Air Purifier CADR Explained: How to Pick the Right Size
CADR stands for clean air delivery rate. It is the volume of filtered air a cleaner delivers, measured in cubic feet per minute (cfm), for smoke, pollen and dust. Use it to match a unit to a room, and read it as a particle rating only.
The rating comes from AHAM, the trade association behind the AHAM Verifide seal. A CADR is a number on a rating, not a promise about every room. The EPA says the CADR rating system is for particles only, and King County notes that the listed figure usually applies to the highest fan speed. The sections below set out what the number covers, the room-size rules of thumb from the sources we opened on 17 October 2026, and a worked example.
What CADR measures
- Three scores. AHAM’s seal lists “three Clean Air Delivery Rate (CADR) numbers for common pollutants”: smoke, pollen and dust.
- Particles only. The EPA says: “The CADR rating system is for particles only.” A CADR figure does not measure gas or odor removal.
- Units. King County gives CADR in cfm, for example “above 200 cfm” for larger spaces. The AHAM pages we opened do not print a unit.
- Fan speed. King County says the listed CADR usually applies to the highest fan speed, so a lower or auto setting may clean less air.
How CADR is used for room size
The EPA chart gives the minimum CADR for three room sizes, assuming an 8-foot ceiling. King County uses the same ratio as a rule of thumb. AHAM’s rule is stricter for smoke: the smoke CADR should be at least two-thirds of the room’s floor area in square feet.
| Room floor area | EPA chart minimum (cfm) | King County ratio, 65 per 100 sq ft (cfm) | AHAM two-thirds rule, smoke CADR (cfm) |
|---|---|---|---|
| 100 sq ft | 65 | 65 | 67 |
| 200 sq ft | 130 | 130 | 133 |
| 300 sq ft | 195 | 195 | 200 |
The AHAM column is our arithmetic from the two-thirds rule. The EPA and King County columns are the figures those pages give.
Worked example: a 12 ft by 14 ft room
Take a room 12 ft wide and 14 ft long. Its floor is 12 x 14 = 168 sq ft. By the King County ratio, 168 / 100 x 65 is about 109 cfm. By AHAM’s two-thirds rule, 168 x 2/3 is about 112 cfm of smoke CADR. The two rules of thumb differ by about 3 cfm for this room, so a unit with a smoke CADR of about 112 cfm meets both. Both figures are our arithmetic from the sources; neither source prints this room.
Where CADR can mislead
- Ceiling height. Both the EPA chart and the AHAM rule assume an 8-foot ceiling. AHAM says a higher ceiling needs “an air cleaner rated for a larger room”.
- Odors. Because the rating is for particles, the number says nothing about how well a unit removes odors or gases.
- Missing figures. Not every listing states a CADR. Check the listing and the manufacturer’s page before you compare two units.
How to use CADR when you choose
- Measure the room’s floor area in square feet (length x width).
- Work out a minimum CADR with the King County ratio, and check it against the two-thirds rule for smoke.
- Choose the higher of the two numbers if the room has a high ceiling or is used for smoke.
- Look for an AHAM Verifide mark or a CADR stated on the listing.
Frequently asked questions
What does CADR stand for?
Clean air delivery rate. AHAM’s standard for measuring an air cleaner is called the Clean Air Delivery Rate.
What units is CADR measured in?
King County gives it in cfm. The AHAM pages we opened leave the unit out.
Is a higher CADR always better?
A higher CADR moves more filtered air for particles. King County says CADRs above 200 cfm are more efficient and better for larger spaces. Choose a CADR that matches the room; a larger figure does not add value to a small room.
Does CADR measure odor?
No. The EPA says the CADR rating system is for particles only.
What CADR do I need for a bedroom?
It depends on floor area. For a 120 sq ft bedroom the King County ratio gives about 78 cfm, and the two-thirds rule gives about 80 cfm of smoke CADR.
Sources
US EPA, Guide to Air Cleaners in the Home (page dated October 1, 2026; opened 17 October 2026).
AHAM Verifide, Air Filtration Standards (no page date shown; opened 17 October 2026).
King County, Washington, HEPA technology page (no page date shown; opened 17 October 2026).
