About this tool
The Air Purifier CADR & Room Size Calculator is a free, in-browser tool that sizes an air purifier to a room. It tells you the minimum Clean Air Delivery Rate (CADR) a room needs, or in reverse the largest room a given CADR can handle, and reports the air changes per hour (ACH) you achieve. It follows AHAM's widely cited guidance so you can match a purifier to your space with confidence. Everything is computed locally in your browser and nothing you enter is uploaded.
The method is AHAM's 2/3 rule: for a standard 8 ft ceiling, minimum smoke CADR (in cfm) = room area (sq ft) × 0.67, which delivers roughly 4.8–5 air changes per hour. If your ceiling is not 8 ft, the required CADR scales by (ceiling height / 8). ACH is CADR × 60 / room volume (ft³), and higher targets suit sensitive users — about 4–5 for general use, 5+ for allergies, and 6–8 for asthma or wildfire smoke — so the tool also computes the CADR your chosen target ACH requires. In reverse mode it converts a known CADR into the maximum area it covers, area = (CADR / 0.67) × (8 / ceiling). Note AHAM CADR tops out around 450, so if the requirement exceeds that, use two units.
As a worked example, a 15 ft × 12 ft room (180 sq ft) with an 8 ft ceiling needs a minimum smoke CADR of 180 × 0.67 ≈ 121 cfm, which gives about 5 ACH; targeting 8 ACH for smoke would need 8 × 1,440 / 60 = 192 cfm. Going the other way, a 200 cfm purifier covers up to about 200 / 0.67 ≈ 299 sq ft at an 8 ft ceiling. CADR is measured with fresh filters, so real-world performance drops as filters load — size up for a margin.