Read the airflow unit first
CADR is a clean-air delivery rate for a specified particle type. It combines flow with particle removal performance, so it is more informative than fan airflow alone. Check whether the specification uses cubic meters per hour (m³/h) or cubic feet per minute (CFM).
The conversion is approximately 1 CFM = 1.699 m³/h. Cubic meters per minute would be 60 times cubic meters per hour for the same numeric value. Missing the time unit can create a very large sizing error.
Use room volume, not just floor area
For a rectangular room, multiply length by width by ceiling height. A room 4 m long, 5 m wide and 2.7 m high contains 54 m³. At a selected 5 ACH target, the corresponding clean-air delivery is 54 × 5 = 270 m³/h.
For an irregular room, divide the floor plan into simple shapes and add their areas. Include openly connected space when air can move freely between it and the purifier. Do not assume a unit in one closed room gives its full rated service to the next room.
Treat coverage as a conditional number
The same 300 m³/h purifier corresponds to 60 m³ at 5 ACH or 150 m³ at 2 ACH. With a 2.5 m ceiling, those become 24 m² and 60 m². Neither area is meaningful without the underlying ACH target.
Use the CADR calculator when you know your room and target. Use the room-size calculator when you already own a purifier. Use the ACH calculator to compare an existing setup with a chosen target. These three tools rearrange the same equation rather than using different sizing rules.
Keep the model separate from a health promise
Equivalent clean-air ACH from a particle filter is not outdoor-air ventilation. It does not by itself show that carbon dioxide, gases or every pollutant are controlled. Also, air mixes: one theoretical room volume passing through a filter does not mean every portion of room air has been filtered once.
Check the CADR at a fan setting you can actually use. A high maximum rating is less useful if noise causes you to run the unit much slower. Filter condition, placement and open doors can all change practical performance. Use source control and appropriate ventilation alongside filtration.
Sources & methodology
- Guide to Air Cleaners in the Home, second edition — U.S. Environmental Protection Agency
- Air Cleaners and Air Filters in the Home — U.S. Environmental Protection Agency