Air Purifier CADR Room Sizing Calculator
Volume I · May 2026 · 712 words
Determine the Clean Air Delivery Rate (CADR) required to achieve your target air changes per hour for any room. CADR is the most important specification on an air purifier — it tells you how quickly the unit removes particles of a given size from the air. For a detailed explanation of CADR and room sizing, see our CADR and room sizing guide.
CADR (CFM) = (Room Length × Width × Ceiling Height) × ACH ÷ 60
Recommended Room Size (sq ft) = Smoke CADR × 1.55
| Application | Recommended ACH |
|---|---|
| General dust control | 2–3 |
| Allergy management (AHAM recommendation) | 4.8 |
| Asthma management | 6 |
| Wildfire smoke / severe allergies | 6–8 |
| Hospital airborne infection isolation rooms (reference) | 12 |
A unit with a smoke CADR matching the calculated value will achieve the target ACH with a clean filter under ideal air mixing. Real-world performance is typically 15–25% below the calculated figure due to furniture obstruction, imperfect air circulation, and filter loading. Derating the required CADR by 20% — using the AHAM room size formula — is the conservative approach recommended by most manufacturers.