Duct Size Calculator
Determine the correct duct size for airflow.
How It Works
Area (ft²) = CFM ÷ 900. Round diameter = 2 × sqrt(Area × 144 ÷ π). Rectangular: W×H at a 2:1 ratio.
Example
1,000 CFM: area = 1.11 sq ft → 14.3 in round duct or ~9 × 18 in rectangular.
Frequently Asked Questions
What is CFM?
Cubic feet per minute — the airflow a duct must carry to supply the room’s heating or cooling.
Why 900 ft/min?
It is a common residential duct velocity that balances noise and pressure loss.
Round or rectangular?
Round ducts move air most efficiently; rectangular is easier to fit in walls and dropped ceilings.