Furnace Size Calculator
Size a furnace for your home: enter the heated area and climate factor to get the required BTU output and fuel input, adjusted for AFUE efficiency.
How this calculator works
Sizing a furnace starts with the heat your home loses on a cold day. You take the heated floor area and multiply it by a climate factor measured in BTU per square foot. That factor runs from about 30 in mild regions to 45 in cold ones, with 40 as a moderate default. The result is the heat the furnace must deliver every hour.
Delivered heat is not the same as fuel burned. Every furnace loses some energy up the flue, so it has to burn more fuel than it delivers. Dividing the output by the AFUE efficiency gives the fuel input. Enter your area, climate factor, and efficiency, and the calculator returns both the output and the input.
Formula: Heat delivered (BTU/hr) = area × climate factor (≈30-45 BTU/sq ft; 40 is a moderate default). Fuel input = output ÷ AFUE efficiency (e.g. 0.95 for a 95% furnace).
Worked example
A 1500 sq ft home with a moderate climate factor and a 95% furnace:
- Heated area: 1500 sq ft
- Climate factor: 40 BTU/sq ft
- Heat output = 1500 × 40 = 60,000 BTU/hr
- Fuel input = 60,000 ÷ 0.95 ≈ 63,158 BTU
Notes
Colder climates use a higher BTU per square foot factor, because the home loses more heat on a hard winter day and the furnace has to replace it. A milder region uses a lower factor for the same floor area.
A higher AFUE burns less fuel for the same delivered heat. The efficiency rating sets how much of the burned fuel becomes usable warmth, so a more efficient furnace needs a smaller fuel input to hit the same output.
How to use
Measure the heated floor area of your home in square feet and pick a climate factor that matches your winters. Enter both along with your furnace’s AFUE rating as a decimal, such as 0.95 for a 95% unit. The calculator returns the required heat output and the fuel input so you can match a furnace to your home.
Sizing here is a planning estimate. A professional Manual J load calculation accounts for windows, insulation, and climate — confirm equipment size with an HVAC contractor.
Frequently asked questions
How do I size a furnace for my home?
Start with the heated floor area in square feet and multiply it by a climate factor in BTU per square foot. That factor runs from about 30 in mild regions to 45 in cold ones, with 40 as a moderate default. The result is the heat the furnace must deliver each hour. This calculator does that math and then adjusts for efficiency so you also see the fuel input.
What is the difference between BTU output and fuel input?
Output is the heat the furnace actually delivers into your home each hour. Input is the fuel energy it has to burn to produce that heat. No furnace is perfect, so input is always higher than output. The gap depends on the AFUE rating: a 95% furnace turns 95 of every 100 BTU of fuel into delivered heat and loses the rest up the flue.
What climate factor should I use?
The BTU per square foot factor reflects how hard your winters are. Mild southern climates sit near the low end, around 30 BTU per square foot, while cold northern climates run toward 45. If you are unsure, 40 is a reasonable moderate default. Insulation quality, window area, and ceiling height also shift the true number, which is why a professional load calculation is the final word.
Does a higher AFUE furnace need less fuel?
Yes. AFUE measures how much of the burned fuel becomes usable heat. A higher AFUE furnace wastes less energy up the flue, so it burns less fuel to deliver the same heat output. That lowers the fuel input figure and your running costs. The trade-off is a higher purchase price, which a high-efficiency unit recovers over time through lower fuel bills.
Estimates only. Verify quantities with your supplier before purchasing.