Duct Velocity Calculator
Enter airflow and duct diameter to get velocity and friction rate — the companion check to the duct sizing calculator for existing systems.
Open calculatorLast updated: 2026-09-15
How the calculation works
- Velocity is airflow divided by duct area — an 8-inch round duct is 0.349 ft², so 400 CFM runs at 1,146 fpm.
- The tool inverts the equal-friction fit to estimate what friction rate this velocity implies.
- The interpretation flags whether the velocity suits branches or trunks.
Worked example
Interpreting the result
Velocity is the noise and energy dial of duct design: every extra 100 fpm adds perceptible noise and squared friction loss. Residential targets: 600 fpm branches, 700–900 fpm trunks, 1000 fpm ceiling. Commercial systems run faster with acoustic treatment. Return-side grilles should stay near 500 fpm at the face.
Assumptions
- Round duct with uniform flow — flex duct and fittings alter the effective velocity.
- Standard air density.
Limitations
- Rectangular ducts need equivalent diameter conversion first.
- Entry/exit loss coefficients (fittings) dominate total pressure and aren't captured.
Air velocity (ft/min) in round ducts by airflow and diameter
Each cell is the calculator's result for that airflow and inside diameter.
| Airflow | 6 in duct | 8 in duct | 10 in duct | 12 in duct | 14 in duct |
|---|---|---|---|---|---|
| 100 CFM | 509 | 286 | 183 | 127 | 94 |
| 200 CFM | 1,019 | 573 | 367 | 255 | 187 |
| 400 CFM | 2,037 | 1,146 | 733 | 509 | 374 |
| 600 CFM | 3,056 | 1,719 | 1,100 | 764 | 561 |
| 800 CFM | 4,074 | 2,292 | 1,467 | 1,019 | 748 |
| 1,000 CFM | 5,093 | 2,865 | 1,833 | 1,273 | 935 |
| 1,500 CFM | 7,639 | 4,297 | 2,750 | 1,910 | 1,403 |
Velocity = CFM ÷ duct area, with area = π × d² ÷ 4 (in square feet).
Worked scenario: 400 CFM through an 8 in round duct
The duct area is 0.349 ft², so the air moves at 1,146 ft/min (5.82 m/s). The same airflow in a 6 in duct would travel at 2,037 ft/min, which shows how quickly velocity climbs as the duct shrinks.
Velocity, noise and pressure loss
Faster air means more noise and more pressure loss per length of duct, so a quiet system uses larger ducts or lower velocities, especially near occupied rooms. Acceptable velocity depends on the application and on the design guidance you follow.
Mistakes to avoid
- Entering the radius instead of the diameter.
- Using the nominal diameter of flexible duct when its inside diameter is smaller once it is compressed.
- Applying the fan's total airflow to a branch that carries only part of it.
Frequently asked questions
What is a good duct velocity?
600 fpm in branches, 700–900 fpm in supply trunks, and under 1000 fpm anywhere in a house. Above that, noise complaints are near-certain.
How do I convert rectangular to round velocity?
Compute the rectangular duct's area (W × H ÷ 144 ft²) and divide CFM by it — velocity follows from area, not shape.