Enter the length, width, and height of the room along with the insulation factor of the room into the calculator to determine the correct heater size.

Heater Size Formula

The following equation is used to calculate the correct Heater Size for a room.


  • Where HS is the heater size (BTUs)
  • L is the length of the room (ft)
  • W is the width of the room (ft)
  • H is the height of the room (ft)
  • IF is the insulation factor
    • 7.5 for poor insulation
    • 5.5 for average insulation
    • 4.5 for good insulation
    • 3.5 for excellent insulation

What is the Correct Heater Size for a Room?


The correct heater size for a room primarily depends on 2 factors:

  1. The size of the room. The larger the room, the larger the heater needs to be.
  2. The quality of insulation in the room. Better insulation leads to less heat loss and less energy/heat required from the heater.

How to Calculate Heater Size?

Example Problem:

The following example outlines the steps and information needed to calculate the correct heater size.

First, determine the length, width, and height of the room. In this example, these are measured as 10ft, 15ft, and 8ft respectively.

Next, determine the quality of insulation in the room. The room is in an old building and has poor insulation for this problem. This gives an insulation factor of 7.5.

Finally, calculate the heater size using the formula above:


HS = 10*15*8*7.5

HS = 9,000 BTUs