CFM Calculator

Size an exhaust fan or ventilation system in cubic feet per minute (CFM) from room volume and the air changes per hour you need.

Bathrooms 8, kitchens 15, workshops 6–10, garages 4–6 — check local code.

Required airflow

128 CFM

  • Room volume960 ft³
  • Bathroom rule (1 CFM per ft² of floor)120 CFM

Estimates only. Round up when you order, and confirm structural details with your local building code.

How to use it

  1. Enter room dimensions.
  2. Enter air changes per hour for the room type.

The formula

CFM = room volume × air changes per hour ÷ 60

Worked example

Room length12 ft
Room width10 ft
Ceiling height8 ft
Air changes per hour8

Result

Required airflow128 CFM
Room volume960 ft³
Bathroom rule (1 CFM per ft² of floor)120 CFM

Tips before you order

  • For bathrooms up to 100 ft², a common rule is 1 CFM per square foot with a 50 CFM minimum.
  • Duct length and elbows reduce a fan's real airflow — choose a fan rated at the right static pressure.

Questions

What size bathroom fan do I need?

For a standard 8 ft ceiling, about 1 CFM per square foot of floor (a 5 × 10 ft bathroom needs a 50 CFM fan).