Furnace Size Calculator: What Size Furnace Do I Need?

How many BTU furnace would fit your home perfectly?

Most homeowners have difficulties figuring out what size of furnace do they need. Fortunately, calculating furnace size (be it natural gas, propane, or oil) is not rocket science.

To help future furnace owners out, we have developed a simple furnace sizing calculator.

You only need to know two things; your home square footage and the climate zone you’re in. 

After the furnace sizing calculation, you’ll find how the size of the furnace is calculated. We also made 3 examples for:

  1. What size furnace do I need for a 1000 sq ft house? (Example 1)
  2. What size furnace do I need for a 2000 square foot home? (Example 2)
  3. What size furnace do I need for a 3000 sq ft home? (Example 3)

You can freely use the calculator to roughly estimate the capacity of a furnace you need. Use this map to determine the climate zone (or climate region) you’re living in:

climate zones to calculate how big a furnace you need
5 US climate zones: Size of the furnace heavily depends on where you live.

Furnace BTU Calculator (Input Square Footage And Climate Zone)

0.00 BTU

Furnace Capacity


The basic principles the furnace sizing calculator is based on are:

  • Larger homes need more BTU than smaller homes (directly proportional to square footage).
  • Homes in colder climates (Region 5) need to generate more heat than holmes in warmer climates (Region 1).

To properly size a furnace, we need to use BTU heating needed per square foot for each climate zone:

Region 1 Region 2 Region 3 Region 4 Region 5
35 BTU per sq ft 40 BTU per sq ft 45 BTU per sq ft 50 BTU per sq ft 60 BTU per sq ft

Essentially, homes in the far North (Minessota, for example) need almost twice as much heating as homes in the far South (Texas, for example).

For an exact estimation of furnace size, you should ask your local furnace installers.

Here are the 3 examples that illustrate how the calculator works.

What Size Furnace Do I Need For A 1000 Sq Ft House? (Example 1)

Let’s say we have a nice 1,000 square foot home in Los Angeles, California. We need two data points:

  1. Home square footage: 1,000 sq ft.
  2. Climate zone: Los Angeles, California, is in Region 2.

We input both of these data points into the calculator. Here is what we get:

for 1000 sq ft home we need 40000 btu furnace

In short, we need a 40,000 BTU furnace.

What about if we have a 1,000 sq ft home in the heart of Minnesota? That’s Region 5. Here is what we get with the furnace sizing calculator:

in cold climate 1000 sq ft home needs 60000 btu furnace

We see that for a 1,000 sq ft home we need:

  • 40,000 BTU furnace in California.
  • 60,000 BTU furnace in Minnesota.

Here are the results for 1000 square foot houses in all climate zones:

Region 1 Region 2 Region 3 Region 4 Region 5
35,000 BTU 40,000 BTU 45,000 BTU 50,000 BTU 60,000 BTU

What Size Furnace Do I Need For A 2000 Square Foot Home? (Example 2)

2000 sq ft houses need double the furnace capacity compared to 1000 sq ft house.

Here are calculated 2000 sq ft home estimates for furnace size:

Region 1 Region 2 Region 3 Region 4 Region 5
70,000 BTU 80,000 BTU 90,000 BTU 100,000 BTU 120,000 BTU

What Size Furnace Do I Need For A 3000 Square Foot Home? (Example 3)

Furnace size for a 3,000 square foot home range from 105,000 BTU to 180,000 BTU, depending on the climate zone you’re in. Here is a table with calculated 3,000 sq ft furnace sizes for all 5 regions:

Region 1 Region 2 Region 3 Region 4 Region 5
105,000 BTU 120,000 BTU 135,000 BTU 150,000 BTU 180,000 BTU

We hope everybody will be able to use the furnace capacity calculator and get a good estimate of the size of the furnace they need for the winter heating

You can check our articles about different types of heating furnaces:

14 thoughts on “Furnace Size Calculator: What Size Furnace Do I Need?”

  1. Many factors other than building square footage need to be considered before choosing a furnace but it gives you a ballpark estimate to start with.
    The size and type of duct work also needs considered.
    The ceiling height and the amount and type of insulation in the attic and walls.
    A 3000 sqft home with 11 foot ceilings will need a much larger furnace than one with 8ft ceilings.
    And those with 20ft high cathedral ceilings and “open” concepts may result in all the money you spend for heat merely keeping the ceiling warm instead of you.
    The type of windows and doors also need to be factored in.
    Are you on a hill top or in a valley or in town where winds are buffered and blocked.
    What is the coldest temperature you have had in the past.
    HVAC companies often tell homeowners their old furnace was oversized and recommend a much smaller furnace.
    Then when those nights hit 25 below, their new furnace is running constantly and can’t keep up.
    It really takes a professional with decades of experience to make these determinations so it’s not a DIY project.

    • Sadly when they give an estimate it’s in and out. I’ve never had one ask about or determine the insulation or what the coldest temp is ( we’re in Boston). It’ helpful for the homeowner to have a starting point as they are running around giving estimates all day and are not going to spend a lot of time given that there many others competing for the job. at least that’s my experience in a busy and cold ( in the winter) city.

    • Hello Jane, basements, on average, need a bit more heating per square root. The calculations above are rough estimates; every home is different and that’s why the BTU output should be adjusted accordingly to the requirements of your home.

  2. How do I convert the BTU requiem to to tonnage. My HVAC guy always speaks in tonnage. I have a 1650 sq ft home with a 3 ton furnace.

    • Hello Dan, you’re right, HVAC people prefer to speak about tons. The ton to BTU conversion is fairly easy; 1 ton equals 12,000 BTU. So, when an HVAC guy speaks about 3-ton furnace, that’s 3*12,000 BTU = 36,000 BTU heating output.

  3. Does the required size change for baseboard heat vs. forced air heating? Also, are there systems that will switch automatically between heating water for baseboard heat and heating water for household usage (e.g., shower, washing machine, etc.)? If so, can you comment on them?

    • Hello Alex, an intriguing question. Currently, we don’t really have the adequate knowledge to answer this question. We might think about creating an article about baseboard heat vs. forced air heating and the size changes that may incur. Thank you for the question; sorry we can’t be more helpful.

    • Hello Shelly, that’s a very good question. In the furnace BTU rating, the efficiency is already figured in. So you don’t have to additionally factor in the efficiency.

    • Not if you’re looking at the “Output BTU” value of the furnace. If you’re looking at the input BTU rating, then yes you need to factor in the BTU rating. An 80% efficent furnace rated at 100,000 BTU input will produce 80%, or 80,000 BTU’s on the output side. The output is the important value here.

      • Hello KB, that’s exactly the right assessment. Some producers list input BTUs and others output BTUs; as KB pointed out, it’s important to note the difference when it comes to furnace sizing.

    • Hello Billie, it does make a considerable difference. Usually, all the calculations are made for standard 8 ft ceiling height. If you have a 10 ft ceiling, you need to add +25% to the calculation. So, if the calculator tells outputs 50,000 BTU, you actually have to add 25%; so your total is 62,500 BTU.


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