Cooling BTU Calculation Table to Select the Right AC

When it comes to selecting your portable or window air conditioner, choosing the right size based on BTUs is the most important decision you will make, before you dive into comparing different brands, models, and energy ratings. A unit that’s too small won’t have enough power to reach your desired temperature, while a unit that’s too big will cool quickly but also inefficiently cycle on and off, costing you extra money.

In order to know that perfect size of AC unit, you first need to understand what BTUs are, how they affect heating and cooling, and the measurements of the rooms in your home, along with other facts. Get these answers and explore our chart below to help you pinpoint the BTUs you’ll need.

What Are Air Conditioning BTUs and What Do They Mean?

BTU stands for British thermal unit, which is the unit used to measure thermal (heat) energy. Specifically, it’s the amount of energy needed for 1 pound of water at sea level to rise 1°F.

When used in relation to air conditioning systems or heaters, the measurement is expressed in terms of how many BTUs per hour the product can add or remove from the air. The higher the BTUs, the more “powerful” the system is at cooling or heating the air.

Both “too many” and “too few” BTUs can cause issues down the line. Here’s how:

Too Many BTUs Can Cause Constant On-Off Cycling

If high BTUs mean more power, then why not get the AC unit with the highest BTUs and call it a day? Unfortunately, the biggest AC isn’t always the answer. If the number isn’t right for the room, you’ll not only waste money on energy bills but could also have to replace your system sooner.

When an air conditioner has a BTU level higher than needed, the unit cools quickly and cycles off. But to maintain your desired temperature, it will cycle on again pretty soon after, blast the room, and cycle off again. It creates a situation where the unit is turning itself on and off again in fitful spurts.

This causes the air conditioner’s compressor to overwork itself. When done excessively and frequently, the unit’s lifespan will likely shorten, causing you to need a replacement much sooner than you might otherwise. This goes for all AC units: portable, window and central.

Plus, window air conditioners also dehumidify the air on top of cooling it. An overpowered unit will run less often, leaving more humidity in the air and your room feeling muggy.

Too Few BTUs Can Cause Continuous Running

Using an AC with not enough recommended BTUs will keep the room from reaching your desired comfort level because the heat load will be too much for your unit to handle. This will cause your AC unit to run continuously without ever reaching your set temperature level—which will also shorten the unit’s lifespan.

Calculating the Goldilocks BTU for your home relies on two variables: room size and heat load.

First, figure out the room size in square feet that you would like to keep cool and then match that number to BTUs in our chart below.

After that, figure out the amount of heat or “heat load” the room receives and, if necessary, adjust that BTU number up or down. For example, if the room has a lot of windows or is next to a kitchen, it would probably have a high heat load.

How to Calculate Square Footage of Your Room

Calculating square feet takes a little measuring and a dash of geometry. Explore the simple geometric formulas below to find the square footage of rooms shaped like squares, rectangles, triangles and more.

How to Measure a Square or Rectangular Room

The simplest formula of length times width works for square and rectangular rooms:

L (length) x B (width) = square footage

So if we have a room that measures 10 feet by 12 feet, our calculations are 10 ft. x 12 ft. = 120 sq. ft.

How to Measure a Triangle-Shaped Room

The formula for the area of a triangle is less commonly needed for rooms but still comes in handy:

L x B x 0.5 = square footage

In our example below of a triangular space, our calculations are 10 ft. x 12 ft. x 0.5 = 60 sq. ft.

How to Measure an Odd-Shaped Room

Calculating the total square footage of odd-shaped rooms requires a few extra steps—finding the area of each “piece” and then adding them all together. Walk through these steps in detail below:

First, measure each wall and map that out. 

Next, break the room into smaller pieces and calculate the square footage of each of those pieces.

We’ll calculate area “A” first. These walls are 15 feet each, so calculating this section is easy:

15 ft. x 15 ft. = 225 sq. ft.

Calculating “B” requires the same formula. Each wall in this space measures 4.5 feet:

4.5 ft. x 4.5 ft. = 20.25 sq. ft.

And we’ll use the triangle formula to calculate area “C.” Note that the two “legs” (non-diagonal walls) are used for the formula:

6 ft. x 7.5 ft. x 0.5 = 22.5 sq. ft.

Note: We’re using decimals instead of fractions to make the calculations easier to manage. To convert to fractions, measure your space and keep the footage as a whole number. Take your number of inches and divide them by 12. So if you have a wall 10 feet 8 inches it would calculate to: 10 ft. + 8/12 in. (.67) = 10.67 feet.

Now that we have calculated the areas for each of our three pieces in our example space, we’re ready to add them up to get our overall square footage of the room:

Area “A” + Area “B” + Area “C” = 267.75 sq. ft.

Match Your Room Size in the BTU Calculation Table

Our chart below shows how many BTUs are recommended for rooms as small as 150 sq. ft. up to 2,700 sq. ft.

Heat Load and Other Factors in Calculating BTU Cooling Capacity

Square footage recommendations will generally work for most rooms, but considering heat load and other variables can help you get closer to that magic number.

The chart assumes traditional 8-foot ceilings; if your ceilings are higher than that, you’ll want to increase your BTU level.

If you want to cool a second story that is typically warmer than the ground floor or a space that gets a lot of direct sunlight (for example, a room with a large, west-facing window), add 10% of the recommended BTUs.

Adding 10% provides a better estimate of the cooling power you will need for this space due to the specific environment’s added ambient heat. This may mean going up to the next available cooling power.

On the other hand, if the area you want to cool has lots of shade and is well insulated from outdoor heat, you could consider subtracting 10% of the recommended BTU. This is not always recommended, as it is better to err on having too many BTUs than not enough.

BTU recommendations typically account for the body heat of two people in a room. If you anticipate family game night or other gatherings in the den, for example, add approximately 600 BTUs per additional person.

If you plan on using your air conditioner in your kitchen, or any area of your home where your appliances will likely generate higher levels of heat, add 4,000 BTUs to the recommended cooling power.

Lastly, consider if your space is open or confined. Our square footage recommendations are meant for confined areas, so if your room opens into another space without the option of closing a door, add in that other space. For example, if you want to cool a kitchen that opens into a living room, you will want to combine the size of those two rooms as if they were one space and use that to determine square footage.

The wrong size is one of the most common mistakes in selecting a room air conditioning unit. With the proper measurements and calculations, you will be able to determine the square footage of your room to find the BTU cooling capacity and make the best choice for your next room AC.