Basement Ventilation CFM Calculator
Fan size for a basement or cellar.
Enter your values
Enter the floor area in sq ft.
Enter the ceiling height in ft.
Choose one of: Storage (3 ACH), Finished living (5 ACH), Workshop (8 ACH), Damp/radon (10 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 fan
- 563 CFM
- With 20% duct allowance
- 675 CFM
- Air changes per hour
- 5
- Volume
- 6,750 cu ft
- In L/s
- 265
Quick answer
Basements typically need 3-6 air changes per hour, more if there is a workshop, radon risk or damp.
CFM = area x height x ACH / 60
At a glance
| What it does | Fan size for a basement or cellar. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Floor area, Ceiling height, Use |
| Main output | Required fan |
| Formula | CFM = area x height x ACH / 60 |
| Cost | Free — no sign-up, no download |
How to use the Basement Ventilation CFM Calculator
- 1Measure the basement area and ceiling height.
- 2Pick the use case that matches how the space is occupied.
- 3Add makeup air by cracking a window or fitting a passive inlet on the far side.
Inputs explained
- Floor area(sq ft)
- Enter the floor area in sq ft.
- Ceiling height(ft)
- Enter the ceiling height in ft.
- Use
- Choose one of: Storage (3 ACH), Finished living (5 ACH), Workshop (8 ACH), Damp/radon (10 ACH).
Worked example
Using the values the calculator loads with:
Inputs
- Floor area900 sq ft
- Ceiling height7.5 ft
- UseFinished living (5 ACH)
Results
- Required fan563 CFM
- With 20% duct allowance675 CFM
- Air changes per hour5
- Volume6,750 cu ft
- In L/s265
Frequently asked questions
Exhaust or supply?
Exhaust works for damp and fumes; supply slightly pressurises and keeps radon out better.
Continuous or intermittent?
Continuous at low speed controls humidity far better than short bursts.
What do I need to enter into the Basement Ventilation CFM Calculator?
Just 3 values: floor area, ceiling height, use. 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 Basement Ventilation CFM Calculator work out the answer?
Basements typically need 3-6 air changes per hour, more if there is a workshop, radon risk or damp. It applies the formula CFM = area x height x ACH / 60 and shows the working so you can check each step by hand.
Is the Basement Ventilation 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.