How to Size a Space Heater

by Charlie Rainer Gaston
Published April 29, 2010 | Updated November 26, 2014

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:

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  • Determine the size of the room where the space heater will be used:

    Measure the length and width of the room, and multiply the two measurements to determine room's size in square feet. For example, a room that is 12 feet long and 12 feet wide equals 144 square feet.

  • Calculate the wattage your room needs:

    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.

  • 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.

  • Adjust your number after considering the room:

    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 these factors.

  • If the room has a lot of windows.

  • If the room isn't well insulated.

  • If the room has high ceilings or a staircase.

  • If the room has cement or tile floors.

  • If the room doesn't have many windows.

  • If the room is well insulated.

  • If the room has low ceilings.

  • If the room is usually used by several people at once.

  • Use space heaters safely:

    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.

  • Consider heater types as you shop:

    Different types of space heaters have different benefits. Be sure to read our guide on the best space heaters for bedrooms to learn more.