Duct Velocity Calculator
Air moving too fast is noisy; too slow wastes metal. Find the air velocity in a duct from its size and airflow, or size a duct to hit a target velocity — with a check against recommended ranges for supply, return and branch runs.
Why Duct Velocity Matters
Air velocity is the speed the air travels through a duct — airflow divided by the duct’s cross-sectional area. Get it wrong in either direction and you pay: too fast and the system whistles, roars and burns fan power; too slow and the ducts are needlessly large and expensive. The sweet spot balances noise, energy and material cost, which is why designers size ducts to a target velocity.
Recommended Velocities
| Run | Typical velocity (fpm) |
|---|---|
| Residential supply main | 700 to 900 |
| Residential branch | 600 to 700 |
| Commercial supply main | 1000 to 1500 |
| Commercial branch | 600 to 1000 |
| Return air | 500 to 800 |
Velocity, Friction and Noise
Friction loss climbs roughly with the square of velocity, so doubling speed quadruples the pressure the fan must overcome. Noise rises even faster. As a rule of thumb, residential ducts above about 900 fpm start to be heard and above 2000 fpm are clearly noisy. Slowing the air is one of the cheapest ways to quiet a system, at the cost of larger ducts.
Sizing a Duct to a Velocity
Working backward, the duct area you need is simply the airflow divided by the target velocity, and the round diameter follows from that area. Choose a target in the recommended band for the run, compute the size, then round up to the next available duct. This calculator does both directions, so you can check an existing duct or size a new one.
Frequently Asked Questions
How do you calculate air velocity in a duct?
Divide the airflow by the duct cross-sectional area. In US units, velocity in feet per minute equals CFM divided by the duct area in square feet.
What is a good air velocity for a duct?
About 700 to 900 fpm for residential supply ducts and 1000 to 1500 fpm for commercial trunks. Returns run a little slower; above roughly 2000 fpm ducts become noisy.
What duct size do I need for a given CFM?
Divide the CFM by your target velocity to get the required area, then find the round diameter for that area and round up to the next standard size.
Does higher velocity mean more noise?
Yes. Both noise and friction loss rise steeply with velocity, so keeping speed in the recommended range keeps a system quiet and efficient.
Related calculators
- CFM & Air Changes Calculator — set the airflow first.
- Duct Static Pressure Calculator — friction loss at that velocity.
- Fan Laws Calculator — match the fan to the duct.
- Duct Size Calculator — quick residential duct sizing.