How to Calculate for a Heater by Room Size

Written by g.k. bayne
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How to Calculate for a Heater by Room Size
Measuring individual rooms will accurately calculate heater sizes. (bed room image by Stephen Orsillo from

Making your home comfortable during cold weather is a balancing act. The heater for a room must be efficient in overall energy cost and yet still provide enough energy to heat the room properly. Calculating the heater by the room size is a quick method in obtaining enough information to make a heater selection.

Skill level:
Moderately Easy

Things you need

  • Tape measure

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  1. 1

    Measure the room to find the width and length. Multiply those measurements together to find the square footage of the room. As an example a 10-by-10 foot room equals 100 square feet.

  2. 2

    Use the following wattage multiplier for the insulation value of your home. If the home has poor insulation, multiply the square footage times 12.5 watts per foot. The total wattage for the 1,000 square foot room is equal to 1,250 watts. An average insulation rating is a room with R-11 in the walls and R-19 in the ceilings. This is equal to 10 watts per foot. The 100 square foot room is equal to 1,000 watts. A room with full insulation that contains R-19 in the walls and R-38 in the ceiling will require 7.5 watts per square foot. This room will need 750 watts of heating power.

  3. 3

    Understand that the wattage calculations are for 8-foot ceilings. Increase the wattage by 25 per cent for every 2 feet above an 8-foot ceiling height.

  4. 4

    Convert the wattage measurement to BTU (British Thermal Unit). Gas heaters are sold by the amount of BTUs emitted from the ductwork. One watt is equal to 3.41 BTU. Multiply the average insulated room of 1,000 watts times 3.41 BTU per watt. The result is 3,410 BTUs.

  5. 5

    Perform the above steps for each room in the home. Add all of the rooms together for the total heating requirement of the home.

Tips and warnings

  • Bathrooms are small and require special heating needs. Add 1,000 watts per bathroom to the final calculation.
  • Consult with your local heating contractor on new energy efficient models once you have all of the information. New energy efficient heating units are being manufactured each year.

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