Free Tool
CADR Calculator
Find the right air purifier for your room size. Enter your dimensions below and we'll calculate the minimum CADR rating you need using the AHAM 2/3 rule.
Enter Your Room Dimensions
Your Results
Based on the AHAM 2/3 rule
Floor Area
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Volume
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Min CADR
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Ideal CADR
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Air Changes Per Hour (ACH)
Higher ACH means your air gets cleaned more often. The EPA recommends at least 4 ACH for typical rooms, and 6+ for allergy or asthma sufferers.
| ACH | CADR Needed | Best For |
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What is CADR? Clean Air Delivery Rate measures cubic feet of clean air produced per minute. A higher CADR means faster air cleaning. Learn more about CADR →
Air Purifiers That Match Your Room
These products meet or exceed your minimum CADR requirement. Affiliate disclosure.
No products in our database match this CADR requirement. For very large spaces, consider running multiple air purifiers.
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How the CADR Calculator Works
This calculator uses the AHAM 2/3 rule, the industry-standard formula endorsed by the Association of Home Appliance Manufacturers. The rule states:
An air purifier's CADR should be at least two-thirds of the room's floor area (in square feet).
For example, a 200 sq ft room needs a minimum CADR of 133 CFM. We also calculate CADR requirements for different air changes per hour (ACH) rates, which matter if you have allergies, asthma, or are dealing with smoke.
The Formula
For a given ACH rate, the required CADR is:
CADR = (Room Volume in cu ft × ACH) ÷ 60 The AHAM 2/3 rule simplifies this to roughly 4 air changes per hour at standard 8-foot ceilings, which is adequate for general use.
How Many Air Changes Do You Need?
- 2 ACH: Light maintenance — keeping already-clean air fresh
- 4 ACH: Standard recommendation — good for most rooms
- 5 ACH: Better for pet owners, cooking, and moderate allergies
- 6+ ACH: Recommended for asthma, severe allergies, wildfire smoke, or VOC concerns
Not sure what CADR means? Read our complete guide to CADR. Or check out our air purifier buying guide for help choosing the right model.