Air Purifier Size Calculator
Enter your room size to get the exact CADR you need — and the right-size HEPA purifier to match.
Your Room
Length × width of the room. Not sure? Tap a common room below to size it instantly:
Standard is 8 ft. Higher ceilings mean more air volume to clean.
What size air purifier do I need?
The right size air purifier is decided by CADR (Clean Air Delivery Rate), not by the "up to X sq ft" number on the box. CADR — measured in cubic feet per minute (CFM) and verified by AHAM — tells you how much filtered air a unit actually delivers. To clean a room well you want enough CADR to cycle all the air in the room several times an hour (air changes per hour, or ACH).
- General cleaning: ~4 air changes/hr is a sensible baseline.
- Allergies & asthma: aim for ~5 air changes/hr so pollen and dust are cleared quickly.
- Pets: ~5 air changes/hr, and choose a unit with an activated-carbon layer for odor.
- Wildfire smoke: 6–7 air changes/hr to keep a clean-air room during smoke events.
Multiply your room's square footage by the ceiling height to get its volume, then use the formula above. Higher ceilings and higher air-change targets both push the CADR you need upward. When a single unit can't deliver enough clean air for a large or open-plan space, two smaller purifiers running together will out-clean one oversized claim — and let you cover more than one zone.
Air Purifier Sizing FAQ
What size air purifier do I need for my room?
Size by CADR (Clean Air Delivery Rate in CFM), not the vague “up to X sq ft” claim on the box. Multiply your room’s square footage by ceiling height to get its volume, then use CADR = air changes per hour × volume ÷ 60. Aim for about 4 air changes per hour for general cleaning, 5 for allergies or pets, and 6–7 during wildfire smoke. Pick a purifier whose AHAM-verified CADR meets or beats that number.
What size air purifier do I need for a 300 sq ft room?
A 300 sq ft room with an 8 ft ceiling is 2,400 cubic feet. At 5 air changes per hour (a good target for allergies) that’s 5 × 2,400 ÷ 60 = 200 CFM of CADR. A single Coway AP-1512HH Mighty (~246 CFM True HEPA + carbon) comfortably covers it; a Levoit Core 400S (~141 CFM) fits a smaller bedroom of roughly 170 sq ft at the same target.
How many air changes per hour (ACH) does an air purifier need?
Air changes per hour is how many times the purifier cycles all the air in the room each hour. Use ~4 ACH for everyday cleaning, ~5 ACH if anyone has allergies or asthma or you have pets, and 6–7 ACH to hold a clean-air room during wildfire smoke (EPA/AHAM clean-room guidance). Higher ceilings and higher ACH targets both raise the CADR you need.
What CADR do I need for wildfire smoke?
For heavy smoke, target 6–7 air changes per hour instead of the usual 4. That 300 sq ft / 8 ft room (2,400 cu ft) needs 7 × 2,400 ÷ 60 = 280 CFM — more than one mid-size unit delivers, so either step up to a larger purifier or run two units in the room. A True HEPA filter captures up to 99.97% of the fine PM2.5 particles in wildfire smoke.
Can one air purifier clean a large or open-plan space?
Often not at a useful air-change rate. Clean-air delivery is additive: two purifiers running together in different zones clear a large or open-plan area far faster than one oversized “up to 1,800 sq ft” claim, which usually assumes just one air change per hour. When the required CADR exceeds a single unit, this calculator recommends doubling up and placing the units apart rather than side by side.