# Greenhouse Heating Load Calculator

> Estimate the heater size a greenhouse needs from its covering area, U-value, the inside–outside temperature difference and air leakage.

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

Heat loss is the covering area × U-value × temperature difference, plus the heat carried out by leaking air. Size the heater for the coldest night you plan for.

Also called: Greenhouse heater size calculator, Greenhouse BTU calculator.

## Inputs
- Units (options: Metric (m, °C, W/m²K), US (ft, °F, BTU/h·ft²·°F); default metric)
- Length (default 30)
- Width (default 9)
- Side-wall (eave) height (default 3)
- Ridge height (default 5)
- U-value of the covering (default 4)
- Inside temperature to hold (default 15)
- Coldest outside temperature (default -5)
- Air leakage (unit air changes/h; default 1)

## Results
- Heating load
- Heating load
- Loss through the covering
- Loss by air leakage
- Covering area
- Greenhouse volume

## Formula

```
Q = U × A × ΔT + 1206 J/(m³·K) × V × ACH × ΔT ÷ 3600;  A = walls + gable ends + roof
```

Where:
- U: heat transfer coefficient of the covering
- A: covering area
- ΔT: inside minus outside temperature
- V: house volume
- ACH: air changes per hour; air holds 1.2 kg/m³ × 1005 J/(kg·K)

## Worked example
1. Area = walls 2 × 39 × 3 = 234 m² + gables 9 × 2 = 18 m² + roof 2 × 30 × √(4.5² + 2²) = 295.47 m² → 547.47 m².
2. Covering loss = 4 × 547.47 × 20 = 43.80 kW.
3. Leakage = 1206 × 1080 × 1 × 20 ÷ 3600 = 7.24 kW; total 51.03 kW (174,133 BTU/h).

**Result:** Holding 15 °C at −5 °C outside needs about 51 kW (174,000 BTU/h) of heat for this 30 × 9 m house.

## What the result means

Choose a heater with output at or above the load (allowing for its efficiency), and consider a second skin or thermal screen to cut the covering loss.

## Assumptions and limits
- Steady-state loss at the design temperature; no solar gain.
- The floor and soil heat exchange is ignored.
- Wind, radiation to a clear night sky and thermal screens change the real load.

## Questions

### Why does the covering dominate?

Glazing and films conduct heat far more readily than insulated walls, so area × U-value is usually most of the load.

### How do I convert kW to BTU/h?

Multiply by 3,412.14.

## Related calculators
- [Greenhouse Ventilation Calculator](https://calculators.geekmidst.com/calculators/greenhouse-ventilation-calculator)
- [Temperature Converter](https://calculators.geekmidst.com/calculators/temperature-converter)

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