The short answer
Compare smoke CADR and the air-change rate behind the room-size claim. For a typical room with an 8-foot ceiling, AHAM’s quick starting rule is smoke CADR at least two-thirds of floor area. A 180-square-foot room therefore starts at 120 cfm. Higher ceilings need more capacity.
This guide helps you shortlist a room purifier and read its specifications. It is not a prediction of your home’s air quality, a whole-house design or a medical recommendation.
“Covers 1,748 ft²” needs a second number.
Here is a real example, not a product ranking. The US page for Coway Airmega Mighty AP-1512HH lists three coverage figures for the same model. The large number describes a different air-change rate.
Its specifications list 233 smoke CADR, 246 dust CADR and 240 pollen CADR. Those are manufacturer-listed ratings. For the room-size comparison, keep the smoke number and the stated air-change basis together.
Our take: put the air-change rate next to every square-foot figure in your shortlist. Comparing a one-change claim with a roughly five-change claim makes an ordinary specification difference look like a huge performance advantage.
Three terms worth separating
- CADR
- Clean air delivery rate: filtered-air delivery for a test particle type. Keep units consistent. This guide uses cubic feet per minute, or cfm.
- Room area
- Length × width, measured in square feet. Ceiling height adds a third dimension when you calculate the volume of air.
- Equivalent air changes
- Filtered-air delivery relative to room volume. A purifier recirculates indoor air; this is not a count of fresh outdoor-air replacements.
AHAM’s suggested-room-size convention is approximately 4.8 equivalent changes per hour. The simple two-thirds rule works out to 5 at an 8-foot ceiling. They are nearby sizing conventions, not identical formulas. Neither promises that every particle is captured five times.
Size the room you actually use.
Enter one rectangular room in feet. This estimate extends AHAM’s two-thirds rule in proportion to ceiling height. It rounds up to a whole cfm.
Illustrative comparison aid. No data is sent or saved. For irregular or open-plan spaces, measure the full connected area being served; this rectangle tool does not map airflow.
Starting smoke CADR estimate
cfm
180 ft² × 8 ft ceiling
180 × ⅔ × (8 ÷ 8) = 120 cfm
A taller room changes the shortlist.
A 300-square-foot room with a 10-foot ceiling contains 25% more air than the same floor plan with an 8-foot ceiling. Preserving the same calculated cleaning rate raises the two-thirds estimate from 200 to 250 cfm.
| Floor area | 8 ft ceiling | 10 ft ceiling | 12 ft ceiling |
|---|---|---|---|
| 150 ft² | 100 | 125 | 150 |
| 200 ft² | 134 | 167 | 200 |
| 300 ft² | 200 | 250 | 300 |
| 400 ft² | 267 | 334 | 400 |
| 500 ft² | 334 | 417 | 500 |
Original arithmetic from area × ⅔ × height ÷ 8. This is not EPA’s approximate sizing table, an AHAM certification or a measured result. Compare like units and manufacturer-stated operating conditions.
AHAM: air filtration standards · AHAM: equivalent air changes and room size
What makes a good fit?
Start with the room
Measure the space, then compare the same CADR type and air-change basis. A purifier for a bedroom is a different purchase from a device expected to serve a connected living-and-dining area.
Check the speed you can live with
Published CADR is typically measured at the highest setting. Do not pair maximum CADR with a low-speed noise claim as if they describe one operating mode. Ask for speed-specific figures when quiet operation is essential.
Keep the daily friction low
Before buying, find the exact replacement-filter part number, the maker’s replacement instructions and an unobstructed place for the unit. Compare ongoing filter needs as well as the machine itself.
Trade-off: higher published capacity can give you more options, but a bigger housing or extra filter expense may not suit the space. Bigger is not automatically the better everyday fit.
The number has boundaries.
Particle sizing
Smoke / dust / pollen CADR
Every air problem
Gases, moisture or fresh-air supply
Particle CADR does not rate gas or odor removal.
Filtration is a supplement to controlling pollution sources and ventilating with clean outdoor air. A room purifier cannot remove every pollutant or turn a room-size calculation into a health guarantee.
EPA: air cleaners and air filters
Avoid devices that deliberately generate ozone as a cleaning method. Ozone can harm the respiratory system; a clean-smelling room is not proof of safe air.
Before you add it to your cart
Does “HEPA” tell me the room size?
No. Filter terminology alone does not give the unit’s clean-air delivery rate. Look for the actual CADR and the conditions behind the stated room area.
Can I compare 300 m³/h with 300 cfm?
No. They are different units. Convert them before comparing; 1 cfm is approximately 1.699 m³/h. A 300 m³/h rating is approximately 177 cfm. These are unit conversions, not new test results.
Why not just choose the largest square-foot number?
It may use a lower air-change rate. The Coway example shows how one model can have very different coverage numbers. Ask which rate and operating mode the number represents.
Will the calculator tell me whether my air is safe?
No. It estimates a specification to compare. Pollution sources, outdoor conditions, airflow, placement and operation still matter. A portable purifier is not a substitute for fresh-air ventilation when appropriate or for addressing the source.
Sources & further reading
Checked October 10, 2026. Coway values are manufacturer specifications. Diagrams, calculations and buyer-fit interpretation are original editorial work.