Air Purifier CADR Calculator
CADR needed for a room and resulting ACH.
Enter your values
ⓘ Live results
Results
- CADR required
- 200 CFM
- Two-thirds rule check
- 198 CFM
- Your unit's ACH
- 6.25
- Units needed
- 0.8
- Room volume
- 2,400 cu ft
Quick answer
A purifier's CADR should be at least two-thirds of the room's floor area in square feet for 4-5 air changes per hour.
CADR ≈ volume x ACH / 60
How to use the Air Purifier CADR Calculator
- 1Enter the room dimensions and how many air changes you want.
- 2Optionally enter a specific model's smoke CADR to check it.
- 3Run the unit on a speed you can live with — rated CADR is usually the loudest setting.
Worked example
Using the values the calculator loads with:
Inputs
- Room area300 sq ft
- Ceiling height8 ft
- Target air changes5 ACH
- Purifier CADR (optional)250 CFM
Results
- CADR required200 CFM
- Two-thirds rule check198 CFM
- Your unit's ACH6.25
- Units needed0.8
- Room volume2,400 cu ft
Frequently asked questions
Which CADR number matters?
Smoke CADR is the toughest test and best proxy for fine particles.
Two small units or one big one?
Two smaller units usually mix the room better and run quieter.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.