How To Calculate BTU At Home DIY? BTU Calculator

How To Calculate BTU At Home DIY

Finding the perfect heating or cooling system unit that can fit seamlessly into your home is a challenge of a different kind.

Whether it is an AC, furnace, or a baseboard you’re looking for, after all, all of them have qualifications and specifications of their own that need to be matched up against your personal requirements and preferences as homeowners.

Then, considering the wide classification of models available to make a choice from- the selection process is a tough nut to crack, indeed. Typically factors that influence the making of different units also significantly impact the buying process of a customer.

The BTU, standing for British Thermal Unit, has made the buying process for homeowners all around by its easy and instant method of reference and comparison which informs a customer of the square footage coverage capacity of a unit.

The BTU is a relatively simple concept and an even easier measure to calculate. It is, in fact, the standard in the world of heating and cooling systems, and you will find knowledge on the topic without much difficulty. This guide aims to do the same.

What is the BTU?

Before we set about telling you how you can attain a required BTU, let’s begin by forming a distinct understanding of the concept and its helpfulness in the practical world.

If you’re a homeowner experienced in the purchase and replacement of a heating or cooling unit, you’re probably already familiar with the term.

The BTU, in essence, is as important as the heating or cooling system itself. It is the first detail that comes after the name and model number of the unit, rarely secondary after the price tag (if you’re running on a budget).

To define ‘BTU or the British Thermal Unit in official terms, it can be said that 1 BTU is the heat needed to increase the temperature of 1 lb of water by 1 F.

As you can see, the BTU is assumed to be in terms of a heating system, although it applies to cooling systems as well under variable conditions. However, the more pressing issue in this definition is that it dictates the meaning of BTU when it comes to water-retentive systems.

However, HVAC heating or cooling systems are more concerned with air, and how its temperature and humidity levels can be manipulated to achieve a desired heating or cooling effect. Then,

1 BTU is the energy required to heat or cool up to 30 ft3 of air by a degree of Fahrenheit.

1 BTU is also equal to 1055 Joules, and the value increases as the BTU increases.

This BTU figure is, then, attached to different heating or cooling devices including furnaces, radiators, ACs, and so on. Often you’ll have to look for a particular BTU rating to suit the area or the size of the room in which you intend on placing the unit.

Let’s delve into this further by understanding why the BTU becomes so relevant in such a purchase.

Why is the ‘BTU’ important?

To put it simply, the BTU is a way of making sure that the heating or cooling unit you are looking to buy will be able to meet your heating or cooling requirements, particularly that of your living space.

The BTU, as you know, implies a size or square footage of the area that it can heat or cool efficiently. When there is a disparity between these factors, it usually leads to unsatisfactory performance on the part of your AC, heat pump, or radiator.

In fact, choosing a unit that is ‘too small, i.e, has a reduced capacity (smaller BTU) that is not able to accommodate the area of your room can lead to a consequence where your room is never heated or cooled enough. 

Additionally, a too-small BTU can even cause overheating in the system, cause quick wear and tear in the equipment, and so on.

Adversely, when a unit is ‘too big or has a big BTU that is more than what is required for a given room, this can have effects like short-cycling, where the room is heated or cooled too quickly and not thoroughly.

In the case of Air Conditioners, short cycling means that the room is cooled but the moisture is not depleted. Thus, even though the temperature has been lowered, the nature of the air still remains sticky, humid, and uncomfortable.

Thus, the BTU becomes of utmost significance in the context of avoiding all the above consequences and finding a heating or cooling device that molds well into your living space and gives you the heating or cooling you require without any hindrances.

How to Calculate your BTU?

Every heating or cooling device and its different models produce different BTU sizes. In fact, most manufacturers label their products with a clear BTU size number that cannot be missed.

However, as a rational and self-responsible customer, it is still your job to ensure that you appropriately size an AC or furnace so that you can make the right choice. For this, the most crucial step is to calculate the BTU or to interpret a BTU rating correctly.

Luckily, this is a very simple procedure that will not take more than a few minutes. Often, BTU calculation is said to involve 3 main steps, as follows:

  • To measure the dimensions of the room/space that is to be cooled or heated. For this, values like the square footage area of the room, the total volume of space, the length, width, and height of the room are to be measured accurately and noted.
  • The second step in the process needs you to refer to certain recommendations by the EPA, which operates per a ‘rule of thumb’. This answers the question of ‘how many BTUs per square ft of the area?’. This gives the official BTU values for different area dimensions under the assumption that the area is maintained at ideal conditions for the unit to operate.
  • The final step tries to ensure that the BTU unit retrieved is closer to supporting the realistic conditions prevailing in a room, rather than ideal conventions. Thus, aspects like outdoor climates and temperature, window adjustments, gaps and openings in the rooms, kitchen or garage area, presence of sunlight, etc, are taken into consideration and the BTU value is tweaked accordingly.

Step 1

As you know, the first step is concerned with gathering all the measurements of the area. While you can call in a professional to help you, this job can be done equally well with a handy measuring tape and a notebook to put down your numbers.

First, the length and width of your room are to be calculated in feet and inches. The resulting value will be noted as per the norm that 1 foot is equal to 12 inches. So when you are left with an additional 3, 6, or 9 inches, you can convert them into ¼, ½, or ¾ feet.

To get the total square footage of the entire area, you can use the formula:

Total Square ft = Length x Width.

The Total Volume = Length x Width x Height

Typically, most homes have a standard 8 ft ceiling which is also considered to be the norm ruling over the BTU sizing. However, when the height of your ceiling differs from this, you will have to regulate the changes in BTU sizes accordingly.

Home Area For Cooling(In Square Feet):BTU No. Estimate:
100 to 150 square feet5,000 BTU
150 to 250 square feet6,000 BTU
250 to 300 square feet7,000 BTU
300 to 350 square feet8,000 BTU
350 to 400 square feet9,000 BTU
400 to 450 square feet10,000 BTU
450 to 550 square feet12,000 BTU
550 to 700 square feet14,000 BTU
700 to 1,000 square feet18,000 BTU
1,000 to 1,200 square feet21,000 BTU
1,200 to 1,400 square feet23,000 BTU
1,400 to 1,500 square feet24,000 BTU
1,500 to 2,000 square feet30,000 BTU
2,000 to 2,500 square feet34,000 BTU

Step 2

Once you know the total area or square footage of your room, the required BTU value can be derived as per certain EPA norms. The norm, namely, is that every 1 sq ft of area demands a BTU size of 20. For the sq ft that you’ve already calculated for your room, the following formula can be used to obtain the BTU needed:

Since 1 sq ft = 20 BTU,

For a calculated sq ft of area, BTU = calculated sq ft x 20 BTU.

Thus, with a total square foot of 800, the BTU size required to support it would be equal to 800 sq ft x 20, which gives a total of 16,000 BTU. It can be assumed that a 16,000 BTU is perfect to heat or cool a room of 800 sq ft.

However, a more accurate estimate would tell you that a slightly higher BTU would do a better job, provided that your room is subjected to influences.

Step 3

In the third step, different circumstances that can affect the cooling or heating demands of the room are taken into consideration so that the BTU value can be increased or decreased as needed. You can refer to different factors to determine any changes that need to be made:

People occupying the room

The number of people occupying the room makes a big difference in the heating or cooling requirement. Generally, if a room is taken up by more than 2 people on a regular basis, the BTU capacity required increases and must be adjusted with an additional 600 BTU for every person. Thus, if a bedroom is to be occupied by 3 people regularly, the calculated BTU can be supplemented by an additional 1200 to obtain a precise BTU value.

Sunlight exposure

When a room is subjected to an increased amount of sunlight than moderate, the heating or cooling capacity increases by about 10%.  Similarly, if you live in a climate where the sunlight received in a room is lesser than what is considered moderate, the BTU can be lessened by 10%.

The room itself

The nature of the room also affects the BTU size. For rooms like kitchens or garages, where there are more electrical appliances, and more fumes in the air, the BTU size can be adjusted with a 4,000 value increase.


The BTU size of a heating or cooling system is, thus, one of the principal ways of knowing what kind of a unit suits you and your lifestyle the best.

So you must pay close attention as you go through the process of finding the right BTU that is needed for your room, and be conscious of other influential factors that may partake in the decision. This way, you will be able to make a purchase that you won’t regret.

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