Duct Area Calculator
Work out the cross-sectional area a duct needs from the airflow it must carry and your target air velocity, then get an equivalent round-duct diameter and square-duct side for the same area.
Enter Values
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 Q in litres per second (L/s).
- Enter the target air velocity v in metres per second (m/s) — lower for quiet spaces, higher for plant rooms.
- Read the required area, then the round diameter or square side that delivers it.
How it works
Airflow, area and velocity are linked by Q = A × v. Rearranged, A = Q / v, with Q converted from L/s to m³/s (divide by 1000). A round duct of area A has diameter d = √(4A/π); a square duct of the same area has side s = √A. The tool reports both so you can choose the duct shape that fits the space.
Worked example
Worked example. For 500 L/s at a target 4 m/s, A = 0.5 ÷ 4 = 0.125 m². That equals a round duct of about 399 mm diameter, or a square duct roughly 354 mm on a side.
Common mistakes
- Entering airflow in m³/s or L/min instead of L/s — the formula here expects litres per second.
- Picking a velocity that is too high, which saves space but drives noise and fan pressure up.
- Forgetting that real installs round up to standard duct sizes and add allowances for fittings and friction.
Frequently asked questions
What velocity should I design for?
Typical comfort systems use around 4–7 m/s in main ducts and less in branches near occupied rooms to control noise. Plant rooms and industrial runs can go higher. Always check the project spec and noise criteria.
Why give both a round and a square duct?
The same area can be built as either shape. Round duct is efficient and quiet; rectangular duct fits flat ceiling voids. Showing both lets you match the available space.
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Tip: Enter any known values to calculate the remaining results.
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