Greenhouse Heater Size Calculator
Surface Area of Structure - This is the total square feet of exposed surface area (this is not length x width) your structure has (don't include floor). To find out the surface area of a greenhouse we sell, click here. To find the surface area of a different greenhouse, click here to use our greenhouse area calculators.
Minimum Outside Temperature - You will want to enter the lowest temperature expected for your area. Not sure? Use the USDA Zone Map to find the average minimum temperature for your area (Use the lower of the 2 numbers in the Temp (F) column).
Inside Temperature - This is the minimum temperature you would want to maintain in your greenhouse when heating.
Heat Loss Value - Check the list below to find the heat loss value for the covering on your greenhouse. Some values may vary with manufacturer. If you know the R-value of your covering, you can convert it to a heat loss value using this formula: Heat loss value = 1 / R-value.
Area x (In - Out) x Heat Loss
Minimum BTU Needed - This is the minimum amount of BTU output the heater you use should have. If the heater only has a BTU input rating, use the following formula. BTU output = heater efficiency * BTU input. View Our Selection of Greenhouse Heaters

Greenhouse Covering Heat Loss Values
4 mil polyethylene
1.20
4 mm (5/32") twinwall polycarbonate
.70
6 mil polyethylene
1.15
6 mm (1/4") twinwall polycarbonate
.62
6 mil poly double layer (inflated)
.70
6mm polycarbonate roof & glass walls
.90
11 mil woven polyethylene
1.05
8 mm (5/16") twinwall polycarbonate
.58
3 mm (1/8") glass (single layer)
1.13
10mm (3/8") twinwall polycarbonate
.53
Double layer insulated glass
.45
10 mm (3/8") triple wall polycarbonate
.48
Polycarbonate / fiberglass (single layer)
1.20
16 mm (5/8") 5 wall polycarbonate
.33

Greenhouse Exposed Surface Area
Solar Harvest Greenhouses
SH7 216 SH7ext 80
Grow More Greenhouses
GM8 (8' x 11') 333 GM8ext (8' x 5') 104
GM10 (10' x 13') 442 GM10ext (10' x 6.5') 149
GM13 (13' x 13) 557 GM13ext (13' x 6.5') 175
GM16 (16' x 13') 681 GM16ext (16' x 6.5') 202
Sunshine Greenhouses
GKP64 - 6' x 4' 195 GKP812 - 8' x 12' 462
GKP68 - 6' x 8' 273 GKP816 - 8' x 16' 555
GKP612 - 6' x 12' 352
Cross Country Greenhouses
Because of the large number of sizes available, you will need to use our surface area calculator
to find the total surface area for the model you are interested in.