Air Change Rate (ACH) Calculator
A 12×12×8 ft room (1,152 cu ft) with 100 CFM gets 5.2 air changes per hour — enter your room and airflow below.
Room volume = length × width × height. ACH figures are for planning — confirm ventilation requirements with your local building code or an HVAC professional.
How the ACH calculator works
Air changes per hour (ACH) is how many times the room's air volume is replaced per hour — the standard way to describe how much ventilation a space is getting.
A CFM rating measures cubic feet of air per minute, so multiplying CFM by 60 gives cubic feet of air moved per hour. Dividing that by the room's volume in cubic feet (length × width × height) gives the air changes per hour.
ACH = CFM × 60 ÷ room cu ft
Related tools
If you're sizing HVAC for a sealed space, run the Mini Split Square Footage Calculator next to match the room's load to the right system, and check the Spray Foam Insulation Cost Calculator if you're tightening the envelope.
Air change rate calculator FAQ
What is a good ACH for a home?
ASHRAE 62.2 guidance targets about 0.35 air changes per hour minimum for living spaces; kitchens, baths and workshops run higher when exhaust fans are on.
What is the ACH formula?
ACH = CFM × 60 ÷ room volume in cubic feet.
How do I measure my room's CFM?
Use an anemometer at the vent, or start from the fan's rated CFM and subtract for long or bendy duct runs.
Does opening windows count?
Yes for comfort, but natural ventilation can't be measured precisely — this calculator is for mechanical ventilation.