Air Changes per Hour Calculator
Calculate actual air changes per hour, target airflow, required fan quantity and operating energy for a rectangular room.
Default project preview
6.67 ACH
Room volume
2,700 ft³
Target airflow
270 CFM
Fans needed
1
Target difference
0.67 ACH
Calculator
Build the material order
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Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Room length
Enter this project assumption in ft. The current imperial default is 20 ft.
Room width
Enter this project assumption in ft. The current imperial default is 15 ft.
Room height
Enter this project assumption in ft. The current imperial default is 9 ft.
Airflow per fan
Enter this project assumption in CFM. The current imperial default is 300 CFM.
Operating fans
Enter this project assumption in a unitless value. The current imperial default is 1. Minimum: 1.
Target air changes
Enter this project assumption in ACH. The current imperial default is 6 ACH.
Power per fan
Enter this project assumption in W. The current imperial default is 120 W. Minimum: 0.
Operating time
Enter this project assumption in hours/day. The current imperial default is 8 hours/day. Minimum: 0. Maximum: 24.
Electricity price
Enter this project assumption in per kWh. The current imperial default is 0.18 per kWh. Minimum: 0.
How to read the outputs
Room volume
rectangular room. Default preview: 2,700 ft³.
Target airflow
6 ACH target. Default preview: 270 CFM.
Fans needed
at entered fan airflow. Default preview: 1.
Target difference
above target. Default preview: 0.67 ACH.
Daily energy
8 operating hours. Default preview: 0.96 kWh.
Daily energy cost
fan power only. Default preview: $0.17.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Imperial ACH = CFM × 60 ÷ room volume in ft³. Metric ACH = airflow in m³/h ÷ room volume in m³. Required fan count is rounded up.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Use this calculator to convert airflow and room volume into air changes per hour, or calculate the airflow needed for a target ACH. It is a ventilation-planning tool and does not establish code compliance or contaminant control by itself.
Calculation steps
- 1Calculate room volume.
- 2Use measured or planned airflow in cubic feet or cubic metres per minute.
- 3Multiply airflow by 60 minutes.
- 4Divide by room volume to obtain ACH.
Worked numerical example
A 20 ft × 15 ft × 8 ft room contains 2,400 ft³. At 400 cfm, ACH = 400 × 60 ÷ 2,400 = 10 air changes per hour.
Purchasing and rounding
Fans are selected in available rated airflow sizes. Choose upward only after accounting for duct resistance, filters, controls and sound constraints.
Interpret the result
ACH describes dilution frequency, not how evenly air reaches every part of the room. Short-circuiting and poor mixing reduce practical effectiveness.
Page-specific assumptions
- Steady airflow.
- Well-mixed room air.
- Room volume is accurate.
- Fan rating is available at the actual system pressure.
Common mistakes
- Using free-air fan rating for a ducted system.
- Confusing ACH with cfm.
- Ignoring ceiling height.
- Claiming health or code compliance from ACH alone.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Air Changes per Hour Calculator questions
What does one ACH mean?+
One air change per hour means airflow equal to the room volume is supplied or removed each hour.
Does fan free-air rating equal installed airflow?+
Not always. Ducts, filters, grilles and pressure resistance can reduce actual airflow.
Can this determine a required ventilation rate?+
No. The target must come from the room use, occupancy, contaminant control and applicable standards.