How to Size a Baseboard Heater's BTUs

Measure the length, width and ceiling height of the room with a steel measuring tape. Round up the dimensions to the nearest foot.

Multiply length by width on a calculator to arrive at the total square footage of the room.

Select your multiplying factor according to the room’s level of insulation as follows: Older homes with poor insulation require 12.5 watts per square foot; homes with average level of insulation including R-11 in the walls and R-19 in the ceiling require 10 watts per square foot; fully insulated homes with R-19 walls and R-38 ceilings require 7.5 watts per square foot.

Calculate the baseboard heater requirements as follows; for example, a room that measures 12 feet by 15 feet, has a standard 8 foot ceiling, and is insulated to an average level, requires the following calculation: 12 x 15 feet = 180 square feet of floor area. 180 square feet x 10 watts per square foot = baseboard heater total requirements of 1,800 watts.

Adjust your multiplying factor further as follows: If you’re living in a cold climate with winter temperatures below 20 degrees F, select the next highest baseboard heater wattage available after running your calculations. If your ceiling height exceeds 8 feet, increase the heater wattage by 25 percent for every 2 foot increment in ceiling height.

Convert watts to BTU hours based on the above example by multiplying watts by 3.4129 as follows: 1800 x 3.4129 = 6143.2 BTUs per hour of total baseboard heater output required.