Air Purifier CADR Calculator
Find the clean air delivery rate (CADR) a portable air cleaner needs for your room, or how many clean-air changes per hour a unit you are considering gives.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Minimum CADR
- 200CFM
A 300 ft² room with 8 ft ceilings needs about 200 CFM CADR for 5 clean-air changes per hour.
- Higher ceilings need a higher CADR, as EPA notes. Run the unit on the speed whose CADR you used, and close the room if you can.
Show the working
- Room volume = area × ceiling height2,400 ft³
- CADR = volume × air changes per hour ÷ 602,400 × 5 ÷ 60 = 200 CFM
Sizing by CADR
CADR is the rate at which an air cleaner delivers filtered air, in cubic feet per minute. EPA’s guide to air cleaners recommends a CADR large enough for the room, and its sizing chart works out to a CADR of two-thirds of the floor area with an 8 ft ceiling — about 5 clean-air changes per hour.
This calculator uses the room volume, so it adjusts for ceilings higher or lower than 8 ft: CADR = area × ceiling height × air changes per hour ÷ 60.
Getting the rated performance
CADR is measured in a test chamber. In a real room, open doors, air leaks and running the unit on a lower speed reduce its effect. For smoke, use the smoke CADR; for pollen or dust, the corresponding rating.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Room floor area
- 300 ft²
- Ceiling height
- 8 ft
- Target clean-air changes per hour
- 5
- CADR of a unit you are considering (optional)
- 0 CFM
Result
- Minimum CADR
- 200 CFM
A 300 ft² room with 8 ft ceilings needs about 200 CFM CADR for 5 clean-air changes per hour.
Assumptions
- Clean-air changes per hour = CADR × 60 ÷ room volume. EPA’s chart assumes an 8 ft ceiling.
Limitations
- CADR ratings are measured in a test chamber; real rooms with doors open or air leaks get less benefit.
Frequently asked questions
What CADR do I need for a 300 sq ft room?
About 200 CFM with an 8 ft ceiling — two-thirds of the floor area. With higher ceilings you need proportionally more.
Is a higher CADR always better?
A higher CADR cleans the air faster. The trade-offs are noise and energy; many people run a larger unit on a quieter setting.
Can my HVAC filter do the same job?
A good filter in a forced-air system helps whenever the fan runs. EPA’s guide covers both portable cleaners and HVAC filters.
Sources
- Guide to Air Cleaners in the Home — U.S. EPA. Choose a portable air cleaner with a CADR large enough for the room; EPA’s sizing chart assumes an 8 ft ceiling, and higher ceilings need a higher CADR.
- Choosing a Portable Air Cleaner — Washington State Department of Health. A smoke CADR of at least two-thirds of the room area cleans the air about 5 times per hour with an 8 ft ceiling.
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