Duct sizing balances two things: a duct big enough to move the air without whistling, and small enough to fit the space and cost. The quick, standard way is the velocity method — pick a target air speed, and the duct area falls out of the airflow. Then round up to the next stock size.
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 rectangular, fix one side (the height you have) and the other grows to make up the area.
What velocity to design to
Lower is quieter but needs bigger ducts; higher saves space but adds noise and friction:
| Run | Velocity (fpm) | m/s |
|---|---|---|
| Residential branch | 600–800 | 3–4 |
| Residential main | 700–1,200 | 4–6 |
| Commercial main | 1,200–1,600 | 6–8 |
Round duct by airflow
Sizing at about 800 fpm (4 m/s), a rough guide for round duct:
| Airflow (CFM) | Round duct |
|---|---|
| 100 | 5 in (125 mm) |
| 200 | 7 in (180 mm) |
| 400 | 9–10 in (250 mm) |
| 800 | 12–14 in (350 mm) |
Duct Sizing Calculator
Enter the airflow in CFM (L/s) and a target velocity — it returns the round diameter or rectangular size, rounded to the next standard, and the actual velocity.
Round or rectangular?
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 needed diameter won’t fit. If the space allows, go round; otherwise fix the height you have and let the width grow.
Standard sizes and rounding
Round duct comes in a standard series — 4, 5, 6, 7, 8, 10, 12, 14, 16, 18 and 20 in (roughly 100–500 mm) — and rectangular in 1–2 in increments. Always round up to the next size, which brings the actual velocity a little under your target — quieter, not louder.
When to do a full calculation
The velocity method sizes straight runs well, but it doesn’t model friction rate, static pressure or fittings. For long runs with many bends, or systems where the fan’s available pressure is tight, do a full friction-rate calculation of the whole duct network as the proper check.