Duct Size Calculator
Determine the correct duct size for airflow.
Last updated 2026-08-21
What Is a Duct Size Calculator?
A duct size calculator determines the correct duct dimensions for the airflow, or CFM, your system needs to move. You enter the airflow, duct type, and friction rate, and it returns a round or rectangular duct size. Correct sizing balances noise, pressure loss, and airflow for comfortable heating and cooling.
How It Works
Area (ft²) = CFM ÷ 900. Round diameter = 2 × sqrt(Area × 144 ÷ π). Rectangular: W×H at a 2:1 ratio.
Example
1,000 CFM: area = 1.11 sq ft → 14.3 in round duct or ~9 × 18 in rectangular.
How to Calculate
- Determine the CFM, or cubic feet per minute, the room needs. A system moving 1,000 CFM is a common residential figure.
- The tool divides CFM by a typical duct velocity of 900 feet per minute to get the cross sectional area.
- For 1,000 CFM, the area is about 1.11 square feet.
- Convert that area to a round duct, which works out to about a 14.3 inch diameter.
- For a rectangular duct at a 2 to 1 ratio, that is about 9 by 18 inches for easier wall or ceiling fit.
Round Duct Sizes for Common Airflows
| Airflow (CFM) | Round Duct Diameter (in) | Equivalent Rectangular (in) |
|---|---|---|
| 200 | 6 | 4 x 8 |
| 400 | 9 | 6 x 10 |
| 600 | 11 | 8 x 12 |
| 1,000 | 14 | 9 x 18 |
| 1,400 | 17 | 12 x 18 |
Sources
Frequently Asked Questions
What is CFM?
Cubic feet per minute — the airflow a duct must carry to supply the room’s heating or cooling.
Why 900 ft/min?
It is a common residential duct velocity that balances noise and pressure loss.
Round or rectangular?
Round ducts move air most efficiently; rectangular is easier to fit in walls and dropped ceilings.
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