duct | 8 | 1575 |
| Branch supply | 6 | 1180 |
| Return / extract | 5 | 984 |
| Low-velocity / acoustic | 4 | 787 |
| Residential supply | 3.5 | 689 |
Exceeding velocity limits causes duct noise, erosion of joints, and increased fan energy. Add fitting allowances (typically 50โ100% of straight-run length equivalent) for elbows and tee pieces.
Worked Example โ 500 L/s Main Supply Duct
Round duct sizing (bisection method):
Try D = 430 mm โ v = 0.5/(ฯร0.215ยฒ) = 3.44 m/s โ f โ 0.018
ฮP/L = 0.018 ร (1/0.43) ร (1.2ร3.44ยฒ/2) = 1.19 Pa/m โ
Velocity check: 3.44 m/s < 8 m/s (main duct limit) โ
Total loss over 20 m: 1.2 ร 20 = 24 Pa
+ fitting allowance (50% equivalent length): +10 Pa
= ~34 Pa for this section
Rectangular alternative (width fixed at 600 mm):
De = 430 mm โ solve for H: 1.30 ร (600รH)^0.625 / (600+H)^0.25 = 430
โ H โ 290 mm โ 600ร290 mm duct, aspect 2.07:1 โ
Common Pitfalls
- Sizing by velocity alone. Hitting a velocity target doesn't guarantee the friction rate you need โ a duct at 6 m/s can still produce 3 Pa/m if the diameter is small. Check both velocity AND pressure drop per metre against the system fan curve.
- Ignoring fitting equivalent length. Elbows, tees, and transitions commonly add 50โ100% to straight-run pressure loss. A 20 m duct run with four elbows may behave like 35โ40 m of straight duct. Always add a fitting allowance before matching fan static pressure.
- Flat oval or high-aspect-ratio rectangles. A 400ร100 mm duct carries less air than its equivalent-area round duct โ aspect ratios above 4:1 increase friction by 25โ40% and reduce acoustic performance. Keep rectangular ducts close to 1:1 where possible.
- Using round-duct equivalents without adjusting material. The equivalent-diameter formula assumes galvanized sheet metal. Flexible ducts, insulated flex, and fabric duct have 2โ4ร higher friction factors โ size them a step larger or check manufacturer data.
- Ignoring air density. At altitude (Denver, 1600 m) or in hot exhaust streams, air density drops 15โ25%, lowering velocity pressure and friction. Rated airflow at 20ยฐC sea level won't match performance at 40ยฐC and 1500 m elevation.
Frequently Asked Questions
Should I use round, rectangular, or oval duct? Round duct has the lowest friction and best rigidity for a given cross-section, so prefer it in mechanical rooms and exposed ceilings. Rectangular works where height is limited (above drop ceilings, in chases). Flat oval splits the difference โ lower height than round, lower friction than rectangular โ but costs more.
What's a reasonable friction rate for sizing? Commercial supply ducts typically target 0.8โ1.2 Pa/m (0.08โ0.12 in.wc/100 ft). Residential systems often use 0.25 in.wc/100 ft. Low-pressure return ducts can run at 0.5 Pa/m. Higher rates save material but require larger fan motors and generate noise.
How do I convert round to rectangular? Use the equivalent-diameter formula De = 1.30 ร (ab)^0.625 / (a+b)^0.25 where a and b are rectangular dimensions. The calculator does this automatically. Check both friction equivalence and physical fit โ a 500 mm round may become a 600ร400 mm rectangle with the same friction loss.
Why does my fan deliver less CFM than calculated? Real systems always have more resistance than paper calculations โ fitting losses get underestimated, filters load up, and flex duct sags. Design to 85โ90% of fan-curve CFM at calculated static pressure, or specify a VFD so you can commission and balance after installation.
What velocity is too noisy? For occupied spaces: 4 m/s (787 fpm) for main ducts, 3 m/s (590 fpm) for branches near diffusers, 2 m/s for terminal boxes. Above these, air rumble and register noise become audible. Mechanical rooms and outdoor ducts can run at 8โ10 m/s.
Related Calculators
Ductwork sizing is part of a broader HVAC airflow analysis workflow. Pair this tool with:
- Airflow & Static Pressure Calculator โ after sizing ducts, sum total system pressure (ducts + fittings + filter + coil) to match against fan curve.
- CFM Converter โ convert between CFM, L/s, mยณ/h, and mยณ/s when equipment specs use different units than your design.
- Cooling Load Calculator โ determine required supply CFM from sensible cooling load (Q_s = 1.08 ร CFM ร ฮT) before sizing ducts.
- Ventilation Requirements โ calculate minimum outdoor air CFM per ASHRAE 62.1 before adding it to supply-duct totals.
- Pressure Drop Calculator โ check fitting, filter, and coil pressure losses that the duct friction calculation alone doesn't capture.
- Psychrometric Calculator โ find supply air density at non-standard temperatures to correct friction and velocity results.
Also in Engineering
- โ ASHRAE Humidity Calculator โ Calculate dew point and verify conditions against ASHRAE TC 9.9 environmental classes
- โ Friction Factor โ Calculate friction factor using Colebrook equation or Swamee-Jain approximation
- โ Pressure Drop (Darcy-Weisbach) โ Calculate pressure drop in pipes using the Darcy-Weisbach equation
- โ Boiler & Hydronic Sizing Calculator โ Size a boiler from heat load, calculate design flow rate, pipe velocity, expansion tank volume, and radiator output correction per ASHRAE / CIBSE Guide B / EN 442.