Duct Size from CFM Calculator
Ductwork must be large enough to carry the required airflow (in cubic feet per minute, CFM) without excessive velocity, which causes noise and pressure loss. The duct cross-sectional area equals the airflow divided by the design air velocity. This calculator finds the required area, the equivalent round duct diameter, and a matching rectangular duct height for a width you choose.
Duct sizing formula
Area (sq ft) = CFM / velocity (ft/min)
Area (sq in) = area (sq ft) * 144
Round diameter = sqrt( 4 * area (sq in) / pi )
Rect height = area (sq in) / chosen width
This applies the continuity relation: airflow equals velocity times area, so area equals airflow divided by velocity. Typical residential supply trunk velocities are around 700 to 900 ft per minute; branch runs and return ducts use lower velocities to limit noise. The round diameter comes from the area of a circle.
Worked example
To carry 400 CFM at 700 ft per minute: area = 400 / 700 = 0.5714 square feet = 82.29 square inches. Round diameter = sqrt(4 times 82.29 / 3.1416) = sqrt(104.8) = 10.24 inches, so a 10 inch round duct is the practical size. For a 12 inch wide rectangular duct, height = 82.29 / 12 = 6.86 inches, so a 12 by 7 inch duct works.
Duct sizing notes
- Lower velocities mean quieter, lower-resistance ducts but larger, costlier ductwork.
- Typical residential velocities: about 700 to 900 ft/min for supply trunks, lower for branches and returns.
- Rectangular and round ducts of equal area are roughly equivalent, but rectangular has slightly higher friction.
- This is a velocity-based first pass; a full ACCA Manual D design accounts for total friction and fittings.
- Round the calculated diameter up to the next standard duct size.
Duct Size from CFM Calculator: frequently asked questions
What size duct do I need for 400 CFM?
At a typical 700 ft per minute velocity, 400 CFM needs about 82 square inches of duct, which is roughly a 10 inch round duct or a 12 by 7 inch rectangular duct. Lower velocities require a larger duct.
How do I calculate duct size from CFM?
Divide the airflow in CFM by the target air velocity in feet per minute to get the cross-sectional area in square feet. Convert to square inches and use the circle-area formula to find the round diameter, or divide by a chosen width for a rectangular duct.
What air velocity should I design for?
Residential supply trunks are commonly designed around 700 to 900 feet per minute, with lower velocities for branch ducts and returns to keep noise down. Lower velocity means a larger, quieter duct. Velocity is an editable input here.
Is a round or rectangular duct better?
Round ducts have less friction and are more efficient for a given area, but rectangular ducts fit more easily in walls and ceilings. For the same airflow, match the cross-sectional area; this calculator gives both options.
Sources and methodology
- The duct-sizing relation (area equals airflow divided by velocity) is the standard continuity equation; the circle-area formula and 144 square inches per square foot are fixed math.
- U.S. Department of Energy: Ducts.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.