AIR PURIFIERS · BUYING DECISIONS
What size air purifier does your room need?
Measure the room first. Compare clean-air delivery and a stated cleaning rate—not the largest square-foot number on the box.
A practical starting point
AHAM's general rule is a smoke CADR of at least two-thirds of room area in square feet. For ceilings above eight feet, use an air cleaner rated for a larger room. Read AHAM's sizing guidance.
ROOM-SIZING TOOL
Turn your room dimensions into a shortlist
The starting values are an example, not measurements of your home. Enter one room or the connected area you intend a purifier to serve. This tool is for general room screening, not wildfire or medical guidance.
Method and limitations
Area = length × width. The two-thirds area rule gives smoke CADR in cfm. The separate volume calculation is length × width × height × 4.8 ÷ 60, using the 4.8-air-change coverage basis described by AHAM. Both results are rounded upward to whole cfm.
Taking the higher result is The Home Briefing's transparent screening choice, not an AHAM-prescribed combined formula. It assumes ideal mixing and does not account for leakage, sources of pollution, obstructions or an individual unit's low-speed performance. The catalogue filter excludes models whose listed CADR does not identify smoke; that missing information is not proof those models are unsuitable.
Room area to smoke CADR
These calculated values apply the two-thirds rule. They are a screening guide for ordinary room sizing, not a medical recommendation or proof of real-world pollutant removal.
| Room area | Example dimensions | Smoke CADR starting point |
|---|---|---|
| 120 ft² | 10 × 12 ft | 80 cfm |
| 200 ft² | 10 × 20 ft | 134 cfm |
| 300 ft² | 15 × 20 ft | 200 cfm |
| 400 ft² | 20 × 20 ft | 267 cfm |
| 600 ft² | 20 × 30 ft | 400 cfm |
The Home Briefing calculations from AHAM's two-thirds rule, rounded upward to whole cfm. Ceiling height, connected spaces and the type of pollution can require a different choice.
Why coverage claims can look so different
Air changes per hour (ACH) express a rate, not a second room measurement. A product advertised for a large area at one ACH is not necessarily suitable for that area at a faster target cleaning rate. Compare coverage figures at the same ACH.
For 200 cfm in a 300 ft² room with an eight-foot ceiling, the arithmetic is 5 equivalent air changes per hour. This assumes ideal mixing and is not measured performance in your room.
AHAM explains its 4.8-ACH coverage basis in its air-change FAQ. A recirculating purifier is not a source of fresh outdoor air.
Bedrooms: choose for the fan speed you can live with
A model's quietest mode may move less air than its maximum-CADR setting. EPA notes that lower speeds filter less air. Seek airflow or room-coverage evidence for the speed you will actually use overnight, rather than pairing minimum noise with maximum performance. EPA's air-cleaner guide explains the trade-off.
Also check placement clearances, indicator lights and the filter-change process. A larger machine is not automatically a better bedroom purchase if it cannot fit where its air intake remains unobstructed.
Open-plan areas and higher ceilings
Use the connected area you expect one unit to serve, not just the space immediately around it. Walls and doorways mean one purifier's advertised coverage is not a promise of even cleaning across several rooms. An awkward floor plan may call for a separate unit in the occupied room.
For wildfire smoke, AHAM gives a different rule of thumb: smoke CADR at least equal to the room area in square feet. Consult its guidance and applicable public-health advice for that use; do not treat the general table above as sufficient for every situation. AHAM's smoke guidance.