Sheet set M · Mechanical

HVAC

Round and rectangular duct sizes, what a duct carries, reducing trunks.

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Duct Calculator

Round and rectangular duct size from airflow and your friction rate or velocity limit, rounded up to a size you can buy with velocity and friction recomputed, what a duct carries, and a reducing trunk sized after each branch.

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Quick reference

Airflow and pressure units, as arithmetic
Unit	Equals
1 cfm	0.471947 L/s
1 fpm	0.00508 m/s
1 in. wg	249.089 Pa
1 in. wg per 100 ft	8.1722 Pa/m

Every row is arithmetic, not a table anyone published: a foot is 0.3048 m, a minute 60 s, and an inch of water 0.0254 m × 1,000 kg/m³ × 9.80665 m/s². Velocity pressure converts to speed as V = 4005 √VP, V in fpm and VP in in. wg, for standard air at 70 °F (PNNL). Type your own airflow and limit on sheet M-101.