Air Purifier Size Calculator (CADR)
Work out the Clean Air Delivery Rate (CADR) your room actually needs, based on AHAM's air-changes-per-hour guidelines — then check whether a purifier already on your wishlist is big enough.
Check a specific purifier
Already have a CADR figure from a purifier's spec sheet or box? Enter it below to see the largest room it comfortably handles, and the air changes per hour it will actually deliver in the room you entered above.
How to use
CADR beats "room coverage" claims
The square-footage number printed on a purifier's box is often calculated at a higher ACH than you actually need (sometimes even at just 2 ACH, which is too low for real use). The CADR figure in cubic feet per minute (cfm) is the AHAM-tested, comparable spec — use it, not the marketing coverage claim, to size a unit properly.
Match ACH to what you're filtering
4-5 ACH is the standard HVAC guideline for everyday dust, pet dander and pollen. Wildfire smoke, heavy allergies or asthma call for a faster turnover — 6 ACH or more is commonly recommended, since it clears pollutants before they build back up between purifier cycles.
Slightly oversized is not wasted
A purifier rated above your calculated CADR isn't overkill — it just reaches your target ACH on a lower, quieter fan speed and runs less at full power, extending filter life. That's why "round up to the next size" is common, sensible advice rather than a sales tactic.
Frequently asked questions
What does CADR mean?
CADR stands for Clean Air Delivery Rate, a testing standard developed by AHAM (the Association of Home Appliance Manufacturers). It measures how many cubic feet of air per minute (cfm) a purifier can filter to remove smoke, dust and pollen particles, tested under controlled lab conditions. A higher CADR means the unit can clean a given volume of air faster — it is a far more reliable spec than the vague "room size" figure printed on the box, which manufacturers can calculate using different assumptions about air changes per hour.
How many air changes per hour (ACH) do I need?
For general dust, pet dander and everyday allergens, 4-5 air changes per hour is the widely cited HVAC guideline, and this calculator defaults to 4.8 ACH, which is the figure behind AHAM's commonly quoted "two-thirds rule" for smoke (a purifier's CADR should be roughly two-thirds of the room's square footage). If you have severe allergies, asthma, or are dealing with wildfire smoke, aim higher — 6 ACH or more is a common recommendation from clean-air advocates and some clinical guidance, since it clears pollutants faster and copes better with continuous new pollution sources.
Does room shape or an open-plan layout affect the calculation?
The CADR formula only cares about total floor area and ceiling height, not room shape, so a long narrow room and a square room of the same area need the same CADR in theory. In practice, open-plan spaces (a kitchen-diner-lounge, for example) behave less efficiently because air has further to travel back to the purifier and there is more furniture and doorway turbulence to work against. For open-plan layouts, treat the combined square footage as the room size, then choose a unit at the upper end of the recommended tier, or use two smaller units in different zones instead of one large one.
Is a bigger purifier always better?
A purifier rated for a bigger room than yours isn't wasted money — it simply reaches your target air changes per hour on a lower, quieter fan speed and runs less often at full power, which extends filter life and cuts noise. The main downsides of oversizing are purely practical: a larger unit costs more upfront, uses more standby power, and takes up more floor space. Undersizing is the bigger risk, since an underpowered purifier left on its highest (loudest) setting all day still won't hit a useful ACH in a room that's too big for it.