# How is an Air Purifier’s Recommended Room Size Calculated?

An air purifier’s recommended room size is calculated based on its rated CADR. There are two standards for measuring CADR, which means there are two standards for calculating room size.

### #1 AHAM Standard:

The AHAM standard uses what’s called a ‘two thirds’ or 2/3rd rule. A purifier’s CADR (in cubic feet per minute) should be equal to at least two thirds of the CADR. Or the room size is calculated using:

Square feet of room = Smoke CADR value x 3/2

The AHAM rule is for rooms that are 8ft (2.44m) in height. For rooms with ceilings higher than 8 feet, the formula should be adjusted.

### #2 GB-18801 Standard:

The GB-18801 standard specifies: an air purifier’s CADR can be calculated by multiplying the purifier’s CADR value (in cubic meters per hour) by 0.08 or 0.12.

The value ‘0.08’ is used for poorly sealed rooms, that leak a lot of outdoor air into the house. The value of ‘0.12’ is used for better sealed (but still not well sealed) rooms.

Square meters of room = CADR value x (0.08 or 0.12)

The GB-18801 standard is for rooms that are 2.4m (8ft) in height. For rooms with ceilings higher or lower than this, the formula should be adjusted.