How Can I Determine How Big a Furnace I Need?

Furnace size is essentially determined by its BTU output.

Furnace size is measured in BTUs.Furnace size is measured in BTUs.
A BTU, or British thermal unit, is the measure of the heating value of any given fuel. These heating values can be used to determine how much overall heat it will take to heat a room or a whole house. Although there are many advanced and complicated scientific calculations that will determine the exact size of furnace for a particular-sized house, a relatively basic calculation will give you a rough idea of how big a furnace you will need. From there, you can take these measurements a step further and consult with a heating and cooling specialist, or go online to a home furnace calculator for a more precise answer.

Measure the length, width and height of a room, using a tape measure. Multiply these figures together. This figure is the total cubic footage of that room and is the base number.

Multiply the number from Step 1 by a figure of 5 for a standard room with one or two windows and a doorway. As an example, this would be typical of a bedroom, office or a bathroom. The total will give the amount of BTUs needed to comfortably heat the room.

Multiply the base number by a factor of 6 if the room has more windows or more doorways. As an example, a living room with several windows or several doorways that lead into it would be a typical room that would apply in this case.

Multiply the base number by a factor of 7 if the room has north-facing walls and windows.

Measure all of the rooms in a house separately, and use Steps 1 through 4 to find the BTUs of each room. As an example, if the total cubic feet of a room is 2,000, and there is only one window, multiply that figure by 5. The final figure, 10,000, is the amount of BTUs needed to heat that room. A 2,000-cubic-foot room that faces north would be multiplied by 7. The total BTUs needed to heat that room will be 14,000.

Add the totals for the BTU requirements of each room together. This will give you a rough estimate of how big a furnace you will need to heat an entire house.

Things You Will Need

  • Tape measure
  • Calculator


  • This is just a basic calculation on how to size a furnace. There are many other factors that will affect the total, such as double-paned windows, extra insulation, second-story rooms and others. However, knowing these basic calculations will give you an idea of the size of furnace you will need for new construction or furnace replacement.

About the Author

Dale Yalanovsky has been writing professionally since 1978. He has been published in "Woman's Day," "New Home Journal" and on many do-it-yourself websites. He specializes in do-it-yourself projects, household and auto maintenance and property management. Yalanovsky also writes a bimonthly column that provides home improvement advice.