Air purifier room size calculator (CADR)

How the calculation works

CADR stands for clean air delivery rate: the volume of filtered air, in cubic feet per minute (cfm), that an air cleaner delivers for a given kind of particle. The EPA says the number is meant to help you choose an air cleaner for the size of the room. The calculator finds the CADR you need in three steps:

1. Floor area = length × width, in square feet.
2. Room volume = floor area × ceiling height, in cubic feet.
3. CADR needed = volume × air changes per hour (ACH) ÷ 60, rounded up to the next 5 cfm. Dividing by 60 turns hours into minutes.

Two worked examples, which match the calculator’s display:

• Bedroom, 12 × 12 ft, 8 ft ceiling, 4 ACH. Area 144 sq ft, volume 1,152 cu ft. CADR = 1,152 × 4 ÷ 60 = 76.8 cfm, so look for about 80 cfm. The EPA’s two-thirds rule gives 96 cfm for smoke.
• Living room, 15 × 20 ft, 9 ft ceiling, 5 ACH. Area 300 sq ft, volume 2,700 cu ft. CADR = 2,700 × 5 ÷ 60 = 225 cfm. The EPA rule gives 200 cfm at 8 ft, and 225 cfm when scaled to a 9 ft ceiling.

What the EPA says

The EPA’s guide to air cleaners lists minimum CADR by room size, assuming 8 ft ceilings: 65 cfm for 100 sq ft, 130 cfm for 200 sq ft, 195 cfm for 300 sq ft, 260 cfm for 400 sq ft, 325 cfm for 500 sq ft and 390 cfm for 600 sq ft. Its wildfire guidance says to choose a unit whose smoke CADR is at least two-thirds of the room’s area in square feet. At an 8 ft ceiling, two-thirds of the floor area is exactly 5 air changes an hour (area × 8 × 5 ÷ 60), which is why the calculator offers 5 ACH as the smoke setting. The EPA also says that rooms with higher ceilings need a higher CADR; the scaled figure here simply adjusts the rule in proportion to ceiling height, which is our own arithmetic, not an EPA formula.

Common mistakes

• Trusting “covers up to” claims. Box claims can assume fewer air changes than you want. Compare the CADR number with your target.
• Using the dust or pollen CADR for smoke. Smoke particles are smaller. Choose the unit by its smoke CADR where one is listed.
• Forgetting ceiling height. A 300 sq ft room with a 12 ft ceiling holds 50% more air than the same room at 8 ft.
• Running on low. CADR ratings are typically measured at a unit’s high speed. On its quietest setting it delivers less than its rated flow.
• Ignoring doors. The numbers assume a closed room. An open door means you are cleaning part of the house.
• Choosing an ozone generator. The EPA advises avoiding air cleaners that intentionally produce ozone, which is a lung irritant.
• Skipping filter changes. The EPA notes that a dirty, overloaded filter will not work well and that manufacturers typically recommend replacement every 60 to 90 days; follow your model’s schedule.

A calculator cannot promise a result for your health or allergies. It helps you match a unit to the room. Use an air quality monitor to see whether particle readings actually fall. For smoke season, see wildfire smoke and indoor air; for a full shopping list, see the allergy air quality kit.