What CADR does your room need? Air purifier size.

Short answer

A 300 sq ft room with an 8 ft ceiling needs a portable air cleaner with a CADR of at least 195 CFM, per the EPA and ENERGY STAR sizing chart (65 CFM per 100 sq ft). The popular two-thirds rule of thumb gives 200 CFM. With a 10 ft ceiling this tool scales the 195 up to about 244 CFM.

How it works, with a worked example

EPA and ENERGY STAR publish the same particle-removal sizing chart for portable air cleaners: minimum CADR of 65 CFM at 100 sq ft, 130 at 200, 195 at 300, 260 at 400, 325 at 500 and 390 at 600 sq ft, calculated for an 8 ft ceiling. That is 0.65 CFM per square foot. The informal two-thirds rule of thumb, which neither EPA nor ENERGY STAR publishes (CADR at least two-thirds of the floor area in square feet), lands within 3% of it, 200 versus 195 at 300 sq ft.

Worked example. A 300 sq ft bedroom with an 8 ft ceiling: 300 x 0.65 = 195 CFM minimum. Two-thirds rule: 300 x 2/3 = 200 CFM. A 300 sq ft room with a 10 ft ceiling: 195 x 10 / 8 = 243.75, about 244 CFM, or 414 m3/h. 1 CFM = 1.699 m3/h.

Read the CADR on the box. CADR measures how fast a unit delivers filtered air and is a particle rating only. EPA notes there is no widely used rating system for gas removal. EPA says the chart is for estimation, advises a unit sized for a larger area when ceilings are above 8 ft, says to consider the whole space in an open floor plan, and suggests more than one air cleaner for an area larger than any one product serves.