Greenhouse Heater Calculator — Embeddable Widget

Calculate Your Greenhouse Heater Size

Greenhouse shape

Units

Length (ft)

Width (diameter) (ft)

A semicircular hoop house’s peak height is automatically half the width, about 10 ft here.

Glazing type

Heater fuel type

Desired inside temperature (°F)

Record low outside temperature (°F)

Some professionals use the ASHRAE 99% design temperature instead of the historical record low, since a record low can be an outlier. Record low is a fine, simple default for a home greenhouse.

The math:

Heat Loss (Q) = U-factor × Surface Area × ΔT

Recommended Heater Size = (Q × 1.15) ÷ (Fuel Efficiency ÷ 100)

Estimated heat loss

70,152

BTU/hr (20.56 kW)

Recommended heater size

100,843

BTU/hr (29.55 kW)

Includes safety factor + efficiency

Breakdown

Surface area: 1,822 sq ft
Temp. differential (ΔT): 55°F
U-factor: 0.7
Heat loss (U × A × ΔT): 70,152 BTU/hr
+ 15% safety factor: 80,675 BTU/hr
÷ 80% fuel efficiency: 100,843 BTU/hr

Estimate assumes a well-sealed structure. Add 10–15% in windy/exposed sites.

U-factor by glazing type
Glazing	U-factor (BTU/hr·ft²·°F)
Single-layer poly film	1.2
Double-layer inflated poly	0.7
Single-pane glass	1.1
Twin-wall polycarbonate	0.65
Triple-wall polycarbonate	0.55