Air purifier sizing Last reviewed June 30, 2026
What Is CADR? Air Purifier Sizing Explained
By the WhatSize editorial team 4 min read Standards-reviewed
CADR (Clean Air Delivery Rate) measures how much filtered air an air purifier delivers, in cubic feet per minute (CFM). The higher the CADR, the faster it cleans your room. AHAM Verifide is the only independent CADR certification in the US.
How CADR is measured
CADR is measured by the ANSI/AHAM AC-1 standard. An independent lab introduces three challenge aerosols — smoke, dust, and pollen — into a test chamber and measures how fast the air purifier removes them. The result is a CADR number for each pollutant, and the overall CADR is the lowest of the three (to ensure balanced performance).
Look for the AHAM Verifide seal on a product listing — that means the CADR is independently tested and certified, not just claimed by the manufacturer.
Top picks for this size
These Amazon-available picks meet the calculated spec. For region-specific or licensed contractor-installed options, consult a local professional. See our Disclaimers.
How to use CADR to size your air purifier
The AHAM 2/3 rule
For a room of size X square feet, you need a CADR of at least 2/3 × X (about 67% of the room's square footage). Example: a 300 sq ft bedroom needs ≥ 200 CFM CADR.
Concern multipliers
- General air quality: 1.0× the 2/3 base
- Pet dander: 1.1× the base
- Allergies / pollen: 1.25× the base
- Smoke / VOCs: 1.35× the base
What CADR doesn't measure
CADR measures particle removal (smoke, dust, pollen). It does not directly measure gas-phase removal (VOCs, odors). For VOC concerns, look for a purifier with an activated carbon filter in addition to the HEPA filter.
Use our free CADR calculator for your exact room.
Frequently asked questions
What does CADR stand for?
CADR stands for Clean Air Delivery Rate. It's measured in CFM (cubic feet per minute) — how much filtered air the purifier delivers per minute.
What is a good CADR rating for a bedroom?
For a standard 300 sq ft bedroom, look for a CADR of 200 CFM or higher (the AHAM 2/3 rule). For larger bedrooms or allergy needs, go higher — our calculator handles the multiplier.
Is higher CADR always better?
For the same room, yes — higher CADR cleans the air faster. But for a given room size, oversized CADR just costs more without proportional benefit. Match CADR to room size, not max it out.
You might also need
More air purifier sizing guides
- What Size Air Purifier for a Bedroom? CADR Guide
- What Size Air Purifier for a Large Room (500+ Sq Ft)?
- What Size Air Purifier for a Living Room?
- What Size Air Purifier Do Pet Owners Need?
- What Size Air Purifier Do You Need for Allergies?
- How Many Air Changes per Hour Do I Need? (ACH Chart)
- What Size Air Purifier for Smoke and Wildfire? CADR Guide
- What Size Air Purifier for an Office or Home Office?
- What Size Air Purifier for a Basement?
Still choosing between sizes? Compare: CADR vs ACH