CFM Calculator
Airflow in cubic feet per minute from room size and air changes.
Enter your values
Enter the room length in ft.
Enter the room width in ft.
Enter the ceiling height in ft.
Enter the air changes per hour in ACH.
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.
- Required airflow
- 224 CFM
- Room volume
- 2,240 cu ft
- In m³/h
- 381 m³/h
- In L/s
- 106 L/s
- Minutes per full air change
- 10
Working
- 1volume 2,240 cu ft x 6 ACH / 60 = 224 CFM
Quick answer
CFM is room volume in cubic feet multiplied by air changes per hour, divided by 60.
CFM = (length x width x height x ACH) / 60
At a glance
| What it does | Airflow in cubic feet per minute from room size and air changes. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Room length, Room width, Ceiling height, Air changes per hour |
| Main output | Required airflow |
| Formula | CFM = (length x width x height x ACH) / 60 |
| Cost | Free — no sign-up, no download |
How to use the CFM Calculator
- 1Measure the room length, width and ceiling height in feet.
- 2Pick the air changes per hour your space needs — 6 ACH suits a living space, 8–15 a kitchen or workshop.
- 3Size the fan or duct for at least the CFM shown, then add 10–20% for duct and filter losses.
Inputs explained
- Room length(ft)
- Enter the room length in ft.
- Room width(ft)
- Enter the room width in ft.
- Ceiling height(ft)
- Enter the ceiling height in ft.
- Air changes per hour(ACH)
- Enter the air changes per hour in ACH.
Worked example
Using the values the calculator loads with:
Inputs
- Room length20 ft
- Room width14 ft
- Ceiling height8 ft
- Air changes per hour6 ACH
Results
- Required airflow224 CFM
- Room volume2,240 cu ft
- In m³/h381 m³/h
- In L/s106 L/s
- Minutes per full air change10
- volume 2,240 cu ft x 6 ACH / 60 = 224 CFM
Frequently asked questions
What is a good CFM for a room?
Most habitable rooms are comfortable at 4–6 air changes per hour, which for a 20 x 14 ft room with 8 ft ceilings is roughly 190–280 CFM.
Does duct length change the CFM I need?
It does not change the air the room needs, but long or restrictive duct runs cut the fan's delivered CFM, so oversize the fan by 10–25%.
What do I need to enter into the CFM Calculator?
Just 4 values: room length, room width, ceiling height, air changes per hour. 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 CFM Calculator work out the answer?
CFM is room volume in cubic feet multiplied by air changes per hour, divided by 60. It applies the formula CFM = (length x width x height x ACH) / 60 and shows the working so you can check each step by hand.
Is the 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.