How to Calculate Heater Size. To heat an area like an outbuilding or new building, you must calculate what size heater you need. Once you make your calculations, you can decide on the best choice for a type of heater and how many BTUs (British Thermal Unit, the standard heat measurement) you need. Follow these steps to calculate heater size.

Step 1

Measure the length, width and height of each room you want to heat. Multiply the length of a room by the width and the height to learn the total cubic feet of area. A room that is 10 feet long, 12 feet wide and 12 feet high would be have an area of 1440 cubic feet (10 by 12 by 12).

Step 2

Add the areas of all rooms to be heated to get the total cubic feet. In this example, assume you have a total of 7 rooms with the same dimensions given in Step 1. Your total cubic feet would be 10,080.

Step 3

Estimate your desired temperature increase. Research the lowest area temperature. Subtract this from the temperature you want to maintain in order to calculate the desired temperature increase. For example, if area lows average 38 degrees, and you want to maintain a temperature of 68 degrees, you will need a temperature increase of 30 degrees.

Step 4

Calculate the BTUs you need. First multiply by .133 the total cubic feet of area determined in Step 2. Multiply that result by the temperature increase you calculated in Step 3. The result, rounded off to the nearest thousand, is the total BTUs or heater size you need. In the example, this would be 40,000 BTUs.

Step 5

Recognize situations that may increase your heater size needs. Large windows, drafty doorways and minimal installation all increase your heating needs. Alternatively, a well-insulated room does not need as much heat.