10 × 12 × 8 ft: 80 CFM
The room is 120 sq ft. The general rule gives 120 × 2/3 = 80 CFM, reproducing AHAM’s example.
Air purifier size calculator
Find the smoke CADR needed for one room, using its dimensions and ceiling height. Optionally check a purifier’s rating against the result.
Room size is only useful with a cleaning rate
AHAM’s general room rule calls for smoke CADR at least two-thirds of floor area in square feet. Its wildfire-smoke guidance calls for smoke CADR equal to that area. This page applies those household sizing rules and shows the result in CFM and m³/h.
| Selection | Applied rule | Limit |
|---|---|---|
| General particle cleaning | Floor area in sq ft × 2/3 | Smoke CADR in CFM, based on an eight-foot ceiling. |
| Wildfire smoke particles | Floor area in sq ft × 1 | A particle-cleaning sizing rule, not a gas-removal claim. |
| Ceiling above 8 ft | Multiply the area-rule CADR by ceiling height ÷ 8 | Explicit volume scaling used here, rather than a separately certified room rating. |
| Ceiling at or below 8 ft | Keep the area-rule minimum | No reduction for a lower ceiling. |
Three examples you can reproduce
12 × 15 × 10 ft: 150 CFM
The general area rule gives 120 CFM. Our height adjustment gives 120 × 10/8 = 150 CFM, equivalent to five clean-air changes per hour.
12 × 15 × 8 ft, wildfire rule: 180 CFM
The selected AHAM wildfire-smoke rule uses the 180 sq ft floor area directly. It gives 7.5 equivalent clean-air changes per hour in this room.
What “air changes” means here
CADR × 60 ÷ room volume gives equivalent particle-cleaning air changes per hour. A purifier recirculates room air; this is not an outdoor-air ventilation rate. AHAM certification’s suggested room area uses 4.8 air changes per hour. The two-thirds rule used here gives five at an eight-foot ceiling, so the two figures are not treated as identical.
Check the rating at the speed you can live with
A listed maximum CADR may be measured at the loudest setting. Compare a documented lower-speed CADR if you plan to use sleep or quiet mode. If no lower-speed rating is available, this calculator cannot establish its capacity at that speed.
Air cleaning complements pollutant-source control and ventilation when outdoor air is suitable. Follow the manufacturer’s filter-replacement and placement guidance. This tool does not recommend a particular product or estimate medical risk.
Air purifier sizing questions
Can I add the ratings of two purifiers?
You can enter their total smoke CADR as a planning comparison when both run in the same room. Their placement, speed and room mixing affect real performance. Do not add units serving separate closed rooms.
Why does a taller ceiling raise the target?
The room contains more air. The stated height adjustment keeps the area rule’s equivalent cleaning rate for ceilings above eight feet. It is transparent volume scaling, not a manufacturer certification.
Does smoke CADR measure odour or gas removal?
No. The smoke test is a particle-removal rating. Gas-removal performance needs separate product evidence; a CADR result does not establish carbon-monoxide protection.
Sources and scope
Primary guidance checked October 12, 2026. Inputs and comparisons are calculated in your browser.
- AHAM Air Filtration Standards
General two-thirds rule, wildfire smoke rule, 80 CFM example and higher-ceiling caveat.
- AHAM Portable Air Cleaners and Air Changes per Hour
The 4.8 air-change basis of the certified suggested-room-size rating.
- EPA Air Cleaners and Air Filters in the Home
Filtration scope, source control and ventilation context.