Air changes per hour is how many times a room's full volume of air is moved, replaced or filtered in an hour. It's a way of comparing airflow across rooms of different sizes, whether the air comes from outdoors, an exhaust fan or an air purifier.
ACH = airflow in cubic feet per minute × 60 ÷ room volume in cubic feet. A 12 × 15 foot room with an 8-foot ceiling holds 1,440 cubic feet, so 120 CFM moves its air 5 times an hour. Ventilation ACH (fresh outdoor air) and an air purifier's ACH (filtered room air) measure different things.
Air Changes Per Hour (ACH) Calculator
Ventilation rate from airflow and room size.
The formula
- ACH = CFM × 60 ÷ volume.
- CFM needed = volume × target ACH ÷ 60.
Volume is length × width × ceiling height: a 12 × 15 ft room with an 8-foot ceiling holds 1,440 cubic feet. The 60 converts cubic feet per minute to cubic feet per hour.
From a target back to CFM
The calculator also runs the formula in reverse: set a target and it returns the airflow. Rearranged, CFM needed = volume × target ACH ÷ 60.
Take a 200 sq ft room with a 9-foot ceiling, so a volume of 1,800 cubic feet. To move its air 6 times an hour, you need 1,800 × 6 ÷ 60 = 180 CFM. Halving the target to 3 ACH halves the airflow to 90 CFM, because the two are directly proportional.
Ventilation vs. filtration
- Ventilation brings in outdoor air and exhausts indoor air. ASHRAE Standard 62.2 sets a home's continuous ventilation in cfm: 0.03 per square foot plus 7.5 per person, counted as bedrooms plus one. For a 2,000 sq ft, three-bedroom home that's 90 cfm, which in a 16,000-cubic-foot house is about 0.34 ACH. See whole-house ventilation.
- Filtration recirculates room air through a filter. AHAM's two-thirds rule for air purifiers, a smoke CADR of two-thirds of the floor area, works out to about 5 ACH of filtered air at an 8-foot ceiling. See the air purifier sizing guide.
The calculator's per-room targets are common rules of thumb for comparing rooms, not code requirements.
Turning a purifier's CADR into ACH
A purifier's clean air delivery rate (CADR) is just an airflow in CFM, so it drops straight into the formula: filtered ACH = CADR × 60 ÷ room volume. The table below works that out for three CADR values at an 8-foot ceiling. See the air purifier sizing guide for matching CADR to a room.
| Room (8-ft ceiling) | CADR 100 | CADR 200 | CADR 300 |
|---|---|---|---|
| 150 sq ft (1,200 cf) | 5.0 ACH | 10.0 ACH | 15.0 ACH |
| 300 sq ft (2,400 cf) | 2.5 ACH | 5.0 ACH | 7.5 ACH |
| 500 sq ft (4,000 cf) | 1.5 ACH | 3.0 ACH | 4.5 ACH |
AHAM's two-thirds rule lands on about 5 ACH: a 300 sq ft room wants a smoke CADR near 200, and 200 × 60 ÷ 2,400 is 5.0. Doubling the CADR doubles the air changes, and doubling the room roughly halves them.
How fast a given ACH clears the air
In a well-mixed room where the incoming air is clean and nothing new is being added, particle concentration falls exponentially. The fraction still in the air after a given time is e raised to the power of minus ACH times the hours elapsed. So one air change does not empty the room; it clears about 63% of what was there, because e to the minus 1 is 0.37.
The table shows the percent removed at 2, 5 and 8 ACH. At 5 ACH, for instance, 30 minutes is half an hour, so the exponent is minus 2.5, about 8% remains, and 92% is gone.
| Air changes per hour | 15 min | 30 min | 60 min |
|---|---|---|---|
| 2 ACH | 39% | 63% | 86% |
| 5 ACH | 71% | 92% | 99% |
| 8 ACH | 86% | 98% | ~100% |
Real rooms clear more slowly because some air short-circuits between the inlet and outlet and misses the corners, so treat these as an upper bound. Higher ACH mainly buys speed: 8 ACH reaches in 15 minutes what 2 ACH takes a full hour to do.
ACH50: measuring air leakage
Energy auditors use a blower door to depressurize a house to 50 pascals and measure the airflow needed to hold it. That airflow × 60 ÷ the house volume is ACH50, a measure of how leaky the building is. The 2021 International Energy Conservation Code requires new homes to test at no more than 3 ACH50 in most climate zones, and 5 in the warmest.
Frequently asked questions
How do I calculate air changes per hour?
Multiply the airflow in CFM by 60 and divide by the room's volume in cubic feet. 120 CFM in a 1,440-cubic-foot room is 5 ACH.
How many air changes per hour does a house need?
For fresh air, ASHRAE 62.2 sets ventilation in cfm, not ACH: about 90 cfm for a 2,000 sq ft, three-bedroom home, roughly 0.34 ACH.
How many air changes per hour should an air purifier provide?
AHAM's two-thirds rule works out to about 5 ACH of filtered air at an 8-foot ceiling. Aim higher for allergies or smoke.
What is ACH50?
The air changes per hour measured with a blower door at 50 pascals of pressure, used to rate how airtight a house is.
How many air changes per hour clear smoke or a cooking smell?
Faster is better, and the exponential decay sets the pace: at 5 air changes an hour a well-mixed room is about 92% clear in 30 minutes, and at 8 ACH about 98%. A purifier only pulls out the smoke particles, so for cooking odors and gases you also need activated carbon or an exhaust fan venting outdoors. AHAM's two-thirds rule gives roughly 5 ACH of filtered air, and smoke is a case to aim higher.
What ACH does a bedroom need?
Fresh-air codes set bedroom ventilation in CFM, not ACH, so there is no single required number. For an air purifier, AHAM's two-thirds rule works out to about 5 filtered air changes an hour at an 8-foot ceiling, and light sleepers with allergies often run more. A larger CADR reaches that target on a quieter fan setting, which matters most at night.