How to size an air purifier for a bedroom: the CADR number decides it, not the room label
Short answer: size by CADR against your bedroom’s actual volume and the air changes per hour you want, not by the square-footage number printed on the box.
Start with air changes per hour
The useful metric is how many times the unit can process the room’s entire volume in an hour. Allergy and smoke advice generally lands around four or five air changes per hour; general freshening is fine at two.
The manufacturer’s “recommended room size” is usually quoted at the lowest useful air-change rate, which is why a unit rated for your room can still underperform for allergies. Halving the stated room coverage is the crude correction, and it is closer to right than the box is.
Work out the CADR your room actually needs
The number the box does not print. Everything here is the arithmetic from the section above.
CADR, and why the three numbers differ
Clean Air Delivery Rate is measured separately for smoke, dust and pollen because particle sizes differ. Match the number to your problem: smoke for wildfire or cooking, pollen for seasonal allergy, dust for a renovation or a shedding pet.
A unit with a high pollen figure and a weak smoke figure is not a bad unit. It is a unit for a different problem.
Filters are the real running cost
The purchase price is the small number. Work out the annual filter cost before choosing, since a cheap unit with proprietary filters replaced twice a year is routinely the more expensive option over three years. Check whether pre-filters are washable, because a washable pre-filter meaningfully extends the life of the expensive HEPA layer.
What we would ignore
Ozone-generating modes. Ozone is a lung irritant. Some regions restrict these devices for that reason, and no purifier needs one.
Ioniser-only devices. Without a fan and a physical filter there is no measured delivery rate to compare, so there is nothing to hold the manufacturer to.
App dashboards with a coloured air-quality score from a sensor of unstated accuracy. Pretty, unfalsifiable, and no substitute for a delivery rate.
Noise decides whether it runs
A purifier on its top setting is often too loud to sleep beside, so it gets turned down and the sizing you paid for evaporates. Size the unit so your room’s target air changes are met on a setting you will actually leave running overnight, and check the noise figure at that setting rather than the minimum.
The order, one line
Room volume, then target air changes, then the CADR matching your problem, then the noise level at the setting you will really use, then three years of filters.
Common questions
What CADR do I need for a bedroom?
Work it from volume and air changes per hour rather than from a room label. Multiply floor area by ceiling height, decide how many air changes you want, and convert that to a CADR figure the machine has to meet at a noise level you can sleep through.
Is the square-footage number on the box wrong?
It is not wrong so much as unstated. It usually assumes a ceiling height and an air change rate that nobody prints, so two machines with the same coverage claim can differ by a wide margin in practice.
How often do the filters really need changing?
Filters are the running cost of the machine, and the interval depends on how dirty the air is and how many hours a day it runs. Price the replacement filters before buying the unit, because that is the number you pay again.
Where to look on Amazon
These are searches, not picks. We do not quote prices or specifications here: Amazon’s terms allow those only where Amazon itself serves the data, and a hard-coded number is wrong the week after it is written. Compare the listings against the order above.
- true hepa air purifier large room , start here with the room volume you measured, and read the CADR figures on the listing rather than the headline room size
- hepa air purifier smoke cadr , if your problem is cooking or wildfire smoke, the smoke CADR is the number to compare
- quiet hepa air purifier bedroom , for a unit that has to run overnight, where the noise figure at your working setting decides everything
- true hepa replacement filter , the running cost above, priced for your specific model before you commit to it rather than after
Built from published standards and manufacturer documentation. We do not claim laboratory testing we have not performed.