Air Purifier Room Size Calculator
Find the CADR an air purifier needs for your room and how large a purifier that rating covers.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
CADR is the filtered-air delivery rate in cubic feet per minute. This estimate uses the selected air-changes-per-hour (ACH) profile: 3 for basic, 4.8 standard, 6 allergy, or 8 smoke. Room size, ceiling height, and ACH determine the result; use manufacturer guidance for a specific room.
Worked example
calculated with the default values below
Room length 15 ft · Room width 12 ft · Ceiling height 8 ft · Air changes per hour Standard (4.8 ACH) CADR needed 115 CFM · Room area 180.0 sq ft · Room volume 1,440 cu ft · CADR needed 196 m³/h · Max room for a CADR 300 unit 469 sq ft
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
CADR = L × W × H × ACH ÷ 60
- CADR
- required CADR (ft³/min)
- L
- room length (ft)
- W
- room width (ft)
- H
- ceiling height (ft)
- ACH
- air changes per hour
ACH presets: 3 basic, 4.8 standard, 6 allergy, 8 smoke; multiply CADR by 1.699 for m³/h.
Frequently asked questions
Why is the smoke CADR higher than the standard setting?
The model uses 8 air changes per hour for smoke and 4.8 for standard use. Because CADR is room volume multiplied by the selected air-change rate and divided by 60, the smoke result is 8 ÷ 4.8, or about 1.67 times the standard result for the same room. For a 180 sq ft room with an 8 ft ceiling, that is 192 CFM instead of 115.2 CFM.