Air Purifier Size Calculator: What CADR Do You Need?

The calculator turns your room into a number you can compare on the box: the smoke CADR (Clean Air Delivery Rate, in cubic feet per minute). Smoke CADR is the one to use for sizing because smoke particles are the smallest and hardest of the three AHAM test particles to capture.

How the calculator works

It multiplies your room’s volume by the number of air changes per hour (ACH) you want, and divides by 60 minutes:

Minimum CADR (cfm) = air changes per hour × room volume (cu ft) ÷ 60

and, when you enter a purifier’s CADR:

Air changes per hour = CADR × 60 ÷ room volume

This is the same formula the CDC and the EPA use. Metric sizes are converted to feet first, because CADR is published in cubic feet per minute.

Which goal to choose

Goal	Air changes per hour	Where it comes from
Minimum	4.8	The room size printed on AHAM-rated boxes: smoke CADR × 1.55 at an 8 ft ceiling
Everyday use	5	The EPA and AHAM “two-thirds rule”: CADR ≥ 2/3 of the floor area; the CDC also suggests 5 or more
Wildfire smoke	7.5	AHAM’s recommendation: smoke CADR at least equal to the floor area

Worked examples

• Bedroom, 12 x 15 ft, 8 ft ceiling (180 sq ft, 1,440 cu ft): 5 ACH × 1,440 ÷ 60 = 120 cfm. That matches the two-thirds rule (180 × 2/3 = 120). For wildfire smoke: 7.5 × 1,440 ÷ 60 = 180 cfm.
• Same room, 10 ft ceiling (1,800 cu ft): 5 × 1,800 ÷ 60 = 150 cfm. Taller rooms need more CADR than the floor area suggests, which is why AHAM says to choose a unit rated for a larger room when your ceiling is above 8 ft.
• Checking a purifier: a model with a smoke CADR of 233 cfm in that 1,440 cu ft bedroom gives 233 × 60 ÷ 1,440 = 9.7 air changes per hour on its highest speed. Its AHAM room rating is 233 × 1.55 = 361 sq ft.

How to read the result

• CADR is measured on the highest fan speed. The EPA notes that CADR on lower, quieter speeds is usually not published and can be much lower. If you want to run the purifier quietly at night, pick one with a CADR well above the minimum.
• Real rooms are messier than test chambers. Open doors, furniture and air leaks all change how well a purifier performs, so treat the number as a minimum.
• Use the right CADR. Boxes list smoke, dust and pollen CADR. The smoke figure is the lowest and the most demanding, so it’s the safest one to compare.

Tip: Placement matters almost as much as size. Read where to put an air purifier before you set yours up.

Limitations

The calculator sizes a purifier for particles (smoke, dust, pollen, pet dander, mold spores). It says nothing about gases and odors: those depend on how much activated carbon a filter holds, which no standard rates. See HEPA vs carbon filters for the difference, and our guides to the best air purifier for smoke and the best air purifier for pets for models that meet these numbers. To see what running your purifier will cost, use the air purifier electricity cost calculator.