Greenhouse Ventilation Calculator
Exhaust fan sizing for a greenhouse.
Enter your values
Enter the length in ft.
Enter the width in ft.
Enter the altitude in ft.
Choose one of: No shade cloth, 30% shade, 50% shade.
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.
- Summer exhaust
- 2,304 CFM
- Winter minimum (2 CFM/sq ft)
- 576 CFM
- Floor area
- 288 sq ft
- Intake louvre area
- 4.6 sq ft
- Suggested fans
- 1 unit(s)
Quick answer
A greenhouse typically needs 8-12 CFM per square foot of floor in summer, corrected for altitude and shading.
CFM = area x 8 x altitude factor / shade factor
At a glance
| What it does | Exhaust fan sizing for a greenhouse. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Length, Width, Altitude, Shading |
| Main output | Summer exhaust |
| Formula | CFM = area x 8 x altitude factor / shade factor |
| Cost | Free — no sign-up, no download |
How to use the Greenhouse Ventilation Calculator
- 1Measure the greenhouse footprint.
- 2Add altitude and any shade cloth already fitted.
- 3Size intake louvres at roughly 1.25 times the fan area so the fans are not starved.
Inputs explained
- Length(ft)
- Enter the length in ft.
- Width(ft)
- Enter the width in ft.
- Altitude(ft)
- Enter the altitude in ft.
- Shading
- Choose one of: No shade cloth, 30% shade, 50% shade.
Worked example
Using the values the calculator loads with:
Inputs
- Length24 ft
- Width12 ft
- Altitude0 ft
- ShadingNo shade cloth
Results
- Summer exhaust2,304 CFM
- Winter minimum (2 CFM/sq ft)576 CFM
- Floor area288 sq ft
- Intake louvre area4.6 sq ft
- Suggested fans1 unit(s)
Frequently asked questions
Two-speed fans?
Yes — a low speed near 2 CFM per sq ft handles spring and autumn without chilling plants.
Where do fans go?
Exhaust at one gable end, intake louvres at the opposite end for end-to-end sweep.
What do I need to enter into the Greenhouse Ventilation Calculator?
Just 4 values: length, width, altitude, shading. 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 Greenhouse Ventilation Calculator work out the answer?
A greenhouse typically needs 8-12 CFM per square foot of floor in summer, corrected for altitude and shading. It applies the formula CFM = area x 8 x altitude factor / shade factor and shows the working so you can check each step by hand.
Is the Greenhouse 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.