Free tool · air purifier sizing
Air Changes Per Hour (ACH) Calculator
Pick a medical or a commercial setting, enter the room’s length, width, and ceiling height in feet, and the calculator shows how many MedicAir® air purifiers that room needs to reach its recommended air changes per hour: 10 for medical spaces, 5 for commercial ones.
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MedicAir® Air Changes Per Hour Calculator
For specific air changes per hour calculations, adjust the room dimensions to match your own (if the calculator did not load, please refresh your page).
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Want a room-by-room recommendation?
The calculator sizes one room at a time. Our technicians measure your air on-site and go through the readings with you — about 30 minutes, no cost, and no obligation. Based on the results and your interest, we show you the options best suited for your home.
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- Since 1999 based in London, Ontario
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Commercial systems are sized from your square footage and ceiling height, your ACH targets, occupancy, and HVAC layout.
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The air purifier in the calculator
The MedicAir® medical-grade HEPA air purifier
The MedicAir® is a medical-grade HEPA air purifier that is extremely easy to maintain with a simple swap of a single all-in-one cartridge housing a HEPA H13 Filter, Activated Carbon Filter, and Ultra-Violet Type-C light. The MedicAir® is used in thousands of medical offices and commercial offices globally.
- 605 m³/h CADR (Clean Air Delivery Rate)
- ReAKT mode changes the fan speed to match your air quality
- 3-year warranty
About the calculator
What is the ACH calculator for?
The ACH (Air Changes Per Hour) calculator helps determine how often the air inside a room is replaced or filtered by your air purifier, HVAC system, or ventilation setup. This is essential for maintaining indoor air quality in homes, offices, medical clinics, and schools.
Originally popularized during COVID-19 for air purifiers, ACH is now a critical metric in choosing effective ventilation. This tool uses room volume and the CADR (Clean Air Delivery Rate) of the MedicAir® to work out how many units a room needs to reach its recommended ACH — a key factor in reducing airborne contaminants.
The calculation is based on the size of the room, ceiling height, width, and length in feet to work out how many MedicAir® units reach the recommended ACH. For simplicity, the calculations and breakdowns of cubic feet per minute, CFM variable adjustments, and HVAC-related ventilation rates are not shown or used.
What is the recommended ACH, air changes per hour?
10+ ACH
Medical environments
Hospitals, clinics, labs
Recommended ACH is 10+ air changes per hour.
Air purification for dental offices5–6 ACH
Commercial & office spaces
Offices and businesses
Recommended ACH is 5–6 air changes per hour.
Commercial air purification4–6 ACH
Homes & residential spaces
Bedrooms, living rooms, kitchens
ACH of 4–6 is typically sufficient for bedrooms, living rooms, and kitchens.
Air purifiers for your homeA higher ACH means dust, pollen, and pet dander are pulled out of a room’s air more often, so homes with allergy sufferers often aim for the top of the range.
The formula
How to calculate air changes per hour (ACH)
Three steps. All you need is a tape measure and your air purifier’s CADR.
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Measure the volume of your room
Multiply:
Length (ft) × Width (ft) × Ceiling Height (ft)
Example:
25 ft × 20 ft × 8 ft = 4,000 cubic feet
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Convert CADR to CFH (Cubic Feet per Hour)
Multiply the CADR in CFM (cubic feet per minute) by 60. Example:
CADR 605 CFM × 60 = 36,300 CFH
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Calculate ACH using this formula
ACH = CFH ÷ Room Volume
ACH = 36,300 ÷ 4,000 = 9.075
CADR listed in m³/h?
Some air purifiers list their CADR in cubic meters per hour (m³/h). That is already per hour: multiply it by 35.3 to get CFH. The MedicAir® is rated at 605 m³/h: about 21,357 CFH, or about 5.3 ACH in the 4,000 cubic foot room above.
ACH to CFM
Working backwards: to find the airflow a room needs, multiply the target ACH by the room volume and divide by 60.
CFM = ACH × Room Volume ÷ 60
6 × 4,000 ÷ 60 = 400 CFM
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Air changes per hour: frequently asked questions
What does ACH mean in air purification?
ACH stands for Air Changes Per Hour. It refers to how many times the air within a defined space is completely replaced in one hour by an air purifier, HVAC system, or ventilation system.
Why is ACH important for indoor air quality?
A higher ACH rate means your indoor air is filtered or replaced more frequently, reducing airborne contaminants like dust, allergens, viruses, and odors. This is critical in medical offices, schools, and homes with allergy sufferers.
What is a good ACH for a bedroom, office, or clinic?
Residential bedrooms should have at least 4–6 ACH, and offices 5–6. Medical clinics and dental offices should target 10–12 ACH or higher.
How do I calculate ACH for my air purifier?
Measure your room size (length × width × height), convert your air purifier’s CADR to cubic feet per hour (CFM × 60), and divide that number by the room volume. This gives your ACH.
How do I convert ACH to CFM?
Multiply the ACH by the room volume in cubic feet, then divide by 60. A 4,000 cubic foot room at 6 ACH needs 6 × 4,000 ÷ 60 = 400 CFM.
What’s the difference between CFM and CADR?
CFM (Cubic Feet per Minute) is a general airflow measurement, while CADR (Clean Air Delivery Rate) measures the volume of filtered air. CADR is better for assessing air purifier performance.
Can one air purifier cover multiple rooms?
Yes and no — ACH is based on room volume. Air purifiers are most effective when used in a closed space. To achieve the right ACH, you should calculate for each room individually; however, our air purifiers are powerful enough for most homes.
How often should I run my air purifier to maintain ACH?
To maintain your target ACH, your air purifier should run continuously or on a smart sensor mode that adjusts speed based on real-time air quality.
Does a higher ACH help with allergies or asthma?
Yes. A higher ACH means pollen, pet dander, and dust mite particles are pulled out of the room’s air more often. Those are the things most people with allergies or asthma react to, so less of them in the air can only help.
Do HVAC systems have ACH ratings too?
Yes — commercial and residential HVAC systems have ventilation rates that contribute to overall ACH. Combining HVAC and air purifiers helps reach optimal ACH levels.
Is ACH the same as ventilation?
No — ACH refers to how often air is replaced or filtered, while ventilation involves bringing in fresh outdoor air. Both improve indoor air quality but serve different functions. For fresh air in a home, see HRV and ERV ventilation.
Free in-home air test
Not sure how many air purifiers your space needs?
The calculator gives you a starting point. For a home, the free in-home air test is the next step: about 30 minutes, no cost, and no obligation. For an office, clinic, or school, call us.
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