Greenhouse Heater Sizing Calculator
Estimate steady-state heat loss for a simple rectangular gable greenhouse, with each assumption visible.
Your planning estimate
Enable JavaScript to run this calculator. The formula and assumptions are available below.
How this tool works
Uses the conduction, infiltration and perimeter method described in UGA Bulletin 792. A is the roof-and-wall envelope, excluding the floor; V is enclosed air volume. Geometry assumes a symmetric straight-sided gable with vertical walls. ΔT is zero when the outside temperature is at or above the target.
Q = A × U × ΔT + 0.02 × V × ACH × ΔT + P × perimeter × ΔT. Thermal kW = BTU/h ÷ 3412.142.
What this estimate does not tell you
This is not an equipment selection, circuit sizing or fuel-system design. It excludes warm-up recovery, humidity-control loads, extreme leakage, thermal curtains and detailed thermal bridges. It takes no solar gain credit. It is not suitable for bell, hoop, arched or lean-to geometry without a separate area model. Thermal output is not necessarily electrical input or burner input.
Sources & verification
Manufacturer figures are reproduced as documented. Editorial comparisons and calculations are identified separately.
- University of Georgia Extension — Greenhouses: Heating, Ventilation and Cooling (B792) ↗ — checked 2026-09-19.
Source pages can change. The supplier’s current exact-model documentation and local requirements control final decisions. Report a correction.