Furnace Size Calculator
This furnace size calculator finds the heating output a home needs from its floor area and a climate factor. Enter them and the furnace size calculator returns the output in BTU per hour and kilowatts. It works as a furnace size estimator for a house, an addition, or a garage, so you can determine furnace size before you shop.
Results
The Furnace Size Calculator is part of our Cooling & Heating collection. Browse more cooling & heating to size, convert, and check your work alongside this tool.
What the furnace size calculator shows
A furnace has to replace the heat a home loses, and the furnace size calculator estimates how much output that takes. It converts the floor area to square feet, then multiplies by a climate factor in BTU per square foot, so a colder climate or a larger home needs a bigger furnace. As a furnace size estimator it gives a starting output in BTU per hour and kilowatts.
How to determine furnace size
To determine furnace size, the furnace size calculator converts area to square feet and multiplies by the climate factor:
The climate factor rolls insulation quality and outdoor temperature into one number of BTU per square foot. A well sealed home in a mild zone sits near 30, while a drafty home in a cold zone runs 50 or more. The furnace calculation then divides by 3412 to also show the output in kilowatts.
Furnace size calculator example
A 100 m² home at a climate factor of 40 BTU per square foot. The furnace size calculator gives:
So the home needs about 43,000 BTU per hour, or 12.6 kW. Raise the factor for a colder climate and the furnace size estimator returns a larger output at once.
How to use the furnace size calculator
Enter the floor area in square meters and a climate factor in BTU per square foot, then the furnace size calculator returns the heating output. Use around 30 to 40 BTU per square foot for a mild climate and 45 to 60 for a cold one. This furnace btu calculator gives a block estimate; a full heat loss survey refines it for insulation, windows, and ceiling height.
Reading the furnace size estimator result
The BTU per hour is the heat output the furnace must deliver; the kilowatts value is the same figure for metric and electric units. Match it to a furnace output rating, not the input rating, since a gas furnace is not fully efficient. Round to the nearest standard size, and avoid oversizing, because a furnace that is too large short cycles, wastes fuel, and leaves uneven temperatures. The furnace size estimator gives a solid starting point for a whole home.
When to determine furnace size
- Sizing a home unit when you determine furnace size before buying
- Heating a workshop with a garage furnace size calculator
- Checking a quote with a furnace load calculator
- Comparing fuels with a gas furnace size calculator
Common furnace size calculator mistakes
- One factor for every climate. Cold zones need a much higher BTU per square foot than mild ones.
- Oversizing. A furnace past the load short cycles and heats unevenly, so size to the loss.
- Ignoring efficiency. Match the output rating, not the input, since a gas furnace is not fully efficient.
Tips for right-sizing a furnace
- Pick a climate factor that matches your zone and insulation.
- Use a higher factor for tall rooms and drafty spaces.
- Match the furnace output rating, then round to a standard size.
How to determine furnace size by climate
Climate is the biggest lever in how to determine furnace size, which is why the factor is given in BTU per square foot rather than fixed. A mild coastal zone might need only 30 BTU per square foot, while a cold northern zone can demand 60, double the furnace for the same house. Insulation shifts the number too: a tightly sealed, well insulated home lands at the low end, and a drafty older home at the high end. A furnace load calculator that ignores climate and insulation will miss badly, so pick a factor that matches both your zone and your build. Ceiling height matters as well, since the tool works from floor area but the furnace heats the full volume, so tall rooms warrant a higher factor.
Garage furnace size calculator and gas furnace size calculator
Different spaces and fuels change the sizing in practical ways. A garage furnace size calculator usually applies a higher factor, because a garage is often uninsulated, has a large door, and loses heat fast. A gas furnace size calculator and a propane furnace size calculator size the same output, but the difference shows at purchase, where you match the calculated output to the furnace rated output and check the efficiency, since a furnace rated at 80 percent must burn more fuel to deliver the same heat as a 95 percent unit. An electric furnace size calculator converts the output straight to kilowatts, since electric heat is close to fully efficient. Whatever the fuel, size to the heat loss, then choose the unit whose output meets it.
Furnace btu calculator reference chart
Heating output by floor area at 40 BTU per square foot, a quick check on the furnace btu calculator:
| Floor area | Heating output | Output (kW) |
|---|---|---|
| 50 m² | 21,528 BTU/h | 6.3 kW |
| 100 m² | 43,056 BTU/h | 12.6 kW |
| 150 m² | 64,583 BTU/h | 18.9 kW |
| 200 m² | 86,111 BTU/h | 25.2 kW |
Furnace Size Calculator FAQ
How do you determine furnace size?
Convert the floor area to square feet and multiply by a climate factor in BTU per square foot. That is how to determine furnace size, and what the furnace size calculator does.
How many BTU do I need per square foot?
About 30 to 40 for a mild climate and 45 to 60 for a cold one. The furnace btu per sq ft figure rolls insulation and outdoor temperature into a single factor.
Is a bigger furnace better?
No. An oversized furnace short cycles, wastes fuel, and heats unevenly. Size to the load and round to the nearest standard output.
Does it work for a garage?
Yes. A garage furnace size calculator uses the same method; a garage often needs a higher factor since it is usually less insulated than a house.