Air Changes per Hour Calculator
Air Changes per Hour (ACH) is a critical metric that measures how many times the air in a room is completely replaced with fresh air each hour. This calculation is essential for evaluating indoor air quality, ensuring proper ventilation, and maintaining healthy environments in homes, offices, schools, and healthcare facilities.
Using this calculator, you can determine your space’s ACH by inputting room dimensions (length, width, height) and airflow rate (in CFM or m³/min). Understanding your ACH helps you assess whether your ventilation meets recommended standards for your specific room type and usage.
Calculation Results
Air Changes per Hour (ACH): — times/hour
Room Volume: —
Air Changes per Minute (ACM): — times/minute
At — ACH, the air in your space is exchanged about every — minutes.
Understanding Air Changes per Hour (ACH)
ACH is calculated using the formula:
ACH = (Airflow per hour) ÷ Room Volume
Where:
- Airflow per hour = CFM × 60 (for Imperial) or m³/min × 60 (for Metric)
- Room Volume = Length × Width × Height
If you have multiple ventilation devices, the formula becomes:
ACH = (Airflow per hour × Number of devices) ÷ Room Volume
What is a Good ACH?
Recommended ACH values vary depending on room type and usage:
| Room Type | Recommended ACH | Notes |
|---|---|---|
| Residential Living Areas | 4-6 | For general comfort and air quality |
| Classrooms | 4-6 | ASHRARE Standard 62.1 recommendation |
| Offices | 6-8 | For optimal productivity and comfort |
| Kitchens | 8-12 | Higher due to cooking odors and moisture |
| Bathrooms | 8-12 | For effective moisture and odor removal |
| Hospital Patient Rooms | 6-12 | Higher for infection control |
| Laboratories | 8-15 | Depending on chemical usage and safety requirements |
Note: These are general guidelines. Specific requirements may vary based on local building codes, occupancy, and activities in the space.
How to Improve ACH in Your Space
If your calculated ACH is lower than recommended, consider these strategies:
- Increase airflow by upgrading to a more powerful ventilation system
- Add more ventilation devices such as additional exhaust fans or air purifiers
- Ensure proper duct design with minimal bends and appropriate sizing
- Maintain your HVAC system with regular filter changes and cleaning
- Use natural ventilation by opening windows when weather permits
- Reduce room volume by installing partitions or lowering ceilings if feasible
- Seal air leaks to ensure air flows through intended pathways
Frequently Asked Questions
Air Changes per Hour (ACH) is a measurement of how many times the air within a defined space is replaced with fresh air in one hour. It’s a key metric for assessing ventilation effectiveness and indoor air quality.
Proper ACH is crucial for maintaining healthy indoor environments by removing pollutants, controlling moisture, preventing mold growth, diluting airborne contaminants, and ensuring adequate oxygen levels. Inadequate ventilation can lead to health issues, discomfort, and reduced cognitive function.
CFM (Cubic Feet per Minute) can be measured using an anemometer or flow hood at ventilation grilles. Many HVAC professionals use specialized equipment for accurate measurements. Alternatively, you can often find CFM ratings in product specifications for fans and ventilation systems.
While ACH is an important metric, it doesn’t tell the whole story. Effective ventilation also depends on air distribution, filtration efficiency, source control, and the quality of outdoor air. ACH is best used alongside other indoor air quality measurements.
ACH measures air changes per hour, while ACM (Air Changes per Minute) measures air changes per minute. ACM is simply ACH divided by 60. ACH is more commonly used in ventilation standards and guidelines.
Unit Conversion Reference
| Imperial | Metric | Conversion Factor |
|---|---|---|
| 1 foot (ft) | 0.3048 meters (m) | Multiply feet by 0.3048 to get meters |
| 1 CFM | 0.0283 m³/min | Multiply CFM by 0.0283 to get m³/min |
| 1 cubic foot | 0.0283 cubic meters | Multiply cubic feet by 0.0283 to get cubic meters |