Air Changes per Hour (ACH) Calculator

ACH measures how many times a room's entire volume of air is replaced in one hour. At 6 ACH, all the air in the room is theoretically swapped out every 10 minutes. It's the standard way to express ventilation and air-cleaning rates because it scales with room size - 100 CFM is a lot of air for a closet and almost nothing for a gymnasium, but '6 ACH' means the same turnover everywhere.

ACH = (airflow in CFM × 60) ÷ room volume in cubic feet. Example: a 100 CFM fan in a 12×12 room with 8 ft ceilings (1,152 ft³) delivers (100 × 60) ÷ 1,152 ≈ 5.2 ACH. Room volume is just length × width × ceiling height.

CFM = (target ACH × room volume) ÷ 60. Example: to hit 8 ACH in a 10×10 bathroom with 8 ft ceilings (800 ft³), you need (8 × 800) ÷ 60 ≈ 107 CFM of exhaust. The ACH → CFM tab above does this and shows the metric equivalents (m³/h and L/s) alongside.

Multiply the CFM by 60 and divide by the room's volume in cubic feet. If your HVAC register delivers 150 CFM into a 1,500 ft³ living room, that's (150 × 60) ÷ 1,500 = 6 ACH. For a whole house, use total system CFM and total conditioned volume.

For comfort ventilation: bedrooms 5-6 ACH, living areas 6-8, kitchens and bathrooms 7-9, basements 3-4. Note these are air-circulation targets used for HVAC design - the code minimum for fresh outdoor air is far lower (ASHRAE 62.2 works out to roughly 0.35 ACH of outdoor air). Circulated air and fresh air are different things; both matter.

4-5 ACH is the widely used target for meaningful air cleaning - allergies, asthma, smoke, and airborne illness. That's the basis of AHAM's room-size ratings. Example: a purifier with a 200 smoke CADR in a 12×12 room (1,152 ft³) delivers about 10.4 ACH - excellent; the same unit in a 500 sq ft open space delivers only 3 ACH. Use the Air Purifier tab above to check your unit against your actual room.

8 feet. AHAM's CADR room-size ratings and virtually every manufacturer's coverage claim are computed at a standard 8-foot ceiling. If your room has 9 or 10-foot ceilings, the real ACH drops proportionally - a room with 10 ft ceilings has 25% more air than the same floor area at 8 ft, so derate the coverage claim by 25% (or just enter your true ceiling height in the calculator above).

Three options. (1) Look up the equipment: fans, ERVs, and purifiers all publish CFM or CADR ratings. (2) For a building's leakage rate, a blower-door test reports ACH50 (air changes at 50 Pascals depressurization); divide ACH50 by roughly 20 to estimate natural infiltration ACH - a house testing 8 ACH50 leaks about 0.4 ACH naturally. (3) Measure airflow directly with an anemometer at the register (average face velocity × register area = CFM), then convert.

ASHRAE Standard 170 sets healthcare rates: patient rooms 6 total ACH (2 outdoor), airborne-infection isolation rooms 12, operating rooms 20 total (4 outdoor). Laboratories typically run 6-12 ACH depending on hazard level, and commercial kitchens 15-30. These are code-driven exposure-control numbers - hitting them is about diluting contaminants, not comfort.

Convert to CFM first (CFM = ACH × volume ÷ 60), then multiply CFM by 0.000472 for m³/s or 0.4719 for L/s. Example: 6 ACH in a 1,152 ft³ room = 115 CFM = 0.054 m³/s = 54 L/s. The ACH → CFM tab shows m³/h and L/s automatically.

ACH describes normal operating air changes - what a fan or HVAC system actually delivers. ACH50 is a blower-door test result: air changes per hour while the house is depressurized to 50 Pascals, used to grade airtightness (new energy codes require 3-5 ACH50; passive houses hit 0.6). ACH50 is always much higher than natural ACH - divide by about 20 to estimate real-world infiltration.