DUCT SIZE CALCULATOR

Size round and rectangular duct from CFM

Use equal-friction sizing and a velocity limit, then see a buildable round size, rectangular equivalent, and pressure-drop check.

EQUAL-FRICTION SIZING

Enter the design constraints

The larger size required by friction or velocity wins. Use total effective length—not just tape-measured straight duct—for the pressure-drop check.

NEXT STANDARD ROUND SIZE
16 in diameter

Calculated minimum 15.6 in · governed by velocity limit

At selected round size859 FPM0.070 in. w.g./100 ft
Rectangular equivalent21 × 11 in2:1 target aspect ratio
Estimated straight-duct pressure drop0.070 in. w.g.Across 100 ft effective length; filters, coils, grilles and other components are not included
METHOD AND LIMITS

More than a free-area guess, less than a full Manual D

Equal-friction method

Round galvanized duct uses the common friction-chart fit Δp = 0.109136 × CFM1.9 ÷ diameter5.02. The result is rounded up to a standard diameter.

Velocity guardrail

If the velocity limit needs a larger diameter than the friction target, the larger size wins. Rectangular dimensions use an equivalent-diameter relationship, then round up.

Before installation

Derive friction rate from available static pressure and total effective length. Flex compression, fittings, filters, coils, grilles, leakage, and the equipment blower table still require a full design check.