- 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
- 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.