Free greenhouse heating calculator: enter length, width, wall height, glazing and temperatures to get surface area, BTU per hour, kilowatts and heater size.
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Enter the greenhouse length, width and wall height in feet, pick the glazing type, and enter the inside temperature you want to hold and the coldest outside temperature you expect. The calculator returns the glazed surface area, the heating load in BTU per hour and kilowatts, and a recommended heater size rounded up to the next 5,000 BTU. Use the coldest night on record for your area, not the average winter low, because the heater must cope with the worst night. This is a planning estimate: draughty structures, wind exposure and heater efficiency all shift the real figure.
Heat escapes through the covering, so the formula uses surface area, not floor area. Glazed surface area = 2 × (length × wall height + width × wall height) + length × width, which covers the two side walls, the two end walls, and a flat roof allowance. Heating load in BTU per hour = surface area × U-value × temperature difference, where temperature difference is inside temperature minus outside temperature. Kilowatts = BTU per hour / 3412.
Worked example: a 10 ft by 8 ft greenhouse with 8 ft walls has a surface area of 368 square feet (2 × (80 + 64) + 80). With double poly glazing at U 0.7 and a 30 degree temperature difference (60 inside, 30 outside), the load is 368 × 0.7 × 30 = 7,728 BTU per hour, or 2.26 kW, so the recommended heater size rounds up to 10,000 BTU per hour. A second example: a 12 ft by 10 ft greenhouse with 8 ft walls and single poly at U 1.2, holding 55 degrees against 15 outside, has 472 square feet and needs 472 × 1.2 × 40 = 22,656 BTU per hour, or 6.64 kW, rounding up to a 25,000 BTU heater.
The U-value measures how fast heat passes through the covering. The lower the number, the better the insulation. Values vary by source and by product, so treat these planning values as typical, and check your own glazing datasheet where it matters.
| Glazing | U-value used here | Notes | |---|---|---| | Single layer poly film | 1.2 | Cheapest, highest heat loss | | Single pane glass | 1.1 | Similar to single poly | | Double layer inflated poly | 0.7 | About 40 percent less loss than single poly | | 8 mm twin-wall polycarbonate | 0.6 | Good balance of light and insulation | | 16 mm triple-wall polycarbonate | 0.4 | Best insulation of the common options |
The same material gets different U-values across sources: single poly appears as 1.05, 1.15 and 1.2, and double poly as 0.62 and 0.70. That spread is normal, because U-values depend on assembly, air gaps and wind. If your supplier quotes a different U-value, the formula still works: multiply your surface area by their value and your temperature difference.
Measure length, width and wall height in feet. If your greenhouse has a pitched roof, the flat-roof allowance in this calculator slightly understates the true glazed area, so treat the result as a minimum and size up one step for a tall gable roof. Pick the glazing closest to your own covering. Enter the inside night temperature your crop needs, and the coldest outside temperature you realistically expect, not the average winter low.
Read the heating load for the physics, and the recommended heater size for shopping: heaters come in fixed sizes, so always size up to the next model. For gas and oil heaters, remember efficiency: a heater rated at 80 percent efficiency must be fed more fuel energy than the heat it delivers, so divide the load by 0.8 to get the input rating, as one manufacturer guide does when it turns 175,840 BTU of load into a 219,800 BTU input requirement.
Growers search for the same sizes again and again, so here are the numbers this formula gives for them, all at a 55 degree temperature difference with double poly glazing at U 0.7, using an 8 ft wall height and the flat-roof allowance.
| Greenhouse | Surface area | Load at 55 degree difference | |---|---|---| | 8 × 10 ft | 368 sq ft | 14,168 BTU per hour | | 8 × 12 ft | 416 sq ft | 16,016 BTU per hour | | 10 × 12 ft | 408 sq ft | 15,708 BTU per hour | | 10 × 20 ft | 640 sq ft | 24,640 BTU per hour |
Switching the same 10 × 12 ft house from single poly to double poly cuts the load by about 40 percent, which is why a second layer of plastic with an inflation blower is often cheaper than a bigger heater. Adding bubble wrap inside single glazing roughly halves the U-value and the load with it.
Many growers ask how many BTU are needed to heat a greenhouse, and greenhouse heating requirements always start from surface area, U-value and delta T. The common searches, what size heater for an 8x10 greenhouse, what size heater for an 8x12 greenhouse, what size heater for a 10x12 greenhouse, and what size heater for a 10x20 greenhouse, are all answered in the table above. For glazing choice, double poly vs single poly BTU comparisons show about a 40 percent saving, and polycarbonate vs glass greenhouse heating comparisons favour polycarbonate for insulation. For units, greenhouse heater kW to BTU conversion is simple: multiply kilowatts by 3412 to get BTU per hour.
The formula above covers conduction through the covering. It does not cover cold air leaking through gaps, vents, doors and fan openings. On a windy night, infiltration can add 25 to 50 percent to the conductive load depending on how tight the structure is. A common shortcut is to multiply the conductive loss by 1.0 to 1.1 for a tight double-poly house with sealed joints, 1.25 for an average single-poly house, and 1.4 to 1.5 for an older glass house with loose laps and worn weatherstripping.
Add a safety margin of 10 to 20 percent for unexpected cold snaps, and consider a backup heater: a small electric heater on a thermostat set a few degrees below the main heater target can save a crop on the night the main heater fails. Sealing poly overlaps with greenhouse tape, fitting inlet baffles on exhaust-fan shutters, and weather-stripping doors can cut 10 to 15 percent off the heating bill without touching the covering or the furnace.
Electric heaters are the simplest to install and need no ventilation, but they are often the most expensive to run, and standard 120 volt outlets typically support heaters only up to about 5,100 BTU. High-output electric heat usually needs a dedicated 240 volt circuit fitted by an electrician. Propane and natural gas heaters pack a much bigger punch and are generally cheaper to run for high BTU needs, but any unvented combustion heater releases carbon monoxide and excess moisture, so vented models are a must for a serious setup.
For a rough fuel cost, take a published illustration: a 248,124 BTU per hour load on propane at 2.50 dollars per gallon, with 91,500 BTU per gallon at 80 percent efficiency, burns about 3.4 gallons per hour at full output, or about 8.50 dollars per hour, which is about 85 dollars over a 10-hour cold night. Your own cost follows the same maths with your fuel price and your load. Lowering the target inside temperature by just five degrees creates a large drop in heating capacity needed, so only heat to the minimum your crop actually requires.
Multiply glazed surface area by the glazing U-value by the temperature difference: BTU per hour = area × U × delta T. Surface area = 2 × (length × wall height + width × wall height) + length × width. This calculator does the whole chain and converts to kilowatts.
Planning values: single poly 1.2, single glass 1.1, double inflated poly 0.7, 8 mm twin-wall polycarbonate 0.6, 16 mm triple-wall polycarbonate 0.4. Sources vary (single poly appears as 1.05 to 1.2), so use your supplier datasheet where it matters.
It depends on the crop. Frost-free houses hold just above freezing, cool houses 40 to 45 degrees Fahrenheit, warm houses about 55 degrees, and hothouses for tropical plants about 65 degrees at night. Only heat to the minimum your crop needs.
Use the coldest night you realistically expect, ideally the historical record low for your area. Sizing to the average leaves the heater short on the one night that matters, and that night is when crops are lost.
Cold air leaking through gaps, vents and doors can add 25 to 50 percent to the conductive load on a windy night. Multiply the conductive loss by 1.0 to 1.1 for a tight house, 1.25 for an average house, and 1.4 to 1.5 for an older draughty glass house.
Add 10 to 20 percent to the calculated load for unexpected cold snaps, then size up to the next heater model. Heaters come in fixed sizes, so the recommended size in this calculator already rounds up to the next 5,000 BTU.
One correctly sized heater suits most hobby houses. Larger houses benefit from two heaters: heat spreads more evenly, cold spots shrink, and a second unit provides backup if one fails on the coldest night.
Seal gaps and weather-strip doors, add a second poly layer or bubble wrap, insulate the solid north wall, plant a windbreak, and lower the night setpoint to the crop minimum. Each step cuts the load before you buy a bigger heater.
Divide BTU per hour by 3,412. For example, 7,728 BTU per hour is 2.26 kW, and 248,124 BTU per hour is about 73 kW. This calculator shows both figures for every result.
Yes, through surface area. A quonset or hoop house has a different surface-area-to-volume ratio from a gable house of the same floor size, and tall centre ridges add glazed area that a floor-area shortcut misses entirely. Always calculate from the envelope, never from floor area alone.