Calculating How Much Heat Is Needed for Your Greenhouse Properly heating your greenhouse is important as the warmer weather of autumn gives way to the cooler climate of winter.

But if your goal is to heat it efficiently, you’ll want to take these steps to more accurately determine how many BTUs are required to maintain the temperature you want.

To make your calculations, you’ll want to begin with precise measurements. Measuring tape, a calculator, and pencil and paper are requirements to gather this information. But depending the size of your greenhouse, you may also need a ladder or some additional help.

Record the following measurements to help you determine the total square feet of your greenhouse’s surface area:

• Measurement 1: Height of the greenhouse sidewall from the ground to the lowest roof point.
• Measurement 2: Length of the longest greenhouse wall.
• Measurement 3: Length of the shortest greenhouse wall.
• Measurement 4: Height from the ground to the highest point of the greenhouse roof.
• Measurement 5: Length from the highest to lowest roof point along one eave.

To calculate the total surface area, add measurement 1 and 5 and multiply the total times 2, then add the number from measurement 2. Then add measurement 1 and 4, and multiply times measurement 3. Add these two numbers together, and you have the total surface area in square feet.

Next, record the desired temperature in Fahrenheit that you wish to maintain in your greenhouse. Subtract the coldest temperature your greenhouse experiences from this number—or add the two if it is a negative reading.

Make note of the heat loss value of your greenhouse covering. With these three calculations, you can determine the number of BTUs required to heat your greenhouse by multiplying the square feet by the temperature difference by the heat loss value. Or you can apply your measurements to our online greenhouse BTU calculator.

When shopping for a greenhouse heater, keep this number in mind. Purchase a unit that meets this minimum BTU output, but not much higher than what your calculations have suggested.