What size air purifier do you need?
Enter your room, get the CADR you need — or enter a CADR, get the room it can handle.
Use the CADR for tobacco smoke from the box or the AHAM Verifide listing.
4.8 is what AHAM's room-size ratings are based on.
What CADR do I need for my room?
The EPA advises a portable air cleaner with a CADR for tobacco smoke of at least two-thirds of the room area in square feet, and its sizing chart is calculated for 8-foot ceilings. A 12 ft × 15 ft room is 180 ft², so it needs a smoke CADR of at least 120 cfm. The EPA's own sizing chart reads 65 cfm for 100 ft², 130 for 200, 195 for 300, 260 for 400, 325 for 500 and 390 for 600 ft² — about 0.65 cfm per square foot.
How does AHAM size a room from the CADR?
Under ANSI/AHAM AC-1 the suggested room size in square feet is the smoke CADR × 1.55, which is the area where the purifier cuts smoke particles by 80% at about 4.8 air changes an hour with an 8-foot ceiling. A purifier with a smoke CADR of 200 cfm is rated for up to 310 ft². The EPA's two-thirds rule is the same idea rounded: area ÷ 1.5 instead of ÷ 1.55.
How many air changes per hour will my air purifier give?
Air changes per hour = smoke CADR × 60 ÷ room volume in cubic feet. A 200 cfm purifier in a 12 ft × 15 ft room with an 8-foot ceiling (1,440 ft³) gives 200 × 60 ÷ 1,440 = 8.3 air changes an hour. AHAM's room sizes assume 4.8, and it recommends at least that for the room size where you use the purifier. For air changes from a fan or the outdoors, use the CFM calculator.
What if my ceiling is higher than 8 feet?
The EPA's chart is calculated for an 8-foot ceiling, and it says that with higher ceilings you may want a higher CADR — but it gives no number. This calculator scales the CADR up in proportion to the ceiling height (10 ft means 25% more) so that the air changes per hour stay the same; that scaling is plain arithmetic on the room's volume, not an EPA figure. It does not reduce the CADR for ceilings lower than 8 feet.
Is wildfire smoke different, and does CADR cover odors or gases?
For wildfire smoke AHAM changed its room-size guidance from smoke CADR × 1.55 to the smoke CADR itself, so a room should be no larger in square feet than the purifier's smoke CADR — about 7.5 air changes an hour at an 8-foot ceiling. The CADR rating system is for particles only; the EPA notes there is no widely used rating for filters that remove gases, such as volatile organic compounds.
Sources: U.S. EPA, “Guide to Air Cleaners in the Home” (2nd edition) and “How to Create a Cleaner Air Room at Home” (EPA-452/F-26-005); AHAM Verifide, “Portable Air Cleaners and Air Changes per Hour” (October 2021), which also cites ANSI/AHAM AC-1. Ventilation air changes: CFM calculator. How we check our calculators.