Buying a purifier by the sticker is how people end up with a very expensive nightlight that hums. Size it properly instead. The method takes two minutes, a tape measure and one multiplication.
Everything below is a rule of thumb for planning. Your ceiling height, how often doors open and the actual performance of a cheap purifier all change the real result, so leave some headroom.
CADR calculator
- What is CADR?
- The formula
- Three worked examples
- Why 4 to 5 air changes an hour?
- Reading the listing
- Between two sizes? Size up
- FAQ
What is CADR?
CADR stands for clean air delivery rate: how much filtered air a purifier delivers, usually shown in m³/h (or cfm in some markets). A higher number means a faster clean-up in a given room.
The formula
Required CADR (m³/h) = length (m) x width (m) x ceiling height (m) x air changes per hour. For haze, aim for 4 to 5 air changes an hour. Use 5 if you want a faster clean-up, or if the door opens a lot.
Skip the maths with our CADR calculator.
Three worked examples
Small bedroom, 3.0 m x 3.2 m, 2.6 m ceiling. Volume is 25 m³. At 5 air changes an hour, you want about 125 m³/h.
Master bedroom, 3.5 m x 3.5 m, 2.6 m ceiling. Volume is about 32 m³. At 5 air changes an hour, aim for about 160 m³/h.
Living room, 5 m x 4 m, 2.6 m ceiling. Volume is 52 m³. At 5 air changes an hour, you want about 260 m³/h. Open-plan with a dining area? Size up.
Why 4 to 5 air changes an hour?
It is a common planning target for cleaning particles from a room at a reasonable speed. Fewer changes means a slower clean-up, and more means a louder, costlier machine for diminishing returns.
Reading the listing
Look for a CADR figure in m³/h and ideally the test standard behind it. If a listing only gives a coverage claim in m², convert it to airflow yourself using the formula above and see if the numbers make sense. Our bedroom picks and living room picks are sorted by recent orders.
Between two sizes? Size up
A slightly larger purifier running at a lower speed is quieter than a smaller one at full blast. Just make sure the filter cost still makes sense.
Quick answers
What CADR do I need for an HDB bedroom?
Roughly 115 to 160 m³/h for a 9 to 12 m² bedroom at 5 air changes an hour and a 2.6 m ceiling. Measure your own room and use the CADR calculator.
Does ceiling height matter?
Yes. Volume is what you are cleaning, so a taller ceiling means more air to filter. The calculator lets you change it.
Can one purifier cover two rooms?
Only if air moves freely between them, and even then it will be slower. For bedrooms, a unit per room is more reliable.
