Air purifier size calculator

Work out the CADR you actually need for your room — then see what that means for filter cost.

How this calculation works

Air purifier effectiveness comes down to one number: CADR (Clean Air Delivery Rate) — the volume of clean air the unit produces per hour, in cubic metres per hour.

The industry rule of thumb is that a purifier should move at least three times your room's volume per hour. That gives roughly five air changes per hour, which is the point where particle levels drop noticeably rather than drifting down slowly.

• Room volume = width × length × ceiling height
• Base CADR = volume × 3
• Ventilation adjustment — if you keep windows open, outdoor air carries particles in continuously, so the unit has to work harder to hold the same level. This raises the requirement by 15–30%, not lowers it.
• Concern multiplier — for allergies or asthma a higher air-change rate helps more, so the target rises 30%. For odour only, you need less airflow and more carbon filtration, so the target drops 30%.

The mistake most people make

Manufacturers advertise CADR at the highest fan speed, which is loud enough that you will never sleep next to it. On quiet mode, real CADR often drops by half or more.

So when you compare units, look for the quiet-mode figure as well. A unit rated 300 m³/h at full blast might only manage 120 m³/h at the speed you will actually use overnight.