Air Changes per Hour (ACH) Calculator
Convert an airflow in CFM and a room volume into air changes per hour — how many times the air in a space is fully replaced every hour.
Calculator
90 CFM into a 16,000 ft³ space is 0.34 air changes per hour (ACH = CFM × 60 ÷ volume).
Air changes per hour (ACH) tells you how many times the entire volume of air in a room or house is swapped out in one hour. It is the natural way to talk about ventilation: 0.35 ACH is a typical whole-house minimum, a busy kitchen might target 7–8 ACH, and a hospital isolation room can require 12+ ACH. This tool turns an airflow you know (a fan’s CFM rating or the ASHRAE 62.2 rate) and the room’s volume into that ACH figure.
The only slightly fiddly part is the volume: multiply the floor area by the ceiling height. A 2,000 sq ft home with 8 ft ceilings holds 16,000 ft³ of air. Once you have volume and airflow, the conversion is a one-liner.
Formula
ACH = CFM × 60 ÷ volume (ft³)
The × 60 converts cubic feet per minute into cubic feet per hour; dividing by the room volume gives how many whole-room-fulls that is. To go the other way — the airflow needed for a target ACH — rearrange to CFM = ACH × volume ÷ 60.
Worked example
Feed the ASHRAE 62.2 whole-house rate of 90 CFM into a 16,000 ft³ home (2,000 sq ft × 8 ft ceilings):
ACH = 90 × 60 ÷ 16,000 = 5,400 ÷ 16,000 = 0.3375 ACH
So the whole house exchanges its air about 0.34 times per hour — roughly one full air change every three hours, which is right in the range 62.2 targets for general dilution. If you wanted a brisker 4 ACH in a 1,000 ft³ bathroom, you would need 4 × 1,000 ÷ 60 ≈ 67 CFM of exhaust.
Typical ACH targets
ACH is only meaningful next to a target. Whole-house ventilation aims low — around 0.35 ACH — because you are diluting general air, not fighting a point source. Rooms with concentrated loads want much more: a bathroom during a shower, a kitchen during cooking, a workshop with fumes. Higher ACH clears contaminants faster but also drags more conditioned air outside, so it is a balance against energy use.
Two cautions. First, this is a nominal ACH assuming the incoming air mixes perfectly; real rooms have dead spots, so effective ventilation is usually a bit lower. Second, don’t confuse this with a blower-door ACH50 result, which measures air leakage under a 50-pascal test pressure and is a different number entirely. Size real ventilation from the ASHRAE 62.2 rate.
ACH is handy for reasoning about how fast a space recovers after a spike. If a bathroom fills with steam, 8 ACH clears most of it in well under half an hour, while 2 ACH leaves it damp long enough to feed mildew. Indoor CO₂ is a useful real-world proxy: when a closed room full of people creeps past about 1,000–1,200 ppm, the ventilation ACH is too low for the occupancy. Because incoming air never mixes perfectly, treat the calculated ACH as a ceiling and design with a little margin — corners, closets and furniture-packed walls always see less air movement than the room average implies.
Reference table
Common ACH targets by space:
| Space | Typical ACH |
|---|---|
| Whole-house (ASHRAE 62.2) | 0.35 |
| Bedroom / living | 3.00 |
| Bathroom | 8.00 |
| Kitchen | 7.50 |
Guideline ranges; local codes and use may require more.