Quick facts
- Room volume: floor area in m² × ceiling height (usually 2.4m)
- Air changes an hour: 2 for general dust, 5 for hay fever or asthma
- CADR you need: volume × air changes — a 38m³ living room wants about 190 m³/h
- Unit conversion: CADR quoted in CFM × 1.7 = m³/h
The headline number on the box is a room size in square metres.
It is the least useful number on the box.
Floor area ignores your ceiling entirely. A machine sized for a 40m² room in a Victorian conversion is being asked to move a quarter more air than the label assumed.
Work in cubic metres instead. The sum takes a minute and you only do it once.
Sizing a purifier is one multiplication and one comparison
Measure the floor, then multiply by the ceiling height.
Most modern British homes sit at 2.4 metres. Older conversions run to three metres or more, and that difference matters.
A 4m by 4m sitting room with a standard ceiling holds roughly 38 cubic metres of air.
Now decide how often you want that volume cleaned. This is the air change rate, usually written as ACH.
Two changes an hour handles general household dust in a bedroom.
Five is the figure allergy specialists point to, because pollen and pet dander need removing faster than they settle.
Multiply the volume by the air changes and you have the clean air delivery rate you need, in cubic metres an hour.
Thirty-eight times five is 190. That is the number to hunt for in the specification.
The coverage figure on the box usually assumes two air changes
Manufacturers choose the air change rate behind their own headline coverage.
Most pick two. Some pick one. Neither is dishonest, but both are gentler than an allergy sufferer needs.
That is why the same machine is advertised as covering 80m² in one market and half that in another.
A safe habit: halve the stated coverage if anyone in the house has allergies.
Take a third of it if you also want the machine to be quiet.
A purifier bought to exactly match your room will spend its whole life on the loudest setting.
That is the hidden cost of sizing tightly, because the quoted CADR is measured at maximum fan speed.
Nobody actually lives with a purifier at maximum fan speed. It is roughly a hairdryer three metres away.
Buy one size up and the same volume of clean air arrives on speed one, at a level you can sleep through.
This is the most common regret with these machines, and it costs nothing to avoid before you buy.
CFM and m³/h are the same measurement in different clothes
American specifications quote CADR in cubic feet per minute. Multiply by 1.7 for cubic metres an hour.
So a 250 CFM purifier is a 425 m³/h purifier, and comfortably over-specified for that 38m³ sitting room.
Then check which pollutant the figure refers to.
Smoke, dust and pollen each get a separate CADR, and the pollen number is always the flattering one.
Read the pollen figure for hay fever. Read the smoke figure if you have a wood burner or live on a main road, because it is invariably the lowest.
Check those figures against your own room before buying, as in our Blueair Blue Pure 311i Max review: the 5 numbers that decide it.
Doors are walls, so size for one room at a time
Air does not drift between rooms the way most people assume.
A purifier standing in the hallway is cleaning the hallway.
Size for the room the machine actually occupies, which for most households should be the bedroom.
That is where you spend eight uninterrupted hours breathing the same air.
A second unit downstairs beats one oversized machine trying to serve both floors.
If the budget covers one machine only, put it in the bedroom and shut the door.
None of the above is medical advice. If breathing symptoms are the reason you are shopping at all, the charity guidance is the more sensible starting point than any specification sheet.
