Air Changes per Hour Calculator - Calculate Ventilation Rate
Calculate Air Changes per Hour (ACH) to ensure optimal ventilation, improve indoor air quality, and size your fans or HVAC system correctly.
Room Dimensions & Airflow
Results
What is an Air Changes per Hour Calculator?
This Air Changes per Hour (ACH) Calculator is a free tool that determines how many times the air volume in a room is completely replaced within one hour. It calculates ACH based on the room's dimensions and the airflow rate (CFM) of your ventilation system.
This calculator works for:
- Home Ventilation - Sizing bathroom and kitchen fans.
- HVAC Design - Ensuring adequate air turnover for comfort.
- Air Purification - Checking if an air purifier is strong enough for a room.
To determine cooling requirements for your ventilated space, check out our Air Conditioner BTU Calculator to find the right AC size.
For assessing heating needs in colder climates, explore our Boiler Size Calculator to ensure adequate warmth.
To calculate potential energy costs of running ventilation fans, use our Electricity Cost Calculator to estimate monthly expenses.
For measuring room dimensions accurately, try our Feet and Inches Calculator to work with precise measurements.
To convert airflow measurements, check our Cubic Feet Calculator to better understand volume metrics.
How the Calculation Works
The calculation uses the formula:
Where:
- CFM = Airflow in Cubic Feet per Minute
- 60 = Minutes in an hour
- Volume = Room Length × Width × Height (ft³)
Key Concepts Explained
ACH
Air Changes per Hour. A higher number means better ventilation and cleaner air.
CFM
Cubic Feet per Minute. The standard measurement for airflow volume.
How to Use This Calculator
Measure Room
Enter length, width, and ceiling height in feet.
Enter Airflow
Input the fan or system rating in CFM.
View Results
See your ACH and time to exchange air.
Benefits of Adequate Ventilation
- • Healthier Air: Reduces concentration of viruses, bacteria, and CO2.
- • Moisture Control: Prevents mold growth in bathrooms and basements.
- • Odor Removal: Quickly clears cooking smells and other odors.
Factors Affecting Results
1. Obstructions
Furniture and layout can block airflow, effectively reducing the real-world ACH.
2. Duct Resistance
Long or bent ducts can reduce a fan's actual CFM performance below its rating.
Frequently Asked Questions (FAQ)
Q: What is a good ACH for a home?
A: For residential homes, an ACH of 0.35 to 1.0 is typical for natural ventilation. With mechanical ventilation, 3-5 ACH is common for living areas, while kitchens and bathrooms may require higher rates.
Q: How do I calculate room volume?
A: Multiply the room's length by its width and ceiling height (Length × Width × Height). Measure in feet to get cubic feet.
Q: Does higher ACH mean better air quality?
A: Generally, yes. Higher ACH means the air is replaced more frequently, reducing pollutants, odors, and allergens. However, extremely high ACH can increase energy costs for heating or cooling.
Q: What CFM do I need for my room?
A: To find the required CFM, multiply the Room Volume by your desired ACH, then divide by 60. Formula: CFM = (Volume × ACH) / 60.