BTU Calculator: How BTU Is Calculated DIY? (With 8.5% Accuracy)

Calculating BTU doesn’t have to be difficult. If you correctly measure the square footage and know the room/home layout, you can calculate the BTU requirements for any air conditioner or heating unit (with under 8.5% accuracy). You can do it yourself using the simple 3-step guide described further on.

BTU or British Thermal Unit is the most commonly used HVAC heat unit (other units include Joules (J), watts (W), tons, calories). BTU is defined as:

1 BTU is the amount of heat required to raise the temperature of 1 lb of water by 1 °F. (Definition source: Wikipedia)

In HVAC, we don’t talk about water; we talk about air. How much air can 1 BTU heat? Compared to water, the air is about 3400 easier to heat (due to lower specific heat of air). If we take into account nominal air density and specific heat, we can calculate that 1 BTU is enough to heat or cool about 30 ft3 of air by 1 °F.

1 BTU is equal to:

  • 1 BTU = 1.054 kJ (kilojoules)
  • 1 BTU = 0.2931 W⋅h (watt-hours)
  • 1 BTU = 0.0000833 tons
  • 1 BTU = 252 calories

Here are instances when you have to accurately estimate the amount of BTU you need:

BTU plays a key role in every cooling and heating device. For example, we differentiate between 5,000 BTU, 8,000 BTU, 10,000 BTU, 12,000 BTU air conditioners, and so on.

To properly size any HVAC device, you have to correctly calculate the BTU heating/cooling capacity your home requires.

Here is the simple 3 step way of how to calculate BTU (Note: you can also use this simple air conditioner BTU calculator here):

How BTU Is Calculated? (3 Steps)

To properly calculate BTU, we can use a simple-to-use 3 step approach, including:

  1. Measuring square footage, and total volume of the space that is to be cooled. That includes measuring length, width, and ceiling height.
  2. Using EPA recommended BTU per sq ft rule of thumb. This will roughly estimate the BTU cooling requirements.
  3. BTU adjustments for sun exposure, the inclusion of kitchen, people occupying the rooms, window adjustments, and so forth.

Let’s start with the basics:

Step 1: Measure The Square Footage (Length, Width, Height Exact To 0.25 Ft)

The key here is to precisely measure the length and width of the room you want to put an air conditioner in. 

Take a measuring tape and:

  • Measure length of the room (in feet and inches). Example: Room length is 16 ft and 9 inches. This is 16 3/4 ft.
  • Measure width of the room (in feet and inches). Example: Room width is 14 ft and 6 inches. This is 14 1/2 ft.

Keep in mind that 1 ft equals 12 inches. That means that 3″, 6″, 9″ equals to 1/4, 1/2, 3/4 ft, respectively. If you get something like 2″ or 7″, round up to the nearest 1/4 ft or 0.25 ft. This is not 100% exactly but it will suffice for BTU estimation purposes.

Based on your measurements, calculate the square footage using this simple equation:

Square Footage = Measured Length x Measured Width


Square Footage = 16 3/4 ft (Length) x 14 1/2 ft (Width) = 243 sq ft

The ceiling height measurement is usually 8 ft. This is the standard ceiling height; all the BTU calculations and EPA and Energy Star recommendation presume you have an 8 ft ceiling height. If you have a lower or higher ceiling, you will have to use this air conditioner BTU calculator to adjust for higher/lower height.

Note: In HVAC, we talk about square footage coverage. In fact, we should be talking about the volume of air (length x width x height). AC airflow, for example, is measured in CFM (cubic feet per minute); it’s a 3D volumetrical unit, not a 2D area unit. We can talk about square footage instead of volume precisely because more than 83% of buildings have 8 ft ceiling height.

Step 2: Use EPA Recommended BTU Per Sq Ft Rule For Roughly Estimating BTU Requirements

When you calculate the total square footage, you can calculate how many BTU you need by using the EPA’s own rule of thumb for cooling.

This rule states that for every 1 sq ft of living space, you should have 20 BTU of cooling output.

Of course, this a very rough estimate. Here is how you can calculate the BTU yourself. Let’s say you’re looking to cool down a 500 sq ft area. Let’s use the EPA’s rule of thumb:

BTU Requirement = 500 sq ft x 20 BTU per sq ft = 10,000 BTU

In short, a rough estimate tells you that you need about 10,000 BTU air conditioner to cool down a 500 sq ft space.

Note: EPA’s rule of thumb is a bit conservative. In reality, you will need closer to 12,000 BTU air conditioner to adequately cool down spaces up to 500 sq ft. For additional corrections, check the 3rd step.

Energy Star organization – the same organization that issues Energy Star certification for most energy-efficient window, portable, central, and mini-split air conditioners – has a designated BTU table for certain room sizes. Here is this table for reference:

BTU Requirements Based On Room Size (By Energy Star)

Here are calculated BTU cooling capacities by the Energy Star organization. As you can see, they roughly follow the EPA’s rule of thumb recommendations:

Cooling Area (In Square Feet): BTU Cooling Capacity:
100 to 150 square feet 5,000 BTU
150 to 250 square feet 6,000 BTU
250 to 300 square feet 7,000 BTU
300 to 350 square feet 8,000 BTU
350 to 400 square feet 9,000 BTU
400 to 450 square feet 10,000 BTU
450 to 550 square feet 12,000 BTU
550 to 700 square feet 14,000 BTU
700 to 1,000 square feet 18,000 BTU
1,000 to 1,200 square feet 21,000 BTU
1,200 to 1,400 square feet 23,000 BTU
1,400 to 1,500 square feet 24,000 BTU
1,500 to 2,000 square feet 30,000 BTU
2,000 to 2,500 square feet 34,000 BTU

This room size-to-BTU table can help you figure out how big an air conditioner you need.

Step 3: Adjust The BTU Calculation For Other Factors (Windows, Sun Exposure, Kitchen)

Let’s say that you have thus far calculated you will need a 20,000 BTU air conditioner. Does it matter if the room you’re buying an air conditioner for is exposed to the sun or not?

Of course, it does. That’s why both Energy Star and EPA give guidance for additional corrections to the BTU calculation.

Here is the full list of BTU adjustors you may want to use to calculate BTU with 8.5% accuracy (or even less):

Room Properties: BTU Adjustment:
Heavily shaded room: -10% BTU calculated capacity
Very sunny room: +10% BTU calculated capacity
If the room is a kitchen (high temp appliances): +4,000 BTU
Room regularly occupied by more than 2 people: +6oo BTU per every additional person

To illustrate how BTU is calculated using adjustments, let’s solve two examples:

1st Example: 20,000 BTU AC In A Sunny Room, Regularly Occupied By 3 People

This is usually a 1,000 sq ft room; it needs 20,000 BTU AC according to the EPA’s rule of thumb.

But we have to add 10% on top of that BTU calculation because the room has heavy sun exposure. Additionally, we have to account for the 3rd person in the room.

Accurate BTU Calculation = 20,000 BTU * 1.1 + 600 BTU = 22,600 BTU

In this specific case, we’re looking for a 22,600 BTU air conditioner. To be safe, you would do well to invest in 24,000 BTU or 2-ton AC unit.

2nd Example: 15,000 BTU AC IN A Heavily Shaded Kitchen

Here we have two adjustments we need to take into account:

  1. Shaded room: -10% BTU adjustment.
  2. The room is a kitchen: +4,000 BTU adjustment.

Here is the more accurate way how to calculate BTU by yourself:

Accurate BTU Calculation = 15,000 BTU * 0.9 + 4,000 BTU = 17,500 BTU

We can see that the final BTU calculation gives a higher number despite the heavily shaded room. The fact that this is a kitchen (a room that has many hot or heat-generating appliances) overpowers the effect of the heavily shaded room.

BTU Conversions

To help you out with calculations, you can use these following BTU conversion:

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