Duct Sizing Calculator
Sizes HVAC ductwork by the equal-friction method: from the airflow and a target friction rate or velocity it gives the round diameter, velocity, friction rate, pressure loss and equivalent rectangular sizes, with a to-scale cross-section.
Airflow & sizing basis
Equal-friction sizing for standard air (ρ = 1.2 kg/m³). Typical target friction is ~0.8–1.0 Pa/m; galvanised duct roughness ≈ 0.09 mm. Everything runs in your browser.
Duct size
Round vs equivalent rectangular
| Height | Width | Aspect |
|---|---|---|
| 150 mm | 337.5 mm | 2.25:1 |
| 200 mm | 243.8 mm | 1.22:1 |
| 250 mm | 195.3 mm | 0.78:1 |
| 300 mm | 165.7 mm | 0.55:1 |
| 400 mm | 130.9 mm | 0.33:1 |
Each rectangular option carries the same airflow at the same friction rate as the round duct (equal-friction equivalent diameter). Keep aspect ratios below about 4:1. A design aid only — add fitting losses and verify against the relevant standards.
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the airflow and pick its units (L/s, m³/h or CFM).
- Choose whether to size to a target friction rate (e.g. ~1 Pa/m) or a target velocity, and enter the value.
- Enter the run length to get the pressure loss, then read the round diameter and the equivalent rectangular options.
How it works
For a target velocity the round area is simply airflow ÷ velocity, giving the diameter directly. For a target friction rate the diameter is found by iteration so the pressure gradient matches the target.
The friction rate uses the Darcy–Weisbach equation, Δp/L = f·(ρ·v²)/(2·D), with the friction factor f from the explicit Swamee–Jain formula (roughness ≈ 0.09 mm for galvanised steel) and standard air at 1.2 kg/m³.
Each rectangular option carries the same airflow at the same friction rate as the round duct — its equal-friction equivalent diameter, De = 1.30·(a·b)^0.625 / (a+b)^0.25, is solved back to the round diameter for the height you choose.
Worked example
Worked example. 200 L/s (0.2 m³/s) sized to 5 m/s: round area = 0.2 ÷ 5 = 0.04 m², so the diameter is √(4·0.04/π) ≈ 226 mm. The friction rate works out near 1.4 Pa/m, and a 300 mm-high rectangular duct about 200 mm wide carries the same air at the same friction.
Common mistakes
- Mixing airflow units — 200 L/s, 720 m³/h and 424 CFM are the same flow; pick the right unit for your figure.
- Sizing purely on velocity and ignoring the friction rate (or vice-versa) — the two are linked, so check both are sensible.
- Forgetting fitting losses — this sizes straight duct at its friction rate; bends, transitions and dampers add pressure that must be added separately.
Frequently asked questions
What is the equal-friction method?
It sizes every duct in a system to the same friction rate (pressure drop per metre), which keeps the design simple and reasonably balanced. A target of roughly 0.8–1.0 Pa/m is common for low-pressure comfort systems.
What air velocity should I aim for?
Lower velocities are quieter. Around 3–5 m/s suits residential and low-noise areas, 5–8 m/s is common in commercial mains, and above ~10 m/s noise and regenerated sound become a problem. The tool flags high velocities.
How do the rectangular sizes relate to the round duct?
They share the same equal-friction equivalent diameter, so a rectangular duct of the listed width and height carries the same airflow at the same friction rate as the round duct. Keep the aspect ratio below about 4:1 for efficiency.
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Tip: Enter any known values to calculate the remaining results.
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