Calculating Sizing for Electric Baseboard Heaters

Heating Chart For New Additions and Existing Rooms Of A Home

Electric Heater on Grey Carpet
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Determining your electric heating needs is roughly calculated using the room size. This includes the total living area of the room to be heated. If you live in a particularly cold climate, check with the local building department or an HVAC contractor for additional recommendations. 

You can estimate your heating needs in a couple of different ways. The chart below lists the recommended total wattage of heater capacity based on your room's square footage.

It also gives you the minimum circuit size you'll likely need for each level of wattage. The other method is to calculate the size of heater you need in lineal feet. This provides a handy reference for choosing heaters and planning for the required amount of wall space. 

As a general rule, don't undersize. When in doubt, oversize the heating capacity to ensure that the heater(s) can keep the room comfortable. Oversizing electric heaters is not an efficiency problem, as is with oversizing a furnace. 

Calculating Heater Length 

This method assumes a standard electric baseboard heater mounted along the bottom of the room's walls. Start with the wattage from the chart below, or measure the width and length of the room to find the square footage, then multiply by 9. Complete the calculation with the following steps: 

  1. Using the base wattage number, add 10% for EACH of the following, as applicable:  
    • window
    • exterior door
    • exterior wall
    • uninsulated space below
    • poorly insulated walls
    • ceiling more than 8 feet tall


  1. Divide the result by 250. This is the typical wattage per foot used by a baseboard heater. 
  2. Round up to the nearest whole number. The result is the total length of baseboard heater needed to heat the room. 

For example, let's say the room is 200 square feet. The walls are well-insulated, but it is over a uninsulated, unheated garage.

It has two windows and one exterior door. The walls are 9 feet high. The calculation starts with a base wattage of 200 x 9 = 1,800 watts.

  1. Add 20 (10% for garage below) + 40 (two windows) + 20 (exterior door) + 20 (9-foot ceiling) = 1,800 + 100 = 1,900.
  2. 1,900/250 = 7.6 
  3. Round up to 8. This room needs at least 8 lineal feet of heater. 

Heating Wattage Needed For Living Area and ​Circuit Size

Manufacturers Recommended Heating Needs
Total Area Of Room (Sq. Feet)Recommended Heater Rating (Watts)Eectrical Circuit Size Needed (240-volts)
10090015 Amps
1501,35015 Amps
2001,80015 Amps
3002,70015 Amps
4003,60020 Amps
5004,50030 Amps
8007,20040 Amps
1,0009,00050 Amps