Air Purifier Room Size Calculator

Buying an air purifier usually comes down to one question: will it keep up with your room? The honest answer depends on the CADR (but more on that in a second), set against the size of the space you are putting it in. I built this air purifier room size calculator to do that math for you. Tell it your room and the purifier’s CADR, and it works out how many air changes an hour you would get, and how long the unit takes to clear most of the particles from the air.

One thing to note before you trust the verdict: the CADR on the box is a full-speed figure. It is measured with the fan flat out, which is also the loudest the machine ever gets, and almost nobody runs a purifier like that constantly, least of all in a bedroom overnight. So the clean-air number that looks generous on paper can halve or more once you drop to the quiet speed you actually live with. To account for this, there is a speed setting under the advanced settings. A real room leaks and mixes unevenly, so the verdict is only a ballpark, and the full assumptions list sits in the “How this is calculated” dropdown.

Put in your room and your purifier below. It runs entirely in your browser, and nothing you enter is saved or sent anywhere.

This is worth saying twice, because people often forget this. A purifier rated at, say, 250 CADR hit that figure in a test chamber with the fan at maximum, which on most machines is loud enough that you would not leave it going while you sleep or watch television. The advanced menu has a “fan speed you actually run” slider for this reason. Set it to the speed you use for most of the day, not the burst you switch to for ten minutes after cooking, and the air changes per hour will fall to something closer to your real life.

For everyday dust and allergens, five air changes an hour is the level AHAM and the EPA land on, and it lines up with the old two-thirds rule of thumb. Push to six or more if wildfire smoke is the reason you bought the device. That twelve-air-change option comes from hospital isolation rooms and sits in the tool only as a reference. Whichever you choose, the clear-90% time is the readout I find most useful, since it turns a concept into an actual waiting time. At five air changes an hour you are looking at roughly half an hour to clear most of the particles after you burn the toast.

This sizes for particles, the kind a filter catches like smoke and pollen. Gases and the CO2 you breathe out are a different problem, and the calculator doesn’t cover them. A purifier recirculates the air already in the room, so it cannot bring in anything fresh, and a stuffy, high-CO2 space still needs a window or proper ventilation. The maths also assumes a sealed, well-mixed room, which yours may not be, so read every figure as a planning estimate rather than a measurement.