Furnace size is essentially determined by its BTU output. 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 particularsized 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.
Things You'll Need
 Tape measure
 Calculator

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 northfacing 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 2,000cubicfoot 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.
Tips & Warnings
 This is just a basic calculation on how to size a furnace. There are many other factors that will affect the total, such as doublepaned windows, extra insulation, secondstory 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.
Related Searches
References
 Photo Credit Jupiterimages/Comstock/Getty Images