ACH to CFM Calculator
Fan size from the air changes you want.
Enter your values
Enter the room volume in cu ft.
Enter the target air changes in ACH.
Enter the duct loss allowance in %.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Fan size from the air changes you want. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Room volume, Target air changes, Duct loss allowance |
| Main output | Fan size to buy |
| Formula | CFM = volume x ACH / 60 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Room volume(cu ft)
- Enter the room volume in cu ft.
- Target air changes(ACH)
- Enter the target air changes in ACH.
- Duct loss allowance(%)
- Enter the duct loss allowance in %.
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.
What do I need to enter into the ACH to CFM Calculator?
Just 3 values: room volume, target air changes, duct loss allowance. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the ACH to CFM Calculator work out the answer?
Required CFM equals room volume times ACH divided by 60. It applies the formula CFM = volume x ACH / 60 and shows the working so you can check each step by hand.
Is the ACH to CFM Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.