Air Purifier comparison
CADR vs ACH vs Room Size: How to Choose the Right Air Purifier
By the WhatSize editorial team 5 min read Standards-reviewed
CADR and ACH are the two most important numbers on any air purifier, but they measure different things. CADR tells you how fast the purifier cleans. ACH tells you how many times per hour it cycles your room's air. Together with room size, these determine whether an air purifier is actually effective for your space.
CADR (Clean Air Delivery Rate) vs ACH (Air Changes per Hour): Side-by-Side
| Factor | CADR (Clean Air Delivery Rate) | ACH (Air Changes per Hour) |
|---|---|---|
| What it measures | Cleaning speed (CFM) | Room air turnover per hour |
| Set by | AHAM testing | Calculation from CADR + room |
| Target for bedrooms | Dust 100+, Pollen 100+, Smoke 80+ | 4+ ACH |
| Target for allergies | Pollen 150+ | 4.8+ ACH |
| Easiest to use | Compare between models | Calculate for your room |
| AHAM seal | Yes, standard rating | Derived from CADR |
When to choose CADR (Clean Air Delivery Rate)
When to Focus on CADR
- You're comparing two models and want an apples-to-apples performance rating
- You want the AHAM Verified seal for independent testing
- You're buying for a standard-sized room where the manufacturer's coverage estimate is accurate
When to choose ACH (Air Changes per Hour)
When to Focus on ACH
- You want to calculate exactly how many times per hour your room air gets cleaned
- You have a specific health need (allergies, asthma) requiring 4.8+ ACH
- Your room is unusually shaped or has high ceilings — manufacturer CADR-to-room-size estimates may not apply
Frequently asked questions
What's the formula for converting CADR to ACH?
ACH = (CADR x 60) u00f7 (Room Area x Ceiling Height). For example, a purifier with 200 CADR in a 300 sq ft room with 8 ft ceilings: ACH = (200 x 60) / (300 x 8) = 12,000 / 2,400 = 5 ACH. This means the air is fully cleaned 5 times per hour.
Is higher CADR always better?
Not necessarily. A CADR that's too high for your room means you're paying for capacity you don't need and the unit may be louder. Use the 2/3 rule: your CADR should be at least 2/3 of your room's square footage.
Do I need different CADR for different pollutant types?
Yes. AHAM rates three CADR numbers: dust, pollen, and smoke (tobacco smoke, the smallest particles). If you have allergies, focus on the pollen CADR. For general air quality, look at the smoke CADR — smaller particles are harder to capture.