What does ACH mean?
ACH stands for Air Changes per Hour and measures how many times per hour the total volume of air in a room is completely replaced with fresh air from your ventilation or air device. It is the standard metric used by HVAC engineers, building inspectors, and health professionals to evaluate the quality and efficiency of ventilation systems.
What does ACH tell us?
The ACH value indicates how well a space is ventilated. Proper ventilation is essential for health and comfort: it removes air pollutants, excess humidity, CO₂, odors, and airborne microbes. A higher ACH means faster air replacement, which is critical in spaces with elevated infection risks, increased humidity, or heavy occupancy.
As a general guideline, ACH should vary between 4–30 depending on the room type and its purpose. Higher ACH values are required for:
- Closed or poorly ventilated spaces — garages, manufacturing halls, storage rooms
- Spaces with increased humidity — kitchens, bathrooms, laundry rooms
- Gathering places — malls, conference rooms, classrooms, gyms
- Rooms storing temperature- or humidity-sensitive materials — pharmacies, wine cellars, server rooms
- Places of increased infection risk — hospitals, laboratories, isolation rooms
How to calculate air changes per hour?
The formula for calculating ACH depends on the measurement system you are using:
Imperial System (US)
When airflow is measured in CFM (Cubic Feet per Minute) and room volume in cubic feet:
ACH = (CFM × 60) ÷ Room Volume (ft³)
Example: A room 20 ft × 15 ft × 9 ft = 2,700 ft³ with a 200 CFM device:
ACH = (200 × 60) ÷ 2,700 = 4.4 ACH
Metric System
When airflow is measured in m³/h and room volume in cubic meters:
ACH = Airflow (m³/h) ÷ Room Volume (m³)
Example: A room 6 m × 4.5 m × 2.7 m = 72.9 m³ with a 340 m³/h device:
ACH = 340 ÷ 72.9 = 4.7 ACH
How to use this ACH calculator
This air changes per hour calculator supports both the US Imperial system and the Metric system. To get your result:
- Choose your measurement system — Imperial (ft, CFM) or Metric (m, m³/h)
- Select input mode — enter room Length × Width × Height, or just Area + Height if you already know the floor area
- Enter the room dimensions — the calculator will compute the volume automatically
- Enter the airflow rating of your ventilation device — this is usually printed on the unit or in its manual (CFM for Imperial, m³/h for Metric)
- Click Calculate — the result shows your ACH value and compares it to recommended ranges
Recommended ACH values by room type
| Room Type | Recommended ACH |
|---|---|
| Bedroom | 6–15 |
| Living Room | 4–10 |
| Kitchen | 15–60 |
| Bathroom | 6–15 |
| Office | 6–10 |
| Garage | 4–6 |
| Hospital Room | 6–12 |
Tips for improving ACH
- If your ACH is too low, consider upgrading to a higher-CFM fan or air purifier
- Check that vents and ducts are unobstructed
- Open windows when outdoor air quality allows — natural ventilation can significantly boost ACH
- For kitchens and bathrooms, use exhaust fans rated for the room volume to meet local building codes
- In offices and classrooms, CO₂ monitors can help identify when ventilation is insufficient before ACH drops