Duct Size (CFM) Calculator
Sizing a duct comes down to a flow relationship: the air volume per minute equals the duct's cross-sectional area times the air velocity, so to find the area you divide the required airflow by the velocity you want. This calculator takes the airflow in cubic feet per minute and a target velocity in feet per minute, returns the area in square feet, and then converts that area to the diameter of an equivalent round duct. Velocity matters because too high a speed makes ducts noisy and wastes fan energy, while too low a speed needs bulky ducts; typical residential supply velocities sit in a moderate range that this tool lets you set. Enter the CFM the branch must carry and your design velocity, and the tool returns both the required area and the round duct diameter in inches, which you can round up to the nearest available size. The same area can be made into a rectangular duct of equivalent capacity. This is a clean first-pass sizing aid; full duct design also balances pressure across the system. Every figure is computed deterministically from the formulas shown below, with a worked example that reconciles exactly to the calculator defaults.
Duct area is airflow / velocity, and diameter follows from the area. Carrying 400 CFM at 700 ft/min needs 0.57 sq ft, a round duct about 10.24 inches across.
Duct size formula
Area (sq ft) = CFM / velocity
Diameter (in) = sqrt(4 x Area / pi) x 12
CFM = airflow in cubic feet per minute
velocity = air speed in feet per minute
Area is in square feet because CFM is cubic feet per minute and velocity is feet per minute. The diameter formula converts a round area to inches; round up to the next available duct size.
Worked example
Suppose a branch must carry 400 CFM at a target velocity of 700 feet per minute.
- Area: 400 / 700 = 0.5714 sq ft
- Diameter in feet: sqrt(4 x 0.5714 / 3.14159) = 0.8530 ft
- Diameter in inches: 0.8530 x 12 = 10.24 in
The required area is 0.57 square feet, which is a round duct about 10.24 inches in diameter. These are the calculator's default inputs, so the result matches the widget exactly.
Duct Size (CFM) Calculator: frequently asked questions
How do I size a duct from CFM?
Divide the required airflow in cubic feet per minute by your chosen air velocity in feet per minute to get the cross-sectional area in square feet, then convert that area to a round diameter or an equivalent rectangular size. Round up to the nearest standard duct size that is available.
What velocity should I use?
Residential supply ducts are often designed around moderate velocities to balance noise, fan energy and duct size; main trunks run faster than branches. Lower velocities are quieter but need larger ducts. This tool leaves velocity editable so you can match your design target.
Why does velocity affect noise?
Fast-moving air through fittings and grilles generates turbulence and sound. Keeping velocity within recommended ranges limits noise and reduces friction losses that the fan must overcome. Oversized fans pushing air too fast waste energy and annoy occupants.
Can I use a rectangular duct instead?
Yes. Once you have the required area, you can choose rectangular dimensions that give an equivalent capacity, using an equivalent-diameter table or chart. Rectangular ducts fit tight spaces but have more surface area and friction than a round duct of the same capacity.
Is this a complete duct design?
No. This sizes a single duct from airflow and velocity. A full design also balances static pressure across the whole system, accounts for fittings and length, and verifies the fan can deliver the total airflow. The Department of Energy publishes guidance on efficient duct systems.
Official sources
- Duct systems and home energy efficiency guidance: US Department of Energy (DOE). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.