Air purifier sizing Last reviewed June 30, 2026
How Many Air Changes per Hour Do I Need? (ACH Chart)
By the WhatSize editorial team 4 min read Standards-reviewed
ACH (air changes per hour) tells you how many times your room's air is fully filtered per hour. Most homes need 4-6 ACH; allergy and asthma sufferers need 6-8 ACH. Multiply your room volume by the target ACH to get the CADR (in CFM) your air purifier must deliver.
Why ACH matters for air purifier sizing
CADR tells you how much clean air a purifier can deliver per minute. ACH tells you how many times the air in your room gets fully cycled through the filter in an hour. The two work together: a 300 sq ft bedroom (with 8 ft ceilings = 2,400 cubic feet) needs 9,600 cubic feet of filtered air per hour to hit 4 ACH, which is exactly 160 CFM of CADR. That's the AHAM 2/3 rule expressed as volume rather than area.
Use the chart below to find the ACH target for your room type, then size your air purifier accordingly.
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.
ACH targets by room and use case
Recommended ACH by room type
- Bedroom (general): 4-5 ACH
- Bedroom (allergies/asthma): 6-8 ACH
- Living room (general): 4-5 ACH
- Living room (smoking or VOCs): 6-8 ACH
- Home office: 4-6 ACH
- Kitchen (smoke/odors): 6-8 ACH
Quick math: how to convert ACH to CADR
CADR (CFM) = Room volume (cu ft) × Target ACH ÷ 60.
Example: 300 sq ft × 8 ft ceiling = 2,400 cu ft. At 5 ACH: 2,400 × 5 / 60 = 200 CFM. That's the minimum CADR for a 300 sq ft bedroom.
Use our free air purifier calculator to apply this for your exact room and concern.
Frequently asked questions
How many air changes per hour do I need for a bedroom?
For a general bedroom, 4-5 ACH is the standard recommendation. For allergy or asthma sufferers, aim for 6-8 ACH. Use our ACH-to-CADR formula to size the air purifier: CADR = Room volume × Target ACH / 60.
Is higher ACH always better?
Not necessarily. Going past 8 ACH in a residential space adds cost and noise without proportional benefit. Match ACH to your specific use case — 4-5 for general air quality, 6-8 for allergies, smoke, or asthma.
How do I calculate ACH from a CADR rating?
ACH = (CADR × 60) / Room volume in cubic feet. If your air purifier has a 200 CFM CADR rating and your room is 2,400 cubic feet, ACH = (200 × 60) / 2,400 = 5 ACH.
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?
- What Is CADR? Air Purifier Sizing Explained
- 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