Heater BTU Calculator

Find the exact heat output to hold your target temperature.

Climate45 secAnswers: “greenhouse heater size / BTU”

Estimate conductive heat loss for a rectangular enclosure. Air leakage, wind and solar gains are not included; this is a planning estimate, not a heater-sizing guarantee.

Length ft Width ft Height ft

Target inside °F Coldest outside °F

Glazing

ƒ output = surface area × ΔT × U-factor