Greenhouse Heater Calculator
Size greenhouse heat in BTU/h and watts from volume and rise.
What it does
A greenhouse heater fights the night sky. Glass and poly shed heat fast, so holding 20 F above outside through a still night takes steady watts, not a small space heater on high.
The tool above multiplies inside volume by temp rise with a 1.1 glazing factor. A 10 by 8 by 7 ft house holding 20 F needs 12,320 BTU/h, or 3,611 W, which is 3 units of 1500 W. The metric match is 3 by 2.4 by 2.1 m holding 11 C for 11,630 BTU/h.
Rise is the judgment call. Seedlings may want 15 F above a cold snap, while tropical pots may want 25 F. Each added degree adds the same block of watts, so pick the coldest night to survive, not the average.
Volume explains why tall houses cost more. A 7 ft ridge holds far more air than a 5 ft cold frame on the same footprint, and that air must all stay warm.
Sealing beats buying. Weatherstrip laps, skirt the base, and close ridge vents before upsizing heaters, since wind leaks can exceed the glazing loss the factor covers.
How to use
- Pick imperial (ft, F) or metric (m, C).
- Enter length, width, and height inside.
- Enter temp rise to hold.
- Read BTU/h first, then watts.
- Read 1500 W unit count.
- Seal gaps before buying the last unit.
Formula
- volume
- Inside volume from length, width, height.
- rise
- Inside minus outside temp.
- 1.1
- Glazing and leak allowance.
Rise is F in imperial mode and C in metric mode, converted at 1.8 F per C. Volume converts at 35.3147 ft3 per m3, so both modes share one band.
Worked example
Example: 10 by 8 by 7 ft, 20 F rise
Imperial defaults: 10 by 8 by 7 ft with a 20 F rise. These are also the default values, so the page opens on this answer.
- Size: 10 x 8 x 7 ft, so volume is 10 x 8 x 7 = 560 ft3.
- Rise: 20 F above outside.
- Glazing factor: 1.1, so BTU is 560 x 20 x 1.1 = 12,320 BTU/h.
- Watts: 12,320 x 0.293071 = 3,611 W.
- Units: ceil(3611 / 1500) = 3 heaters.
- Tool shows 12,320 BTU/h, 3,611 W, 3 heaters (1500 W each).
Reference table
| House | Rise | BTU/h | Watts |
|---|---|---|---|
| 8 by 6 by 6 ft | 15 F | 4,752 BTU/h | 1,393 W |
| 10 by 8 by 7 ft | 20 F | 12,320 BTU/h | 3,611 W |
| 3 by 2.4 by 2.1 m | 11 C | 11,630 BTU/h | 3,408 W |
| 4 by 3 by 2.2 m | 10 C | 18,459 BTU/h | 5,410 W |
Limits
- Shell heat only. Sun, mats, and lights add separately.
- 1.1 fits average glazing. Single glass in wind needs more.
- Ignores floor loss to cold soil.
- Thermostat swing and heater placement shift plant level temps.
Common mistakes
- Sizing to average nights, then freezing the cold snap.
- Heating a leaky house with gaps at laps and doors.
- Placing one heater far from plants and reading wall temps.
- Forgetting a backup thermostat on tender crops.
- Running extension cords past rated watts.
Try it yourself
Quick checks. Answers sit tucked under each question.
8 by 6 by 6 ft, 15 F rise?
Volume is 8 x 6 x 6 = 288 ft3. BTU is 288 x 15 x 1.1 = 4,752 BTU/h. Watts are 4752 x 0.293071 = 1,393 W, so 1 heater.
4 by 3 by 2.2 m, 10 C rise: BTU?
Volume is 4 x 3 x 2.2 = 26.4 m3, or 932.3 ft3. Rise is 10 x 1.8 = 18 F. BTU is 932.3 x 18 x 1.1 = 18,459 BTU/h.
Questions
How many BTU for my greenhouse?
Multiply volume by rise by 1.1. A 560 ft3 house with 20 F rise needs 560 x 20 x 1.1 = 12,320 BTU/h.
How many watts is that?
Multiply BTU by 0.293071. Here 12,320 x 0.293071 = 3,611 W.
How many 1500 W heaters?
Divide watts by 1500 and round up. Here ceil(3611 / 1500) = 3 units.
Does glazing change it?
Single glass and leaky laps lose more than twin poly. Seal gaps first, then trust the 1.1 factor as a base case.
Does wind matter?
Yes. Wind strips heat from small houses fast. Sheltered walls and skirting cut the real load below the math.
Does sun gain count?
No. Seed mats, lights, and sun gain all add heat outside this shell math.