How to use this calculator
- Enter or choose units.
- Enter or choose length.
- Enter or choose width.
- Enter or choose side-wall (eave) height.
- Enter or choose ridge height.
- The required airflow updates as you type. Use Copy result or Share to keep it.
Method
Volume = Length × Width × (Eave height + (Ridge − Eave) ÷ 2); Airflow = Volume × Air changes per hour; Fans = Airflow ÷ Fan capacity, rounded up
- Air changes per hour
- how many times the house air is replaced each hour
- 1 m³
- 35.3147 ft³
Worked example
- Volume = 30 × 9 × (3 + 2 ÷ 2) = 1080 m³.
- Airflow = 1080 × 60 = 64,800 m³/h (38,140 CFM).
- Fans = 64,800 ÷ 15,000 = 4.32, so 5 fans.
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.