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HVAC & Plumbing

CFM Calculator

Convert room volume and air changes per hour into required CFM.

ft
ft
ft
Bathroom 8, kitchen 15, garage 6, living space 4, workshop 10.
Airflow needed —
Room volume — cu ft
Minutes per air change —

How this calculator works

CFM = room volume × air changes per hour ÷ 60

Dividing by 60 converts hourly changes into per-minute airflow. Bathroom fans have a separate rule: 1 CFM per square foot of floor, with a 50 CFM minimum, which usually gives a similar answer.

Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.

Where these numbers come from

Check the edition. Codes are amended every few years, and the one your inspector works from may not be the one named above.

Worked example

With room length 20 ft, room width 10 ft, ceiling height 8 ft, air changes per hour 8 , this calculator returns 213.33 .

Airflow needed for common rooms

Every row here opens the calculator already filled in. Copy that link and the numbers travel with it.

JobSizeAirflow needed
Bathroom 8 ft × 6 ft × 8 ft × 8 51.20
Kitchen 14 ft × 12 ft × 8 ft × 12 268.8
Bedroom 12 ft × 11 ft × 8 ft × 5 88.00
Two-car garage 22 ft × 20 ft × 9 ft × 6 396.0
Small workshop with fumes 20 ft × 16 ft × 10 ft × 20 1,067

Bathrooms have a separate rule of 1 CFM per sq ft with a 50 CFM floor, which usually lands close to this.

Frequently asked questions

How many CFM do I need for a bathroom fan?

One CFM per square foot of floor area, with a 50 CFM minimum, or work from 8 air changes per hour. A 5 by 8 bathroom needs about 50 CFM either way.

What are air changes per hour?

How many times the entire volume of air in a room is replaced each hour. Bathrooms want 8, kitchens 15, living spaces 4, and workshops with fumes 10 or more.

Does duct length affect the fan I need?

Considerably. A fan rated 100 CFM may deliver 60 through 25 ft of flex duct with two elbows. Check the fan curve at the static pressure your run will actually impose.

Can a bathroom fan be too powerful?

In a tight house, yes. A large exhaust fan with no make-up air path can depressurise the building and backdraft a water heater or furnace flue, which is a carbon monoxide risk.

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