Greenhouse Heater Calculator
Size a greenhouse heater from its glazing area, glazing type, and the temperature difference to maintain inside.
How this calculator works
This calculator sizes a greenhouse heater from three inputs: your glazing area, the glazing’s U-factor (how much heat it loses), and the temperature difference between the coldest outdoor design temperature and the indoor temperature you want to maintain. Multiplying the three together gives the heater output needed in BTU per hour.
Formula: Heater size (BTU/hr) = glazing area × glazing U-factor × temperature difference (indoor minus outdoor design temp).
Worked example
- Glazing area: 200 sq ft
- Glazing type: single-layer poly, U = 1.2
- Temperature difference: 40°F
- Heater size: 200 × 1.2 × 40 = 9,600 BTU/hr
Notes
Glazing type is the single biggest factor in greenhouse heat loss. Switching from single-layer to double-layer poly roughly halves the required heater size for the same temperature difference, and the fuel savings often pay back the cost of the second layer within a season or two.
This calculator covers glazing heat loss only — the heat that escapes through the walls and roof. It doesn’t account for floor losses or air infiltration through gaps, doors, and vents, both of which add to the real-world heating requirement beyond this figure.
How to use
Measure your greenhouse’s total glazing area, select or enter your glazing’s U-factor, and set the temperature difference you need to maintain. The calculator returns the heater size in BTU per hour so you can shop for a unit rated to cover it.
A planning estimate covering glazing heat loss only. Floor losses, air infiltration, and wind exposure add to real-world heating needs — size with some margin.
Frequently asked questions
What size heater do I need for my greenhouse?
Multiply your glazing area by your glazing's U-factor and by the temperature difference you need to maintain between inside and outside. A 200 sq ft single-layer poly greenhouse holding a 40°F temperature difference needs about 9,600 BTU/hr, but your own numbers will differ based on glazing area, glazing type, and climate.
What's the difference between single-layer and double-layer poly glazing?
Single-layer polyethylene glazing has a U-factor around 1.2, while double-layer poly with an inflated air gap between the layers runs around 0.7 — roughly half the heat loss. The trapped air layer acts as insulation, so a double-poly greenhouse needs a smaller heater for the same temperature difference.
Does this calculator account for floor heat loss or air leaks?
No — this calculator covers glazing (wall and roof) heat loss only. Floor losses and air infiltration through gaps, doors, and vents add to the real-world heating requirement, so size your heater with some margin above the calculated glazing figure rather than treating it as the complete number.
How do I find my design temperature difference?
Subtract your local winter design outdoor temperature (a cold night your area regularly sees, not the record low) from the minimum indoor temperature your plants need. That difference is what your heater has to maintain continuously, and it's the number this calculator multiplies against your glazing area and U-factor.
Estimates only. Verify quantities with your supplier before purchasing.