Last updated on January 17th, 2023

Air Changes Per Hour a.k.a ACH or ACPH refers to a volume of air added or removed from a specific space in one hour. It helps buyers to figure out how much time an air purifier needs to ventilate air in their room.

Formula for ACH is fairly simple. You have to know:

  • CADR (Clean Air Delivery Rate) of your air purifier which is available in cfm (cubic feet per minute; mostly used in the US) or m³/h (cubic meter per hour). This is how much air is comes through an air purifier in 1 minute/hour.
  • A room dimensions where you either already have an air purifier or planning to use one.

And Air Changes Per Hour formula is the following: cfm x 60 / (Room Area x Height). For example, if an air purifier has 220 cfm airflow and our room is 350 sq. ft. with a standard 8 feet ceiling, the ACH would be 220 x 60  / (350 x 8) = 4.7 ACH. This means that an air purifier will clean up every bit of air in a specified room every 13 minutes.

Air Changes Per Hour Calculator

To calculate air changes per hour for you air purifier you have to fill up data of your air purifier’s CADR rating using cubic feet per minute number which you can find in specifications. Then add your room dimensions in square feet. Finally, enter your ceiling height to estimate ACPH for your air purifier.

Air Changes Per Hour Standard Recommendations

Air ventilation rates differs for living rooms, kitchens, bathrooms etc. Here is a table (based on vent-axia latest research) with recommended ACH under normal conditions for different spaces:

Space Recommended ACH
Bedrooms 2-4
Bathrooms 6-10
Kitchens 15-20
Living Rooms 3-6
Garages 6-8
Utility Rooms 15-20
Churches 1-3
Libraries 3-5
Entrance Halls & Corridors 3-5
Cafes & Bars 10-12
Club Rooms 12
Offices 6-10
Conference Rooms 8-12
Laundries 10-30
Bakeries 20-30
Commercial Kitchens 30
Factories & Workshops 8-10
Stores & Warehouses 3-6
Commercial Kitchens 30
Toilets 6-10
Schoolrooms 5-7
Shops & Supermarkets 8-15
Compressor Rooms 10-20
Engine Rooms 15-30

For asthma or allergy sufferers the minimum recommended air changes per hour means 4.

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