Crawl Space Ventilation Calculator
Vent area or fan CFM for a crawl space.
Enter your values
Enter the crawl space area in sq ft.
Choose one of: Yes, No.
Enter the average height in ft.
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 net free area
- 0.67 sq ft
- In square inches
- 96
- Typical 8x16 vents needed
- 2
- Fan option (5 ACH)
- 250 CFM
- Crawl volume
- 3,000 cu ft
Quick answer
Traditional code asks for 1 square foot of net free vent area per 150 square feet of crawl space, or 1 per 1,500 with a vapour barrier.
NFA = area / 150 (or /1500 with ground cover)
At a glance
| What it does | Vent area or fan CFM for a crawl space. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Crawl space area, Vapour barrier, Average height |
| Main output | Required net free area |
| Formula | NFA = area / 150 (or /1500 with ground cover) |
| Cost | Free — no sign-up, no download |
How to use the Crawl Space Ventilation Calculator
- 1Measure the crawl space footprint and average height.
- 2State whether a class I vapour barrier covers the ground.
- 3Either fit vents totalling the net free area, or seal and fit a fan/dehumidifier.
Inputs explained
- Crawl space area(sq ft)
- Enter the crawl space area in sq ft.
- Vapour barrier
- Choose one of: Yes, No.
- Average height(ft)
- Enter the average height in ft.
Worked example
Using the values the calculator loads with:
Inputs
- Crawl space area1000 sq ft
- Vapour barrierYes
- Average height3 ft
Results
- Required net free area0.67 sq ft
- In square inches96
- Typical 8x16 vents needed2
- Fan option (5 ACH)250 CFM
- Crawl volume3,000 cu ft
Frequently asked questions
Vent or seal?
In humid climates sealing with a liner and dehumidifier usually outperforms venting.
What is net free area?
The open airflow area after louvres and insect screen, roughly 50 sq in for a typical 8x16 vent.
What do I need to enter into the Crawl Space Ventilation Calculator?
Just 3 values: crawl space area, vapour barrier, average height. 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 Crawl Space Ventilation Calculator work out the answer?
Traditional code asks for 1 square foot of net free vent area per 150 square feet of crawl space, or 1 per 1,500 with a vapour barrier. It applies the formula NFA = area / 150 (or /1500 with ground cover) and shows the working so you can check each step by hand.
Is the Crawl Space Ventilation 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.