How to Calculate the Minimum Size of an Electrical Baseboard Heater

Increase the size of your heaters if your space has several older windows.

Determining the size and quantity of baseboard heaters required for your space involves calculating the size of the space, determining how well insulated the space is and then plugging those values into a formula. If you find your values on the borderline between two sizes of heater, opt for larger heaters to avoid overloading a smaller unit.

1. Calculate the square footage of the space you want to heat with baseboard heaters. To calculate the square footage, multiply the length of the room by the width of the room. If a room is 10 feet by 10 feet, for example, you would multiply 10 by 10 and arrive at 100 square feet.

2. Determine the age of your house and how well it is insulated. Older homes with poor insulation require roughly 12.5 watts of baseboard heating per square foot. An average house with R-12 insulation in the walls and R-19 in the ceiling requires 10 watts per square foot. A newer, fully-insulated house with at least R-12 in the walls and R-30 in the ceiling requires 7.5 watts per square foot. Assuming you have a 10-by-10 room with average insulation, you would need 1,000 watts of baseboard heat if the room has an average ceiling height.

3. Adjust your calculations in rooms with high ceilings. If you are heating a room with ceilings higher than 8 feet, add 25 percent for every 2 feet above 8 feet. A room with 12-foot ceilings, for example, would need an extra 50 percent of baseboard heating wattage.

Tip

If you're in a very cold climate with temperatures that dip below 20 degrees Fahrenheit, use the next largest heating interval. For instance, a newer house that would require 7.5 watts per square foot in an average climate should have 10 watts per square foot in a colder climate. Typical baseboard heaters put out 250 watts per foot. For instance, a 2-foot-long baseboard heater would emit around 500 watts. A 4-foot baseboard heater would emit 1,000 watts. Heating power can vary depending on the make and model of the baseboard heater, so consult its documentation for exact heating specifications. Older windows can allow considerable amounts of heat to escape from a room. If your space has several old windows, consider moving up one wattage interval. For instance, a room that would normally require 7.5 watts per square foot should have 10 watts per square foot if it has several older windows. Bathrooms generally require at least 100 watts of heat regardless of their size.

Warning

The vast majority of baseboard heaters require a dedicated electrical circuit, and the size of this circuit will vary depending on the wattage requirements of your heaters. Consult a certified electrician if you're unsure about your wattage requirements.