# Greenhouse Ventilation Calculator

> Size greenhouse exhaust fans from the house volume and the air changes per hour you want, in m³/h and CFM, and count how many fans you need.

Category: Agriculture · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/greenhouse-ventilation-calculator

Required airflow is the greenhouse volume times the air changes per hour. Divide it by one fan’s rated capacity to count fans.

Also called: Greenhouse fan calculator, Greenhouse CFM calculator.

## Inputs
- Units (options: Metres (m³/h), Feet (CFM); default metric)
- Length (default 30)
- Width (default 9)
- Side-wall (eave) height (default 3)
- Ridge height (default 5)
- Air changes per hour (default 60)
- Capacity of one fan (optional) (default 15000; optional)

## Results
- Required airflow
- Required airflow
- Required airflow
- Greenhouse volume
- Greenhouse volume
- Fans needed

## Formula

```
Volume = Length × Width × (Eave height + (Ridge − Eave) ÷ 2);  Airflow = Volume × Air changes per hour;  Fans = Airflow ÷ Fan capacity, rounded up
```

Where:
- Air changes per hour: how many times the house air is replaced each hour
- 1 m³: 35.3147 ft³

## Worked example
1. Volume = 30 × 9 × (3 + 2 ÷ 2) = 1080 m³.
2. Airflow = 1080 × 60 = 64,800 m³/h (38,140 CFM).
3. Fans = 64,800 ÷ 15,000 = 4.32, so 5 fans.

**Result:** A 30 × 9 m gable greenhouse needs 64,800 m³/h for one air change per minute: five 15,000 m³/h fans.

## What the result means

Fan ratings fall as static pressure rises, so use the capacity at the pressure your installation will run at (often 0.1 inch of water or about 25 Pa).

## Assumptions and limits
- A simple gable roof; for a flat roof enter the same eave and ridge height.
- The air change rate is your input; the calculator does not model solar heat gain or evaporative cooling.

## Questions

### How do I size for a hoop house?

Use the average height (roughly two-thirds of the peak for a semicircular hoop) as both eave and ridge height.

### Why use the volume and not the floor area?

Air changes are defined on volume. Some guides use airflow per floor area instead; both lead to similar fan sizes for typical heights.

## Related calculators
- [Greenhouse Heating Load Calculator](https://calculators.geekmidst.com/calculators/greenhouse-heating-load-calculator)
- [Compost Volume Calculator](https://calculators.geekmidst.com/calculators/compost-volume-calculator)

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/greenhouse-ventilation-calculator