ACH Calculator — Air Changes Per Hour
Air changes per hour tells you if a system is actually moving enough air for the space. It's the number that separates 'feels okay' from 'properly ventilated.'
This calculator takes the room volume and your measured or target CFM, and tells you the real ACH — plus how it compares to what the space needs.
Air changes/hr
3.3 ACH
Room volume
3600 ft³
Per-minute volume
200 CFM
Living spaces: 3–4 ACH. Kitchens/baths: 6–8. Below 2 ACH usually means undersized equipment or a restriction somewhere.
The formula
ACH = (CFM × 60) ÷ room volume (sq ft × ceiling height). Living spaces: 3–4 ACH. Kitchens & baths: 6–8 ACH. Below 2 ACH usually means undersized equipment.
Pro tips
- →Measure CFM at the register with a flow hood or anemometer for a real number — don't guess.
- →If ACH comes out under 2, check for restrictions: dirty filters, crushed flex, undersized returns.
- →Bathrooms with exhaust fans should hit 8 ACH by code in many areas — this calculator checks that too.
Frequently Asked Questions
How many air changes per hour is normal?+
Living spaces: 3–4 ACH. Kitchens and bathrooms: 6–8. Commercial spaces often target 4–6. Below 2 is under-ventilated.
How do I calculate ACH?+
Multiply your CFM by 60, then divide by the room volume in cubic feet. Example: 200 CFM × 60 ÷ 3,600 ft³ = 3.3 ACH.
What does ACH mean for indoor air quality?+
Higher ACH means the whole room's air gets replaced more often — fresher air, better odor and humidity control. Too high wastes energy.
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