Duct Sizing Calculator

It works out how big a duct needs to be to carry a given airflow at a sensible air speed. Enter the airflow in CFM (L/s), pick a target velocity for the type of run, choose round or rectangular, and it returns the duct size in inches (mm) — rounded up to the next standard size — along with the actual velocity through it.

It uses the velocity method: the required cross-sectional area is the airflow divided by the target velocity (A = Q ÷ v). For a round duct the diameter follows from d = √(4A ÷ π); for a rectangular duct you fix one side and the calculator finds the other from the same area. It handles the CFM-to-area conversion so the duct comes out in inches.

Lower is quieter, higher needs smaller ducts. Typical design velocities are about 700–1,200 fpm (4–6 m/s) for residential main ducts and 600–800 fpm (3–4 m/s) for branches; commercial mains often run 1,200–1,600 fpm (6–8 m/s). Above that, noise and friction losses climb quickly. The calculator presets a sensible figure for main or branch runs, which you can override.

Round duct is more efficient: for the same area it has less surface, so lower friction, less noise and easier sealing. Rectangular duct exists to fit shallow spaces — ceiling voids and bulkheads — where a round duct of the required diameter won’t fit. If space allows, go round; otherwise fix the height you have and let the width grow.

Round duct comes in a standard series — 4, 5, 6, 7, 8, 10, 12, 14, 16, 18 and 20 in (roughly 100–500 mm) are common — so the calculator rounds your exact diameter up to the next one. Rectangular duct is usually made in 1–2 in increments. Rounding up means the actual velocity comes in a little under your target.

The velocity method is the standard quick way to size straight runs and works well for typical installs. It doesn’t model friction rate, static pressure or fittings, so for long runs with many bends, or systems where the fan’s available pressure is tight, a full friction-rate calculation of the whole duct network is the proper check.