Air purifier size calculator
This air purifier size calculator follows the U.S. EPA sizing chart: at least 0.65 cfm of CADR per square foot of floor area, based on an 8-foot ceiling. Enter your room size and ceiling height to get the minimum CADR, or enter the CADR of a unit to see the largest room it suits.
Example: What do you want to calculate: Minimum CADR for a room · Units: sq ft, ft and cfm · Room area 300 · Ceiling height 8 → Result: Minimum CADR 195 cfm (331 m³/h). Source: U.S. EPA, Guide to Air Cleaners in the Home, 2nd edition (2018), Portable Air Cleaner Sizing for Particle Removal: 100–600 sq ft → minimum CADR 65–390 cfm, based on an 8-foot ceiling; CADR covers particles only; use the tobacco-smoke CADR for small particles (read 10.10.2026). Updated: .
How it is calculated
CADR (clean air delivery rate) tells you how much cleaned air an air purifier delivers. The EPA guide to air cleaners recommends a minimum CADR for each room size, calculated for an 8-foot ceiling: 0.65 cfm per square foot of floor area. In metric units that is about 11.9 m³/h per m².
EPA sizing chart
| Room area | Ceiling | Minimum CADR |
|---|---|---|
| 100 sq ft | 8 ft | 65 cfm |
| 200 sq ft | 8 ft | 130 cfm |
| 300 sq ft | 8 ft | 195 cfm |
| 400 sq ft | 8 ft | 260 cfm |
| 500 sq ft | 8 ft | 325 cfm |
| 600 sq ft | 8 ft | 390 cfm |
For higher ceilings the EPA suggests a purifier with a higher CADR. The calculator raises the CADR in proportion to the height: a 300 sq ft room with a 10 ft ceiling needs 244 cfm. The other way round, a 200 cfm purifier suits up to about 308 sq ft under an 8 ft ceiling. The EPA value works out to almost five air changes per hour (0.65 cfm × 60 ÷ 8 ft = 4.9).
What to look for
- CADR covers particles only. Against gases and odors an activated carbon filter helps; according to the EPA there is no widely used rating system for that.
- Many purifiers list three CADRs (tobacco smoke, dust, pollen). For small particles the tobacco-smoke CADR counts.
- CADR is usually measured at the highest fan speed; at a lower speed the unit filters less.
- For open floor plans count the whole area; for very large spaces consider several units.
How often natural ventilation exchanges the air is shown by the air changes per hour calculator; measure the room with the square footage calculator.
Frequently asked questions
What CADR do I need for a 300 sq ft room?
At least 195 cfm with an 8 ft ceiling, according to the EPA chart. With a 10 ft ceiling the calculator gives 244 cfm.
How big a room can a 200 cfm air purifier handle?
About 308 sq ft with an 8 ft ceiling (200 ÷ 0.65).
What does CADR mean?
Clean air delivery rate: the amount of cleaned air per minute in cfm (or m³/h). The higher it is, the larger the room the purifier can serve.
Which of the three CADR numbers should I use?
The tobacco-smoke CADR. According to the EPA it represents the smallest particles; dust and pollen are larger.
Does a high CADR remove odors?
No, CADR rates particles only. Gases and odors need an activated carbon filter, and there is no comparable rating system for those.
How do I convert cfm to m³/h?
1 cfm = 1.699 m³/h, because one cubic foot is 0.0283 m³ and an hour has 60 minutes.
Sources and legal basis
- U.S. EPA, Guide to Air Cleaners in the Home, 2nd edition (2018), Portable Air Cleaner Sizing for Particle Removal: 100–600 sq ft → minimum CADR 65–390 cfm, based on an 8-foot ceiling; CADR covers particles only; use the tobacco-smoke CADR for small particles (read 10.10.2026)
- NIST SP 811, Appendix B.8: 1 ft² = 0.092 903 04 m², 1 ft³/min = 4.719 474 × 10⁻⁴ m³/s, 1 ft = 0.3048 m (read 10.10.2026)
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