What it calculates
Estimate round duct diameter from airflow and target velocity.
Round Duct Size Planner
Estimate round duct diameter from airflow and target air velocity.
Planning and estimating aid only. Verify project requirements, manufacturer instructions, adopted codes, permits, and field conditions before performing regulated work.
HVAC planning
How to Use This Calculator
Use the field units shown below, then compare your result with the worked example. Do not round the equivalent diameter and assume the resulting system will deliver 400 CFM.
How the calculation works
Required duct area = CFM ÷ target velocity. The area is converted to an equivalent round diameter.
What the result means
Final duct design should also account for friction rate, fittings, static pressure and noise.
Translate airflow and velocity into equivalent area
At 400 CFM and an entered velocity of 800 feet per minute, area is 400 ÷ 800 = 0.5 square feet, or 72 square inches. A circle with that area has a diameter of about 9.6 inches.
Contractor tip: Do not round the equivalent diameter and assume the resulting system will deliver 400 CFM. Check the complete route and design conditions before specifying ducts or equipment.
Verify the final quantity
The output is an area-equivalent diameter, not a selected stock duct or a complete duct design. The model does not include available static pressure, fittings, friction losses, leakage or noise.
Good practice: Compare the calculated quantity with plans, specifications, product packaging and current field conditions before purchasing material.