Your furnace is one of the most important appliances in your home because it provides heat and sometimes cooling. You'll typically find furnaces measured in BTUs, or British Thermal Units, but you could find some models that use tonnage instead. One ton is equal to 12,000 BTUs, so once you know your BTU requirement, calculating the tonnage is a simple process.
Measure the square footage of each room and hallway in your home. For rectangular spaces, multiply the length and width of the room. For triangular spaces, multiply the base of the triangle by its height and divide the result by 2.
Add each room and hallway's square footage together to determine your home's total square footage. As an example, a home has a square footage of 1,000.
Determine which zone your home is located in. As an example, the home is in Zone 1.
Multiply your home's square footage by 35 if you're in Zone 1, by 40 if you're in Zone 2, by 45 if you're in Zone 3, by 50 in you're in Zone 4 and by 60 if you're in Zone 5. The result of this calculation determines the BTU capacity of the furnace you need. In the example, because the home is in Zone 1, you would multiply 1,000 by 35 and get 35,000. This home requires a furnace of 35,000 BTUs.
Divide the result from Step 4 by 12,000 to determine the tonnage requirement of the furnace. In the example, you would divide 35,000 by 12,000 and get 2.92. This home would require a furnace that has a tonnage of approximately 3.