Shopping for an air purifier usually starts with "will this fit my room?" — but room size alone doesn't answer whether the air actually gets clean fast enough. The real question is air changes per hour (ACH): how many times the room's entire volume gets filtered in sixty minutes. This calculator works both directions — telling you the CADR to shop for, or checking whether the purifier you already own is actually keeping up, fan speed included.
Why CADR and ACH matter more than a size label
Most purifier boxes advertise a "covers up to X sq ft" number, but that figure is usually calculated at a single, often optimistic ACH target and a standard 8 ft ceiling. Two rooms of the same floor area can need very different CADR if their ceiling heights differ, and the same purifier that comfortably handles "general use" (2-3 ACH) can fall well short during a wildfire smoke event, which calls for 4-6+ ACH.
Working from Clean Air Delivery Rate (CADR) and your room's actual volume — not just its footprint — gives a number you can shop against directly, cross-checked against the AHAM AC-1 industry standard's 2/3 floor-area rule of thumb.
The fan-speed trap
A purifier's CADR rating is almost always measured at maximum fan speed. In practice, most people run their units on a quieter medium or sleep setting — and that quietly cuts the real, delivered CADR. Sleep mode commonly delivers only 40-60% of the max-speed rating, which means a purifier that comfortably meets a smoke-event ACH target at full blast can fall noticeably short running overnight on its quietest setting.
This calculator bakes that derating into both directions: when sizing a purifier to buy, it shows the higher, max-speed-equivalent CADR you'd need if you plan to run it quietly; when checking a purifier you own, it shows the real ACH at your actual operating speed, not the rating on the box.
What to do when the room is too big
If your room's max size at a purifier's given speed comes back smaller than your actual room, you have three practical options: run the unit at a higher fan speed (accepting more noise), add a second unit rather than hunting for one oversized purifier, or build a DIY Corsi-Rosenthal box — a box fan taped to four or five MERV-13 furnace filters — which can add substantial CADR very cheaply for large or open-plan spaces during a smoke event.
For genuinely large or open floor plans, two mid-size purifiers placed in different zones of the room usually clean air more evenly than a single oversized unit fighting to circulate air across the whole space.