Duct Sizing (CFM) Calculator

What Is It

Duct sizing balances airflow capacity against noise and pressure loss: velocity too high causes noise and excessive static pressure, while velocity too low requires oversized, expensive ductwork. The velocity method sizes ducts directly from required airflow (CFM) and a target design velocity appropriate for the application.

How to Calculate

Enter the required airflow in CFM and a design velocity (700-900 fpm is typical for residential supply ducts, higher for commercial trunk lines). The calculator computes the required cross-sectional area and the equivalent round duct diameter, then rounds up to the nearest standard manufactured size.

Example

400 CFM at a 900 fpm design velocity: required area = 400/900 = 0.444 sq ft = 64 in2. Round duct diameter = square root of (64x4/pi), about 9.0 inches, rounding up to a standard 10-inch duct.

Recommendations

Match design velocity to the application: 700-900 fpm keeps residential ducts quiet, while commercial trunk ducts can run 1,000-1,500 fpm where some noise is acceptable and space is at a premium. Rectangular ducts can be sized to the same equivalent area if round duct doesn't fit the available space.

Frequently Asked Questions