What is BTU in Air Conditioner?

What exactly does BTU in Air Conditioner mean, and why are they so important? The following is a complete guide to what you need to know about BTU.

What BTU Stands For?

BTU stands for British Thermal Unit. It’s a measure of heat energy and that’s the “thermal unit” part of BTU. The “British” part comes from the use of the Imperial Gallon in the calculation.

How is the Imperial Gallon used?

Well, an Imperial Gallon of water weighs 10 pounds. A BTU heats one tenth of this, or one pound of water, one degree Fahrenheit. This is an older system of measurement that has been replaced in the metric system, but BTU is still used in the HVAC field.

Just as you have to add one BTU to a pound of water to raise the temperature one degree F, you could also take away one BTU and lower the temperature by one degree. It’s all about moving heat energy around.

For BTU in air conditioner, we aren’t changing the temperature of water, but of air. But the idea remains the same.

What BTU Mean in Air Conditioner

When comparing air conditioner systems, the BTU is a handy measure. There is a catch though. In HVAC, we aren’t just interested in BTU but how many BTUs can be exchanged in some set time. Although you will see equipment rated by BTU, we usually mean BTU/h (per hour).

When buying a new air conditioner for your home, it is important to choose the size correctly. Sizing an air conditioner means choosing the BTU rating. An air conditioner needs about 20 BTUs for each square foot to cooling. To get an approximate idea for how many BTUs you need, multiply the square footage of your space by 20.

Are BTU measurements accurate?

For residential applications or when comparing different types of air conditioners, they are a good yardstick. However, technically, they are not accurate. The reason is because there are many other elements that influence how hot or cold the results will be.

For example, temperature changes faster when there is a great difference between hot and cold. Your hot coffee will cool faster at first and then more slowly as it approaches room temperature. Air flow is also a consideration – if cool air doesn’t circulate well, “hot spots” will resist temperature changes.

For larger, commercial installations, the air exchange rate, mass volume and humidity all come into play. Humidity changes both how efficiently air can be cooled as well as our perception of temperature.

Choosing The Right BTU For Your Air Conditioner

Air conditioners start at about 5,000 BTU – for a small, window unit, and go up to 24,000 BTU or greater. To put this in perspective, we eat about 2,000 Calories a day in our food. One Calorie is 4 BTUs.

So a smallish window air conditioner (8,000 BTU) is using as much energy every hour as an average adult uses in a day. Actually, it’s more, because the fan and compressor are not 100% efficient and there are losses.

This BTU chart gives you a rough guide to follow based on square footage.

Room Square FootageBTUs Needed

Central AC units are generally use tonnage as a measurement instead of BTU. To convert between the two; 1 ton = 12,000 BTU/hr.


BTUs are an essential factor of choosing the best air conditioner that will fit your needs. Be sure to check out the BTU rating and do some quick calculations to figure out if it right for you. If you need help deciding the right system for your home, get a free quotes from your local HVAC technician who can help you understand the kind of system that’s right for your home.

mas broto avatar
About Mas Broto

Have been in the heating and air conditioning (HVAC) industry for over 20 years. He is person that will grow and thrive to learn more about the HVAC industry throughout his career. Mas Broto is also a blogger, who's dedicated to bringing you the best knowledge to get ahead in the game of life.