How to size an air purifier for your room
The coverage figure on the box is not comparable between brands, because each one picks its own air-change rate. The arithmetic below takes about a minute and gives you a number you can compare.
The arithmetic
Measure the room’s floor area in square feet and multiply by the ceiling height to get its volume in cubic feet. A 12 by 14 foot bedroom with an 8 foot ceiling is 1,344 cubic feet.
Take the unit’s smoke CADR in cubic feet per minute, multiply by 60 to get an hourly figure, and divide by the room volume. That gives you air changes per hour. A unit rated 240 cfm in that bedroom gives 240 × 60 ÷ 1,344, which is about 10.7 air changes an hour at full speed — comfortably above AHAM’s 4.8 convention.
Run it the other way to find the CADR you need: room volume × the air changes you want ÷ 60. For 4.8 changes in that same bedroom, 1,344 × 4.8 ÷ 60 gives 108 cfm. Use the SMOKE CADR for this, not dust or pollen: smoke is the smallest particle size of the three and the most demanding, which is why it is the figure most worth comparing.
What the arithmetic assumes, and where it breaks
Published CADR is measured at maximum fan speed. If a unit is too loud at maximum to leave running — and many are — the real figure in your home is whatever it delivers at the speed you can live with, which manufacturers rarely publish. Treat the calculated air changes as a ceiling rather than an expectation.
The calculation also assumes a sealed room. An open door connects the volume you measured to the rest of the home, and the unit is then working on a much larger space. A door left open changes the arithmetic more than most spec differences between two competing models.
Finally, air changes per hour describes how fast the unit processes a volume, not how clean the result is. A room with a constant particle source needs more turnover than the same room with a one-off load, and no size calculation can tell you how much more.
Reading a listing without being misled
When a listing quotes a very large coverage figure, look for the air-change rate. If it is not stated, the figure is usually at one air change per hour, which is the least demanding assumption a manufacturer can make. The same machine at AHAM’s 4.8 would be sized for roughly a fifth of that area.
When a listing quotes a CADR without mentioning AHAM, it is the brand’s own measurement. That is not automatically wrong, but it has not been checked by anyone else, and it is not the same kind of claim as an AHAM Verifide figure. On every review here we state which of the two we are looking at.
Questions people ask about this
What CADR do I need for a bedroom?
Multiply the room volume in cubic feet by 4.8 and divide by 60. A 12 by 14 foot room with an 8 foot ceiling is 1,344 cubic feet, which works out at about 108 cfm smoke CADR for AHAM’s 4.8 air changes an hour. Aim higher if you want that turnover at a fan speed you can sleep through, since published CADR is a maximum-speed figure.
Is a bigger purifier always better?
A unit with more capacity than the room needs can run at a lower, quieter speed to reach the same turnover, which is a genuine advantage. The costs are size, wattage and usually larger or more frequent filters, so the sensible ceiling is set by the running cost rather than by the room.
Should I use the smoke, dust or pollen CADR?
Smoke. It is the smallest particle size of the three and the most demanding, so sizing against it is the conservative choice, and it is the figure most consistently reported across brands.
Where this comes from
Written from published standards and manufacturer documentation. We do not bench-test units, and nothing here is a health claim — where the honest answer is that something is not documented or not measured, the page says so.