How to Size Furnaces

Getting the right size furnace for the area you want to heat is important.
The wrong furnace size will either waste energy or not heat the entire area. Furnace sizes are measured in British Thermal Units or BTUs. The input BTU is the amount of energy the furnace consumes while output BTU is the amount of heat energy it produces. Appropriately sizing a furnace involves determining how much output BTU the area needs that you want to heat.

Step 1

Determine the square footage

Determine the square footage of the area you want to heat. To determine square footage, multiply an area's length by its width. For a home, it is best to determine the square footage of each room individually, and add them all together.

Step 2

Determine the insulation level

Determine the insulation level of the area. A lot of windows, thin walls, drafty areas or many other factors contributing to the area losing heat is considered poor insulation. If you are unsure of your insulation level, hire an HVAC contractor to do it for you.

Step 3

Determine the output BTU you need

Determine the output BTU you need for the area. Consider square footage and insulation level. For warm climates 30 to 35 output BTUs per square foot is a good measure. If you are in an area where winter temperatures often drop below freezing, you will want to go with 40 to 45 output BTUs per square foot.

Factor in insulation level. If the area is well insulated, go with the lower range of BTUs per square foot; if it is poorly insulated, go with the higher range.

For example, if you live in a cold climate and are heating a well-insulated 3,000 square foot area, you would want 40 to 41 BTUs per square foot, which means you want a furnace with an output BTU of 120,000 to 123,000.

Things You Will Need

  • Calculator


  • Many furnace specification lists will simply list "BTU." This is almost always input BTU, which is not the number you want to go by. To find output BTU from this, multiply input BTU by the furnace's efficiency, which will be listed as a specification.


  • This is a general guide on furnace sizing. A number of factors can affect how much output BTU you need. An HVAC contractor has the equipment and training to provide the most accurate measure.