What Size Duct Do I Need for My CFM?

Updated November 2, 2026 · Foster Tools · How we calculate

Divide the airflow by the air speed you want to get the duct's area: area (ft²) = CFM ÷ FPM. Then turn the area into a round diameter: diameter (in) = √(4 × area in² ÷ π). At a typical 700 FPM, 100 CFM needs a 5.1-inch duct, 200 CFM needs 7.2 inches and 400 CFM needs 10.2 inches. Round up to the next size you can buy.

The formula

area (ft²) = CFM ÷ velocity (FPM)
diameter (in) = √(4 × area × 144 ÷ π)

CFM is how much air the run has to carry. FPM is how fast that air moves through the duct. The same airflow fits in a smaller duct if it moves faster, but faster air is noisier and loses more pressure to friction. That's why sizing is a trade-off, not a single answer.

Duct size chart by CFM

Standard round sizes go up by an inch to 10 inches, then by two (12, 14, 16, 18, 20). A calculated 10.2 inches means buying 12-inch duct, unless you accept a slightly higher velocity in 10-inch.

Step by step

1. Get the CFM for the run. For a supply run, that's the room's share of the system airflow. A common rule of thumb is about 400 CFM per ton of cooling, split between rooms by their share of the load. A proper Manual J load calculation gives the real numbers.
2. Pick a velocity. Branch runs to bedrooms and returns are usually kept slower and quieter (around 500 FPM in this chart). Trunks can run faster.
3. Work out the area: CFM ÷ FPM. 150 CFM at 700 FPM is 0.214 ft², or 30.9 in².
4. Turn the area into a diameter: √(4 × 30.9 ÷ π) = 6.3 inches.
5. Round up to a standard size: 7-inch round.
6. Check the friction rate. The calculator shows inches of water column per 100 feet for the size you picked. Long runs, many fittings and flex duct all push it up.

Rectangular duct

For a rectangular duct, fix one side (often set by the joist or stud space) and solve for the other so it carries the same air at the same friction as the round size. The usual equivalent-diameter formula is 1.30 × (a × b)^0.625 ÷ (a + b)^0.25. An 8-inch-deep duct equivalent to a 10.2-inch round needs to be about 11 inches wide. The calculator works this out when you switch to rectangular.

Common mistakes

• Sizing from velocity alone on long runs. Velocity gets you a starting size; friction over the full run length and fittings decides whether it delivers the air.
• Treating flex like metal. Flex duct is far rougher inside and loses more when compressed or kinked. Pull it tight and check its friction rate.
• Rounding down to save money. A calculated 7.2 inches in 7-inch duct runs faster and louder than you planned.
• Skipping the load calculation. The CFM is the input everything else depends on. Guessing it makes the rest of the math precise but wrong.

This is the equal-friction starting point most duct calculators use, for standard air. It isn't a Manual D design. Code-required installs should follow your local mechanical code and a qualified HVAC professional.