Calculating Sizing for Electric Baseboard Heaters

Close-up of a baseboard heater

 Frank Bean/Getty Images

Electric baseboard heaters are often the easiest and most effective way to add heating to a room addition or when you are converting unheated space, such as an attic or basement, into living space. Ideally, it's always best to extend existing ductwork from your central furnace/air conditioning system, but sometimes it's just not possible to route ductwork. At these times, installing electric baseboard heaters is by far the easiest solution.


Baseboard Heater Basics

Baseboard heaters are mounted at the bottom of walls and are powered by electrical circuits via wiring that is usually routed through wall cavities to the main service panel. While portable baseboard heaters are available that can plug into standard wall outlets, these are best for temporary use only; for permanent heating, it's best to install permanent baseboard heaters mounted to the wall. 

Permanent heaters are available in both 120-volt and 240-volt models. Wherever possible, install 240-volt heaters, as these are more efficient regarding heating costs. 

When shopping for baseboard heaters, you will notice many different specifications, including the length of the baseboard heater, wattage, amperage, and voltage. For the sake of choosing heaters that are sufficient for the space you need to heat, it is the wattage rating that is the most critical. In general, longer baseboard heaters will produce more wattage.

Here is an example from one manufacturer: 

Cadet 240-volt baseboard heater wattages
30 inches500 watts
36 inches750 watts
48 inches1000 watts
72 inches1500 watts
96 inches2000 watts

The immediate practical question is: "how much wattage do I need to heat my room, and how many baseboard heaters should I install?" The answer to this question will involve calculating the heating needs of the space.

Calculating Necessary Heater Wattage: A Quick and Easy Calculation

A very simple method that can be used to calculate how much total heating wattage you need can be found by calculating the square footage of the room, then multiplying this by 10 watts to produce a baseline wattage requirement. 

For example, if you are heating a 12 ft. x 12 ft. bedroom, it will have 144 square feet. Multiplying this by 10 watts tells you that the necessary heater wattage for the room is 1440.

This base wattage measurement presumes that the room is of modern construction with the typical wall, ceiling, and floor insulation and that it has 8 ft. high ceilings. If the room differs from these specifications, it's recommended that you make the following adjustments: 

  • Add 25% more wattage if the ceilings are 10 ft. high rather than 8 ft. 
  • Add 50% more wattage if the ceilings are 12 ft. high rather than 8 ft. 
  • In an older home, multiply the square footage by 12.5 watts, not 10
  • In an ultra-insulated home, multiply the square footage of the room by 7.5 watts, not 10 

For the sake of our example, let's assume that the room has normal specifications. With 144 square feet, the required wattage is 1440 watts, which means you could heat the room with a single 1500 watt baseboard heater, or two 750-watt heaters.

Calculating by Length of Baseboard Heater

In this method, it's assumed that 240-volt baseboard heaters typically produce about 250 watts per linear foot of length. This calculation is designed to tell you how long your baseboard heater should be. 

  1. Begin by measuring the width and length of the room to find the square footage.
  2. Multiply the square footage by 9. 
  3. Using this base wattage number, add 10% for EACH of the following, as applicable:  
    • Each window
    • Each exterior door
    • Each exterior wall
    • An uninsulated space below the room
    • Poorly insulated walls
    • Ceiling more than 8 feet tall
  4. The resulting number will be the total wattage required for baseboard heaters to heat the room. Now, divide by 250 to arrive at the length of baseboard heater required. 

Using the same size room as described in the first calculation method, we'll assume that our 144-square- ft.

bedroom has one window, two exterior walls, but is otherwise typical. The calculation runs like this: 

  1. 144 square feet multiplied by 9 watts equals 1296 watts.
  2. Adding 10% for a window equals 1425.6 watts.
  3. Adding 20% for two exterior walls equals 1710.72 watts.
  4. Dividing by 250 (the normal wattage per linear foot) equals 6.84 feet of baseboard heater required.
  5. Rounding up, this means that 7 feet, or 84 inches, of heater is needed. This is a somewhat unusual number; in this case, a single 8 ft. or two 4 ft. heaters would be the likely choice.

Manufacturer's Recommended Heating Needs

It is always best to slightly oversize when selecting baseboard heaters. There is no loss of efficiency by heating with baseboard heaters slightly larger than minimum requirements. 

Total Area of Room (sq. ft.)Recommended Heater Rating (watts)Electrical Circuit Size Needed (240 volts)
10090015 amps
150135015 amps
200180015 amps
300270015 amps
400360020 amps
500450030 amps
800720040 amps
1000900050 amps