Formula
D = (0.109136 · Q^1.9 / ΔP)^(1/5.02) · Vel (FPM) = CFM ÷ (πD²/4)Sizes round galvanized duct from CFM and target friction rate, then shows the equivalent rectangular sizes for common widths. Verify against ACCA Manual D for a full design.
Round diameter
11in (exact 10.27")
Nearest commercial size (round up)
Velocity
606FPM
At 11" round · target 700–900 FPM
Residential supply trunk 700–900 FPM (branches 600–700). Duct may be oversized — verify branch balancing.
Equivalent rectangular (in × in)
6 × 16
8 × 12
10 × 9
12 × 8
14 × 7
16 × 6
18 × 6
20 × 5
22 × 5
24 × 5
How to use this calculator
Select round or rectangular duct size from airflow using friction rate or velocity.
- Enter the design airflow for the run in CFM.
- Choose the design friction rate, commonly 0.08–0.1 in w.c. per 100 ft.
- Read the required round diameter.
- Convert to an equivalent rectangular size if the space requires it.
Frequently asked questions
- What size duct do I need for 400 CFM?
- At a 0.1 in w.c. per 100 ft friction rate, 400 CFM needs about a 10 in round duct or an equivalent rectangular size.
- What velocity is acceptable in residential duct?
- Aim for about 700–900 FPM in branch runs and up to 1,200 FPM in main trunks; higher velocity creates noise.
- How do I convert round to rectangular?
- Use the equivalent-diameter table rather than matching area — a rectangular duct has more wall surface and higher friction for the same area.