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How to Size a Space Heater

by Charlie Rainer Gaston

Electric heater

Before you purchase a new space heater, size it to make sure it will have enough British thermal units (BTUs) to heat the room where the heater will be used. A BTU is the amount of energy needed to heat 1 pound of water 1 degree Fahrenheit. All space heaters clearly identify this number on the packaging, so you don't have to worry about doing any additional calculations.

Sizing a Space Heater:

  1. Measure the room. Determine the size of the room where the space heater will be used. Measure the length and width of the room. Multiply the two measurements to determine the number of square feet. For example, a room that is 12 feet long and 12 feet wide equals 144 square feet.

  2. Calculate the wattage. To figure out how many watts your space heater will need, estimate 10 watts per square foot and multiply the square footage of the room by 10. A room with 144 square feet will require 1,440 watts.

  3. Convert watts into BTUs. A single watt is equal to 3.41 BTUs. Multiply your total wattage by 3.41. A 144-square foot room with 1,440 watts will require a space heater that provides 4,910 BTUs.

  4. Other factors to consider. A standard space heater provides approximately 5,100 BTUs, or 1,500 watts. That means that a standard size space heater is more than sufficient for a room with 144 square feet. But before you decide on what size space heater you need for your home or office, consider factors such as the number of windows (which allow heat to escape), quality of insulation and amount of open space in the room, as well as whether the room has a staircase or a high ceiling. Any of these factors may require a more powerful heater.


  1. A space heater can be a fire hazard if used improperly. Place the space heater in the room away from long, flowing curtains. Keep your space heater away from open flames and flammable contents and chemicals. Never leave children unattended in a room with a space heater. Always turn your space heater off before leaving the room.

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