Greenhouse heater sizing calculator
PROBTU/hr to hold greenhouse temperature on the coldest night.
Short answer: BTU/hr = surface × ΔT × U-value. 400 sq ft × 40°F × 1.1 = 17,600 BTU/hr. Enter your measurements above and the greenhouse heater sizing calculator returns the quantity, unit conversions and an estimated cost instantly, in imperial or metric — no sign-up needed.
What do you need to measure?
What is the answer?
BTU/hr required
17,600
Watts (electric)
5,158
How does the greenhouse heater sizing calculator work?
BTU/hr = surface × ΔT × U-value.
What does a worked example look like?
400 sq ft × 40°F × 1.1 = 17,600 BTU/hr.
What do people ask most?
›Add a safety margin?
Add 20% for wind chill and infiltration.
Which garden calculator should you use next?
Where can you learn more?
Read field notes on estimating and materials, or browse every calculator we make.