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Free tools › Contractors › Air changes per hour calculator

Free calculator · no sign-up

Air changes per hour (ACH) calculator

Check whether a fan or system moves enough air: ACH = CFM × 60 ÷ room volume.

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Room and airflow

Air changes per hour
Room volume
Minutes to replace the air once
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How it’s calculated

ACH = CFM × 60 ÷ (length × width × height). Building codes set minimum ventilation for some rooms (for example bathrooms and kitchens) — check the Ontario Building Code for the job.

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Questions

Is a higher ACH always better?

Not always — more air changes cost more energy to heat or cool. Match the design target for the room.