Exhaust Fan Sizing Calculator

Work out exhaust fan sizing instantly with clear inputs, formula shown and shareable results.

About Exhaust Fan Sizing Calculator

An exhaust fan is sized by the number of times the room air must be replaced each hour. Volume multiplied by target ACH gives cubic feet per hour, and dividing by 60 converts it to the CFM rating fans are sold by. The result is rounded up to a catalogue size and matched to a duct at 700 fpm to keep the fan quiet.

Formulas

Exhaust airflow

CFM = floor area x ceiling height x ACH / 60

Duct diameter

Diameter in = sqrt(4 x (CFM / 700) / pi) x 12

Worked example

With the default values below, Exhaust Fan Sizing Calculator returns Fan airflow required of 64. Change any input above and every figure here is recalculated in your browser.

Inputs used

Room floor areaminimum 1 ft²: 60 ft²
Ceiling heightminimum 1 ft: 8 ft
Target air changes per hourminimum 1 ACH: 8 ACH

Results

Fan airflow required: 64 CFM
Fan size to specify: 70 CFM
Room volume: 480 ft³
Duct diameter at 700 fpm: 4.3 in

Step by step

1Values used
2Exhaust airflow
3Duct diameter
4Fan airflow required= 64 CFM
5Fan size to specify= 70 CFM
6Room volume= 480 ft³
7Duct diameter at 700 fpm= 4.3 in

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Exhaust Fan Sizing figure worked out?

CFM = floor area x ceiling height x ACH / 60. An exhaust fan is sized by the number of times the room air must be replaced each hour.

Does duct length affect the fan I need?

Yes. Published CFM is measured at low static pressure; a long or elbow-heavy duct can cut delivered airflow by a third, so choose a fan rated at the static pressure your duct actually imposes.

What values do I need to enter?

This calculator takes 3 inputs: Room floor area, Ceiling height, Target air changes per hour. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.