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Furnace Size Calculator

Table of contents

What is BTU in heating?What size of furnace do you need?How to use our furnace size calculatorHow to calculate the furnace size for a house or room?Sample calculation on how to determine furnace sizeFAQs

This furnace size calculator, or furnace BTU calculator, will help you determine what size of furnace you need for any room area, insulation type, or outdoor temperature.

With our tool, you will learn how to calculate furnace size for a house and what BTU is in heating. Keep on reading to start learning how to determine furnace BTU for your home.

What is BTU in heating?

BTU, or the British Thermal Unit, is a unit of heat used for an appliance's cooling or heating capacity. One BTU is the heat energy needed to raise the temperature of one pound of water by 1°F. That means the higher the temperature difference we need, the higher BTU our equipment should be. When we consider how much power we need from a cooling or heating appliance, we need to know how much BTUs per hour, or BTU/h, it can provide.

What size of furnace do you need?

Before we buy an appliance for our homes, we usually ask ourselves questions like "Will this be big or powerful enough?" or "Isn't this too big for our house?" If you plan to purchase a furnace for your house, you are in the right place. Knowing how to determine the furnace BTU size for your home will help you avoid wasting time and money by purchasing a furnace that is not powerful enough or too powerful and expensive that you end up returning to the store for a replacement. Besides, the furnace's installation is quite a complex procedure we want to get it right the first time.

We usually see heating and cooling appliances like furnace heaters, fire pits, and air conditioners classified in terms of their power capacities, like in watts, hp, BTUs, or BTU/h. Though this furnace BTU calculator focuses on how to determine furnace BTU for your home, you can also use this calculator for watt and hp classifications.

However, what size of furnace do you need? The size of the furnace for your house depends on your location's climate or environment's temperature, the size and orientation of the room or house you plan to heat, and the efficiency of the furnace you wish to purchase.

In a place with a cool climate throughout the year, you will need a much more powerful furnace than in a hot place, wherein the environment can help heat the room. A larger room would also need a larger size of furnace to accommodate the entire area.

Having a well-insulated room that can keep a particular temperature inside it with very minimal to no leakages can also mean we require a smaller furnace as there won't be much heat loss in that room. Our heat loss calculator offers you another way to determine how many BTUs you need for your room. The location's heat index can also affect our choice of furnace size. A room shaded by surrounding trees most of the time might need a larger furnace than a room that already gets heated by the sun during the day.

Keeping all these things in mind can be pretty confusing. This is where our furnace BTU calculator comes in handy. Keep on reading to learn how to use our furnace size calculator to determine what size of furnace you need in no time.

🔎 Are you looking for an air conditioner instead? Check out our air conditioner room size calculator to help you choose the right size of air conditioner that will suit your needs.

How to use our furnace size calculator

Using our furnace size calculator is very easy and intuitive. Here are the steps to follow:

  1. Choose your preferred indicator for the recommended BTU per unit area. You can choose between climate zone or average outdoor temperature.
  2. If you choose the climate zone option in the indicator variable, you will see a climate zone map with a color legend. Each color represents a climate zone and its corresponding range of temperature. Choose the climate zone that suits your area. On the other hand, if you choose the average outdoor temperature option, another variable will show up where you have to enter your location's average annual outdoor temperature.
  3. The next step is to enter the floor area of your room into our furnace size calculator. If you need help determining your room area, our square footage calculator can help you with it.
  4. Then, select how much insulation you have for the room you are considering. Learn more about insulation with our insulation calculator.
  5. Finally, choose the average exposure of that room to sunlight. Choose the sunny room option if the room is facing the sun throughout the day. Otherwise, choose shaded room. If your room gets exposed to the sun for half the day, you can select the option neither shaded nor sunny. Once you've done the above steps, you will see what size of furnace you need.

Like any other appliances that convert electricity to other forms of energy, furnaces also have various efficiency levels. Old furnaces can be as little as 70% efficient, and newer ones can be higher than 90%. We've set the efficiency value to 100% by default to consider ideal situations. However, if you know the efficiency of the furnaces available to you, simply input your known efficiency value into our furnace BTU calculator.

A furnace that is rated 100,000 BTU/h with a 90% efficiency can only provide you with a maximum of 90,000 BTU/h of heating power. So it's also worth considering the efficiency of the furnace you are planning to purchase. Choose one with higher efficiency to help you use your electricity better.

🙋 If you're wondering about the effect of your furnace on your electricity bill, you can use our electricity cost calculator for that.

In the next section, you will learn how to determine the furnace size that would fit your needs based on your location's average outdoor temperature and the floor area you are considering.

How to calculate the furnace size for a house or room?

There are a lot of ways to approximate the recommended furnace size for any room size. In this calculator, we find the furnace BTU size using the recommended BTU/h per square foot for your climate zone or your annual average temperature, as shown in the table below:

Climate zone and annual average temperature

BTU Recommendation (BTU/h per ft2)



Zone 1 (21°C and up)



Zone 2 (16 to 20°C)



Zone 3 (11 to 15°C)



Zone 4 (6 to 10°C)



Zone 5 (-4 to 5°C)



Zone 6 (-9 to -5°C)



Zone 7 (-19 to -10°C)



Zone 8 (-24 to -20°C)



Zone 9 (-25°C and below)



Multiplying the recommended BTUhft2\tfrac{\text{BTU}}{\text{h}\cdot\text{ft}^2} by the room's floor area, and by some multiplier constants to consider the situation of the room in terms of its insulation and location with respect to the sun, we can find our recommended furnace BTU capacity, as we can see in the following equation:

size=BTUrec×A×Fs×Fi×e\footnotesize \text{size} = \text{BTU}_\text{rec}\times A\times F_\text{s}\times F_\text{i}\times e


  • size\text{size} is the furnace BTU capacity in BTU per hour or BTU/h;
  • BTUrec\text{BTU}_\text{rec} is the recommended BTU/h per square foot;
  • AA is the floor area of the room to be heated;
  • FsF_\text{s} is the sunlight factor where its values can be:
    • 1.1 for rooms shaded by surrounding trees or buildings;
    • 1.0 for neither shaded nor sunny room location; and
    • 0.9 for rooms receiving direct sunlight throughout the day;
  • FiF_\text{i} is the insulation factor where its values can be:
    • 1.1 for lightly insulated rooms;
    • 1.0 for rooms with average insulation; and
    • 0.9 for a heavily insulated room; and
  • ee is the percentage efficiency of the furnace.

To further understand how to calculate the furnace size for a house or a room, let's consider an example in this text's next section.

Sample calculation on how to determine furnace size

Let's say we want to install a furnace for a heavily insulated room (insulation factor of 0.9\small 0.9) located on the topmost floor of a building receiving direct sunlight throughout the day (sunlight factor of 0.9\small 0.9) with an area of 1,500 square feet\small 1,500\ \text{square feet}. Suppose this room was in Climate Zone 5 in the United States, and our nearby supplier can only provide a furnace with 90%\small 90\% efficiency. In that case, we can calculate its minimum recommended furnace BTU/h capacity, as shown in the following equations:

size=BTUrec×A×Fs×Fi×e=55 BTUhft2×1500 ft2×0.9×0.9×90%=74250 BTUh\footnotesize \begin{align*} \text{size} &= \text{BTU}_\text{rec}\times A\times F_\text{s}\times F_\text{i}\times e\\[1.0em] &= 55\ \tfrac{\text{BTU}}{\text{h}\cdot\text{ft}^2} \times 1500\ \text{ft}^2 \times 0.9\times 0.9\times 90\%\\[1.0em] &= 74250\ \tfrac{\text{BTU}}{\text{h}} \end{align*}

We can also calculate the maximum recommended furnace BTU capacity for this room by using 70 BTUhft2\small 70\ \tfrac{\text{BTU}}{\text{h}\cdot\text{ft}^2} for the BTUrec\small \text{BTU}_\text{rec} based on our table of recommended BTU/h per unit area. Using the same formula, we'll get 94,500 BTUh\small 94,500\ \tfrac{\text{BTU}}{\text{h}} capacity which means that we can sufficiently heat the room using a furnace with heating capacity ranging from 74,25094,500 BTUh\small 74,250 - 94,500\ \tfrac{\text{BTU}}{\text{h}}.


What is a furnace?

A furnace is a heating appliance that can either be powered using fuel or electricity to produce heat using a heat exchanger. As the heat exchanger increases in temperature, a blower fan moves the hot air towards the air duct to distribute the heated air around the house. Furnaces are also used for different industrial purposes like metallurgy, pottery, and glass-making.

What does a furnace do?

A furnace functions as the main heating component of a centralized heating system. This is where the heat comes from and gets distributed around the house for water heating, heating the floors and walls, or whatever equipment you have connected to the furnace.

How do I choose a furnace for home use?

Choosing a furnace depends on the heat energy needed to warm your home. A house in a cold area would require a larger furnace than one in a warmer area. Typically, a larger furnace has a higher heating capacity, expressed in BTU/h or British Thermal Units per hour. It takes 1 BTU to increase the temperature of one pound of water by 1°F. Therefore, heating more water, or a larger room area, would then require more BTUs.

How do I determine the furnace BTU I need?

  1. Determine your recommended BTU/h needed per unit area for your location.
  2. Multiply your recommended BTU/h per unit area to the floor area of your house to estimate how much BTU/h of furnace you need.
  3. Choose a slightly smaller furnace if sunlight heats your house throughout the day.
  4. Select a larger furnace for a house with not much insulation.

Can I warm my house without a furnace?

Yes, as long as you have other means of warming your house, like using a fireplace, heat pumps, or space heaters. It is also worth knowing what BTU is in heating to help you decide which of these other methods to choose to warm your house effectively.

Environment & home details

The illustration of the different climate zones in the world with color legend.
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