Duct Sizing Calculator — CFM, Velocity & Equal Friction

Short answerRound duct area (ft²) = CFM ÷ velocity (fpm); diameter (in) = √(576 × area ÷ π). Keep supply mains around 1,000–1,500 fpm, branches 600–900 fpm; the equal-friction method targets ~0.08–0.10 in.wg per 100 ft.

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How it's calculated

Airflow, velocity and cross-section are linked: area equals CFM divided by velocity. Pick a target velocity (lower is quieter, larger and costlier), compute the area, then the round diameter or an equivalent rectangular size. The equal-friction method instead holds a constant pressure loss per length.

Area(ft²) = CFM ÷ Velocity(fpm) Ø(in) = √( (Area × 144) × 4 ÷ π ) Equal friction ≈ 0.10 in.wg / 100 ft

Worked example

400 CFM branch at 800 fpm.
Area = 400 ÷ 800 = 0.50 ft² (72 in²).
Ø = 9.6 in → 10-inch round.

Reference tables

Round duct size at ~0.1 in.wg/100 ft (approx.)

CFM	Ø (in)
100	6
200	7
400	9
600	11
1,000	13
1,500	16
2,000	18

Typical design velocities (fpm)

Section	Velocity
Main supply	1,000–1,500
Branch	600–900
Return	700–1,000

A number the tables do not give: 400 CFM at 800 fpm equals a 10-inch round or roughly an 8×8-inch rectangular duct.

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FAQ

Velocity or equal-friction method?

Velocity is quick for a single run; equal-friction keeps a whole system balanced at a constant pressure loss. Most designs use equal-friction with velocity as a noise check.

Why keep velocity down?

High velocity means noise and more fan energy. The limits in the table keep ducts quiet without oversizing.

Round vs rectangular?

Round is the most efficient for a given area; rectangular fits tight spaces but needs a larger equivalent size for the same flow.

Does duct length change the size?

With equal-friction, longer runs accumulate more pressure loss, so long runs may need a size up to protect the fan.

This page is a reference aid. Always verify against ASHRAE / SMACNA methods and the applicable mechanical code. Figures shown are for orientation, not a stamped design.

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