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The total area of the covering — roof and walls — not the floor.
Heat leaves through the covering, so this is the surface it crosses: roof plus sidewalls plus both end walls. THIS FIELD ASKED FOR THE FLOOR AREA UNTIL 2026-09-02, which undersized every result, because the factors below are per square foot of covering. For a hoop house, the arc length across the hoop times the length, plus the two ends; for a gable house, the two roof slopes plus two sidewalls plus two gable ends. The covering runs to roughly twice the footprint on a compact hoop and three times or more on a gable house with tall sidewalls, so if you only have the footprint, that is the scaling to apply — and measure it properly before ordering a heater.
Double-layer poly traps an insulating air gap, losing much less heat than single-layer film or glass.
An inflation blower keeps the air gap between the two poly layers, which is what gives double-poly its efficiency advantage.
The difference between your desired inside temperature and the coldest expected outside temperature.
E.g. a 60°F target minus a 20°F winter low gives a 40°F difference.
Recommended heater size (BTU/hr)
20,600 BTU/hr
This estimates glazing heat loss only — wind exposure, air leaks around vents and doors, and the greenhouse's surface-area-to-volume ratio (smaller structures lose heat faster per sq ft) can all push real requirements higher.
- Covering area
- 430 sq ft
- Same output in kW
- 6.05 kW
They open the calculator with your figures already in it
Greenhouse Heating BTU Calculator: 20,638 BTU/hr — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Commonly cited greenhouse heat-loss factors (BTU/hr per sq ft per °F of temperature difference): single-layer poly film ~1.2, double-layer inflated poly ~0.7, glass ~1.1 — air leakage, wind exposure and glazing condition change these materially, and a U-value for the glazing actually installed governs
Inputs used
- Covering Area (glazed surface)
- 430 sq ft
- Glazing Type
- Single-layer poly film
- Temperature Difference
- 40 °F
Intermediate steps
- Covering area
- 430 sq ft
- Same output in kW
- 6.05 kW
Confidence note: This estimates glazing heat loss only — wind exposure, air leaks around vents and doors, and the greenhouse's surface-area-to-volume ratio (smaller structures lose heat faster per sq ft) can all push real requirements higher.
What this calculation does not cover
- Small houses lose proportionally more than this model suggests, because heat loss scales with surface area while the useful growing volume scales with the cube of the dimension. A propagation frame therefore needs more heat per square metre of glazing than a commercial span, and no surface-to-volume penalty is applied here.
- The covering area is taken as typed and nothing checks it against the structure you actually have. This limitation used to say the covering area 'never enters the calculation', which was true of an earlier floor-area version of this page and is now the opposite of the truth — the glazed surface IS the primary input. What remains is that you have to work that area out yourself, from the sidewalls, the gable ends and the developed length of the roof or arch, and it is always larger than the footprint. Two greenhouses on the same slab, a 1.2 m (4 ft) cold frame and a walk-in with 2.4 m (8 ft) sidewalls, are different numbers here only because you enter different areas.
- Air leakage is excluded. Vent louvres, roll-up sides, door gaps, torn film and an unsealed base skirt admit a separate infiltration load on top of conduction through the covering, and on a loosely sealed or ageing structure that load is a large share of the total.
- Site conditions are excluded: wind speed and exposure, which strip heat from a thin covering far faster than still air; perimeter and ground losses at the foundation; and shelter from buildings, walls or hedging. An exposed field and a sheltered walled garden get the same answer.
- Only three coverings are offered. Thermal or blackout screens drawn at night, a bubble-wrap liner, an insulated north wall, twin-wall polycarbonate, double-wall acrylic, or poly overlaid on old glass cannot be represented by picking one of the listed options, and each of those changes the loss rate materially.
- This is a heat-loss rate, not a heater specification. It carries no allowance for burner efficiency, altitude derate, pull-down after the doors have been open, or the fan and thermostat control needed to hold an even temperature end to end. Any fired heater must still be sized, vented and supplied with combustion air to the manufacturer's data and the applicable fuel-gas code.
Add the equipment this sizes
This result is a specification — 20,600 BTU/hr — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Commonly cited greenhouse heat-loss factors (BTU/hr per sq ft per °F of temperature difference): single-layer poly film ~1.2, double-layer inflated poly ~0.7, glass ~1.1 — air leakage, wind exposure and glazing condition change these materially, and a U-value for the glazing actually installed governs
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