Work out exhaust fan sizing instantly with clear inputs, formula shown and shareable results.
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.
Exhaust airflow
CFM = floor area x ceiling height x ACH / 60
Duct diameter
Diameter in = sqrt(4 x (CFM / 700) / pi) x 12
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.
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.
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.