ACH to CFM Calculator
Fan size from the air changes you want.
Enter your values
ⓘ Live results
Results
- Fan size to buy
- 343 CFM
- Theoretical requirement
- 299 CFM
- In m³/h
- 584
- In L/s
- 162.1
Working
- 12,240 cu ft x 8 / 60 = 299 CFM, +15% = 343 CFM
Quick answer
Required CFM equals room volume times ACH divided by 60.
CFM = volume x ACH / 60
How to use the ACH to CFM Calculator
- 1Work out the room volume in cubic feet.
- 2Choose the ACH target for that room type.
- 3Add a duct loss allowance — 10% for short runs, 25% with filters and long flex.
Worked example
Using the values the calculator loads with:
Inputs
- Room volume2240 cu ft
- Target air changes8 ACH
- Duct loss allowance15 %
Results
- Fan size to buy343 CFM
- Theoretical requirement299 CFM
- In m³/h584
- In L/s162.1
- 2,240 cu ft x 8 / 60 = 299 CFM, +15% = 343 CFM
Frequently asked questions
Should I buy exactly the calculated CFM?
Buy the next size up and use a speed controller; a fan running slower is quieter than a small fan at full tilt.
Do carbon filters change it?
Yes — a carbon filter can cut delivered airflow by 20–25%, so include it in the loss allowance.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.