What the calculator is doing
Room area × ceiling height gives room volume in cubic feet. Multiplying by your target air changes per hour tells us how many cubic feet of clean air the room needs each hour. Dividing by 60 converts that to cubic feet per minute.
Formula: required CFM = room area × ceiling height × ACH ÷ 60. We then apply your selected headroom.
Which ACH should you use?
For general particle filtration, 4–5 air changes per hour is a useful comparison range. A higher target can make sense for wildfire smoke, heavy pollen or faster cleanup. A lower target may be acceptable for background filtration when the room is already relatively clean.
The important part is consistency: compare purifiers using the same air-change assumption instead of comparing one brand’s 1-ACH room-size claim with another brand’s 4.8-ACH claim.
Is CFM the same as CADR?
Not exactly. Fan airflow and clean-air delivery rate are not interchangeable. Use the result as a target for published CADR when available, preferably the pollutant-specific CADR most relevant to your problem. For wildfire smoke, smoke CADR is especially useful.
Open floor plans: include the connected air volume you actually expect the purifier to serve. Closed bedrooms usually behave as separate zones.
Next
Read the full CADR sizing guide, compare Levoit Core 300S vs 400S, or see our 500 ft² sizing example.