Duct Size Calculator
Enter the airflow the duct has to carry in CFM and choose a target air speed. You get the round duct size, a rectangular duct with the same airflow, the real velocity and the friction loss per 100 feet.
- Round and rectangular
- Velocity method
- Friction loss check
For galvanised steel duct. Flexible duct has about twice the friction, so go up one size for flex runs.
Duct size is a trade-off between noise and space. Push air through a small duct and it moves fast, makes noise and loses pressure to friction. Use a larger duct and the air moves slowly and quietly, but the duct takes more room in walls and ceilings. Homes usually aim for 600 to 900 feet per minute.
Round duct capacity
Airflow per duct size at common velocities. Search by diameter or CFM.
| Diameter (in) | Area (sq in) | CFM at 600 fpm | CFM at 900 fpm | Friction at 700 fpm |
|---|---|---|---|---|
| 4 | 12.6 | 52 | 79 | 0.26 |
| 5 | 19.6 | 82 | 123 | 0.2 |
| 6 | 28.3 | 118 | 177 | 0.16 |
| 7 | 38.5 | 160 | 241 | 0.13 |
| 8 | 50.3 | 209 | 314 | 0.11 |
| 9 | 63.6 | 265 | 398 | 0.1 |
| 10 | 78.5 | 327 | 491 | 0.08 |
| 12 | 113.1 | 471 | 707 | 0.07 |
| 14 | 153.9 | 641 | 962 | 0.06 |
| 16 | 201.1 | 838 | 1257 | 0.05 |
| 18 | 254.5 | 1060 | 1590 | 0.04 |
| 20 | 314.2 | 1309 | 1963 | 0.04 |
The friction column is for galvanised duct at 700 fpm. Most residential systems are designed around 0.08 to 0.1 inches of water per 100 feet.
Airflow and friction
Duct size for a target of 700 fpm. Slide the airflow and compare duct sizes either side.
Friction rises roughly with the fifth power of a smaller diameter. Going down just two inches can more than double the pressure the blower has to overcome.
How to size a duct
Start from the room, work back to the furnace. Step through the method.
Find the airflow
About 400 CFM per ton of cooling, split between rooms by their load.
The total of every branch must equal the blower's airflow. If the trunk is too small, branches at the far end get starved of air.
Round, rectangular or flex
Same airflow, different shapes. Pick a duct type.
The most efficient shape for airflow and the cheapest per foot.
Flat ducts that fit between joists and above ceilings.
Quick to install but sags and kinks easily. Keep runs short and pulled tight.
A rectangular duct needs more sheet metal and has more friction than a round duct of the same area. Use it where space forces a flat profile.
Ductwork checklist
Good ducts make any system work better. Tick these as you plan.
Tick each item as you go.
Leaky ducts in an unheated attic can waste 20% to 30% of the air the furnace or AC moves. Seal every joint with mastic or foil tape rated for ducts.
What the result shows
Size, speed and pressure. Each tile explains one result.
The nearest standard diameter that carries the air.
Width for your chosen height.
Air speed in the standard size.
Inches of water per 100 feet.
Cross-section in square inches.
The rectangular equivalent uses the ASHRAE equal friction formula, so it carries the same air with the same pressure loss as the round duct.
Frequently Asked Questions
No question matches that search.
What size duct for 400 CFM?
About 10 inches round at 700 fpm, or 12 inches if you want a quieter system.
How many CFM can a 6 inch duct carry?
About 100 to 120 CFM in a home system, at 500 to 600 fpm.
How do I convert round duct to rectangular?
Use the ASHRAE equivalent diameter formula. A 10 inch round duct is about the same as 8 × 11 inches.
What velocity should residential ducts be?
Branches 600 to 700 fpm and trunks up to 900 fpm. Higher speeds cause noise.
How many CFM per ton of AC?
About 400 CFM per ton, so a 3 ton system moves about 1,200 CFM.
Does flex duct need to be bigger?
Yes. Flex has more friction, so go up one size and keep runs short and straight.
What friction rate is normal?
0.08 to 0.1 inches of water per 100 feet for most homes.
Move the air quietly
Find each room's airflow, choose a sensible velocity and let the calculator give the diameter and friction. Seal and insulate the ducts, size the returns as carefully as the supplies, and the system will deliver the comfort it was designed for.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.