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.
Last 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.
Formula
V = Q / A A = π·D²/4 (converted to ft²)
| Symbol | Meaning | Unit |
|---|---|---|
V | Air velocity | fpm |
Q | Airflow | CFM |
A | Duct cross-sectional area | ft² |
Worked example
400 CFM in a 10-inch duct: area = 0.545 ft², velocity = 734 fpm — good trunk velocity. Squeeze it into 8 inches and it jumps to 1,146 fpm: noisy and lossy; that's why the sizing calculator picked 8 in only at lower friction targets.
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.
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.