CFM Calculator

Convert between fan airflow (CFM) and air changes per hour for a room.

Units	Feet / inches
Room length	12
Room width	10
Ceiling height	8
I know	Air changes per hour → CFM
Air changes per hour	8

Units

Room length

Room width

Ceiling height

I know

Air changes per hour

Fan airflow needed

128 CFM(217 m³/h, 60.4 L/s)

Room volume 960 ft³ (27.2 m³). 128 CFM replaces the air 8 times an hour.

Fan airflow for this room at different air-change rates

2 changes an hour32 CFM
4 changes an hour64 CFM
6 changes an hour96 CFM
8 changes an hour128 CFM
10 changes an hour160 CFM
15 changes an hour240 CFM

About the CFM Calculator

Air changes per hour (ACH) is how many times the room's air is replaced each hour. The US building code (ASHRAE 62.2 / IRC) requires bathroom exhaust fans of at least 50 CFM intermittent or 20 CFM continuous, and kitchen range hoods of at least 100 CFM intermittent. A common rule of thumb for bathrooms is 8 ACH, or 1 CFM per square foot of floor.

The formula

CFM = room volume (ft³) × ACH ÷ 60; 1 CFM = 1.699 m³/h = 0.4719 L/s

Worked example

Example: Fan airflow needed = 193 CFM (328 m³/h, 91.1 L/s)

Inputs: Units: Feet / inches; Room length: 13; Room width: 11; Ceiling height: 9; I know: Air changes per hour → CFM; Air changes per hour: 9.
Formula: CFM = room volume (ft³) × ACH ÷ 60; 1 CFM = 1.699 m³/h = 0.4719 L/s
Result: Fan airflow needed = 193 CFM (328 m³/h, 91.1 L/s)

Questions people ask

What size bathroom fan?

At least 50 CFM, or 1 CFM per ft² for bathrooms up to 100 ft².

Does duct length matter?

Yes, long or bent ducts reduce real airflow; check the fan's rating at your static pressure.