Air Purifier Size Calculator

Enter your room size to find the clean air delivery rate (CADR) an air purifier needs to clean the room's air several times an hour.

Source: AHAM Verifide – Air filtration standards (CADR) ↗

This result is an estimate only. Check with your bank or the relevant authority before making decisions.

How it is calculated

Room volume = floor area × ceiling height
CADR needed (m³/h) = room volume × air changes per hour
CADR needed (CFM) = m³/h ÷ 1.699

AHAM’s two-thirds rule (CADR ≥ ⅔ × floor area in ft², for an 8 ft ceiling) works out to about 5 air changes an hour. Use 4 changes for normal use, or 5 during haze or for allergies. To check how bad the air is, use the haze API checker.

Worked example

A 20 m² room with a 2.7 m ceiling and 5 air changes an hour:

Volume: 20 × 2.7 = 54 m³
CADR needed: 54 × 5 = 270 m³/h, which is about 159 CFM

Why your number may differ

This is a guide for a closed room. Open doors and windows, a very leaky room, and heavy pollution need a bigger purifier than this. The CADR on a box is measured in a test chamber, so real results are lower, and filters clog over time. Rooms are assumed to be about the same temperature everywhere with the air well mixed.

Frequently asked questions

What is CADR?

CADR is the clean air delivery rate. It shows how much filtered air a purifier delivers per hour, in m³/h or CFM. Check the smoke or PM2.5 CADR on the box, not the filter's maximum airflow.

How big a purifier do I need?

Its CADR should be at least the figure above. AHAM's rule of thumb is a CADR of two-thirds of the floor area in square feet, which is about 5 air changes an hour with an 8 ft ceiling.

Does a bigger CADR help during haze?

A higher CADR cleans the room's air faster, and a purifier with a true HEPA filter removes fine particles. Keep windows and doors shut, and change filters as the maker says.