# What duct size do I need?

Duct calculator: the round duct size for an airflow in CFM by the equal-friction method or a target velocity, with the velocity, the friction rate and the equivalent rectangular duct.

- Page: https://www.acalculator.org/construction/duct-size-calculator
- JSON spec: https://www.acalculator.org/construction/duct-size-calculator.json
- Version: 4c339de4f6aa

## Default answer

Example with the default inputs (Airflow (CFM) 400, Size by Friction rate, Friction rate, in. w.c. per 100 ft 0.08): Use a 10.21 in round duct (11 in rounded up): 703 ft/min at 0.08 in. w.c. per 100 ft.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| cfm | Airflow (CFM) | The air the duct must carry, in cubic feet per minute. |
| by | Size by | A friction rate (equal friction), or a target air velocity. |
| fr | Friction rate, in. w.c. per 100 ft | The pressure drop allowed per 100 ft of straight duct, in inches of water; 0.08 to 0.1 is usual for homes. |
| v | Velocity, ft/min | The air speed you want in the duct, in feet per minute. |
| a | Rectangular duct side | One side of a rectangular duct, to find the other side with the same friction. Leave empty to skip. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| diameter | Round duct diameter | The exact diameter for the airflow. |
| roundUp | Next whole inch | The diameter rounded up to a whole inch. |
| velocity | Velocity | Airflow ÷ the duct area, in feet per minute. |
| rate | Friction rate | The pressure drop per 100 ft at this diameter, in inches of water. |
| pam | Friction rate, Pa/m | The same pressure drop in pascals per metre. |
| reynolds | Reynolds number | Velocity × diameter ÷ kinematic viscosity. |
| other | Other rectangular side | The side b that gives the round diameter by the Huebscher formula with the side you typed. |

## Method

Velocity method: D = √(4Q ÷ (πV)). Friction method: the D where f × ρ ÷ 2 × V² ÷ D equals the friction rate, with V = 4Q ÷ (πD²) and f from Colebrook–White. Rectangle: b with 1.30 (ab)^0.625 ÷ (a + b)^0.25 = D.

## Assumptions

- Standard air at 20 °C: density 1.2041 kg/m³, kinematic viscosity 15.06 × 10⁻⁶ m²/s.
- Round galvanized steel duct, absolute roughness 0.15 mm. Flex duct is rougher and needs a larger size.
- Straight duct only: fittings add pressure drop, which the friction rate you choose has to allow for.

## Worked examples

1. cfm = 400, by = friction, fr = 0.08 gives diameter = 0.259386, roundUp = 11. Source: Wikipedia, Darcy–Weisbach equation (Δp ÷ L = f × ρ ÷ 2 × V² ÷ D), https://en.wikipedia.org/wiki/Darcy%E2%80%93Weisbach_equation, and Darcy friction factor formulae (Colebrook–White, f = 64 ÷ Re below Re 2,300), https://en.wikipedia.org/wiki/Darcy_friction_factor_formulae (retrieved 2026-10-03); Wikipedia, Density of air (1.2041 kg/m³ at 20 °C, 101.325 kPa), https://en.wikipedia.org/wiki/Density_of_air; Engineering ToolBox, Air – Dynamic and Kinematic Viscosity (15.06 × 10⁻⁶ m²/s at 20 °C), https://www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601.html; Engineering ToolBox, Surface Roughness of Ventilation Ducts (galvanized steel 0.15 mm), https://www.engineeringtoolbox.com/surface-roughness-ventilation-ducts-d_209.html (retrieved 2026-10-03).
2. cfm = 100, by = friction, fr = 0.08 gives diameter = 0.154637, roundUp = 7. Source: Wikipedia, Darcy–Weisbach equation (Δp ÷ L = f × ρ ÷ 2 × V² ÷ D), https://en.wikipedia.org/wiki/Darcy%E2%80%93Weisbach_equation, and Darcy friction factor formulae (Colebrook–White, f = 64 ÷ Re below Re 2,300), https://en.wikipedia.org/wiki/Darcy_friction_factor_formulae (retrieved 2026-10-03); Wikipedia, Density of air (1.2041 kg/m³ at 20 °C, 101.325 kPa), https://en.wikipedia.org/wiki/Density_of_air; Engineering ToolBox, Air – Dynamic and Kinematic Viscosity (15.06 × 10⁻⁶ m²/s at 20 °C), https://www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601.html; Engineering ToolBox, Surface Roughness of Ventilation Ducts (galvanized steel 0.15 mm), https://www.engineeringtoolbox.com/surface-roughness-ventilation-ducts-d_209.html (retrieved 2026-10-03).
3. cfm = 400, by = velocity, v = 700, a = 0.254 gives diameter = 0.259987, v = 700, other = 0.222925. Source: Wikipedia, Darcy–Weisbach equation (Δp ÷ L = f × ρ ÷ 2 × V² ÷ D), https://en.wikipedia.org/wiki/Darcy%E2%80%93Weisbach_equation, and Darcy friction factor formulae (Colebrook–White, f = 64 ÷ Re below Re 2,300), https://en.wikipedia.org/wiki/Darcy_friction_factor_formulae (retrieved 2026-10-03); Engineering ToolBox, Equivalent Diameter (Huebscher: de = 1.30 (a b)^0.625 ÷ (a + b)^0.25; 300 × 500 mm gives 420 mm), https://www.engineeringtoolbox.com/equivalent-diameter-d_205.html (retrieved 2026-10-03).

## FAQ

### How do I size a duct for CFM?

Pick a friction rate, such as 0.08 in. w.c. per 100 ft, and find the round diameter whose pressure drop at that airflow matches it. 400 CFM at 0.08 needs a 10.2 in duct, so a 10 or 11 in duct by your judgement of the system.

### What friction rate should I use?

Home systems are often sized at 0.08 to 0.1 in. w.c. per 100 ft. The right value comes from the fan’s available pressure less the losses of the coil, filter, grilles and fittings, spread over the longest duct run.

### How does the velocity method work?

Divide the airflow by the velocity you want to get the duct area, then find the diameter: D = √(4 × area ÷ π). 400 CFM at 700 ft/min needs 0.571 ft², a 10.24 in duct.

### What is an equivalent rectangular duct?

A rectangle with the same friction and airflow as the round duct. The Huebscher formula, de = 1.30 (ab)^0.625 ÷ (a + b)^0.25, gives it; type one side and the calculator finds the other.

### How many CFM does a 6 in duct carry?

About 100 CFM at 0.08 in. w.c. per 100 ft: the calculator gives 6.09 in for 100 CFM at that rate.

### Does this work for flex duct?

The roughness used is galvanized steel. Flexible duct is rougher and sags, so it needs a larger size for the same airflow.

## Sources

- Wikipedia, Darcy–Weisbach equation: Δp ÷ L = f × ρ ÷ 2 × V² ÷ D, retrieved 2026-10-03. https://en.wikipedia.org/wiki/Darcy%E2%80%93Weisbach_equation
- Wikipedia, Darcy friction factor formulae: Colebrook–White 1/√f = −2 log(ε ÷ 3.7D + 2.51 ÷ (Re √f)); laminar f = 64 ÷ Re below about Re 2,300, retrieved 2026-10-03. https://en.wikipedia.org/wiki/Darcy_friction_factor_formulae
- Wikipedia, Density of air: dry air 1.2041 kg/m³ at 20 °C and 101.325 kPa, retrieved 2026-10-03. https://en.wikipedia.org/wiki/Density_of_air
- Engineering ToolBox, Air – Dynamic and Kinematic Viscosity: 15.06 × 10⁻⁶ m²/s at 20 °C, retrieved 2026-10-03. https://www.engineeringtoolbox.com/air-absolute-kinematic-viscosity-d_601.html
- Engineering ToolBox, Surface Roughness of Ventilation Ducts: galvanized steel 0.15 mm, retrieved 2026-10-03. https://www.engineeringtoolbox.com/surface-roughness-ventilation-ducts-d_209.html
- Engineering ToolBox, Equivalent Diameter: Huebscher de = 1.30 (a b)^0.625 ÷ (a + b)^0.25; 300 × 500 mm gives 420 mm, retrieved 2026-10-03. https://www.engineeringtoolbox.com/equivalent-diameter-d_205.html