How to Use the Duct Sizing Calculator
Sizing a duct too small drives up air velocity, noise, and pressure loss; sizing it too large wastes ceiling space and sheet metal. This calculator takes required airflow and an allowable velocity, then computes the required round duct cross-section and diameter, with a recommended standard SMACNA duct size.
The math is straightforward: airflow (CFM, cubic feet per minute) divided by allowable velocity (fpm, feet per minute) gives the required cross-sectional area in square feet. Solving the circle area formula (A = πr²) in reverse for that area gives the required diameter.
Velocity that's too high increases noise and pressure loss, while velocity that's too low means an oversized, more expensive duct. SMACNA design guidance commonly targets around 1,000-1,500 fpm for main trunk ducts and 600-1,000 fpm for branch ducts, with the right target depending on the application (office vs. industrial) and acoustic requirements.
Based on the calculated diameter, this calculator recommends the nearest standard round duct size. If you're using rectangular duct instead, convert using a SMACNA circular-equivalent (equivalent diameter) chart to find rectangular dimensions with the same effective area — and have an HVAC engineer confirm the final duct design.
Frequently Asked Questions
Too high a velocity increases noise and pressure loss; too low a velocity means an oversized duct that wastes space and material. SMACNA guidance commonly uses around 1,000-1,500 fpm for main trunk ducts and 600-1,000 fpm for branch ducts, depending on application and noise criteria.
This calculator gives the round duct diameter. To convert to rectangular, use a SMACNA circular-equivalent (equivalent diameter) chart to find rectangular dimensions with the same effective cross-sectional area as the calculated round diameter.