What Is BTU? A Simple Guide to British Thermal Units


BTU or British Thermal Unit is a popular topic and regularly used term here on The HVAC Blog. We take a look at what BTU is, how it is calculated, how it applies to our household utilities, and how it helps us pick out our heaters, air conditioners, and furnaces. 

By the definition, 1 BTU is essentially the amount of power or energy needed to change the temperature of a pound of water by one degree Fahrenheit.  There is no difference between measuring BTUs for cooling and warming up.

Calculating BTUs for Air Conditioners

When we see these numbers in the thousands on our air conditioners this indicates the amount of heat the unit can remove in an hour. For example, an 8000 BTU air conditioner means it can remove 8000 units of heat in an hour.

In the case of central air conditioning BTUs are measured in AC tons. One Air-conditioning ton is equal to 12 000 BTUs or 12000 BTUs per hour. From the information gathered so far about BTUs and air conditioners, it is important to take in that bigger is not always better.

Depending on what space you want to cool, you would need to pick the right size and proper power so you do not over or underdo it. A larger air conditioning unit is sure to spike your utility bill and you might not even need one as big.

The same goes for the smaller units since you may want to cool a bigger space but the air conditioner simply cannot output so much power and in the struggle also uses more energy.

If you do not want to think too much about calculations we have a BTU calculator for you where all you have to do is input the space in square feet you wish to cool and a few more details, and the calculator will come up with an answer to what number BTUs air conditioner you need.

In general, BTU ranges vary between 8000 and 12000 for portable air conditioners, 9000 to 36000 for split air conditioners and 3000 to 25000 for compact window-mounted air conditioners.

Simple Air Conditioner BTU Chart

Area To Be Cooled (square feet)Capacity Needed (BTUs per hour)
100 to 1505,000
150 to 2506,000
250 to 3007,000
300 to 3508,000
350 to 4009,000
400 to 45010,000
450 to 55012,000
550 to 70014,000
700 to 1,00018,000
1,000 to 1,20021,000
1,200 to 1,40023,000
1,400 to 1,50024,000
1,500 to 2,00030,000
2,000 to 2,50034,000

Calculating BTUs for Heaters

Now that we covered air conditioning, we can move to heaters and furnaces. With BTUs and heaters, it gets a bit more complicated as there are two BTU numbers to look for and take into consideration.

One is the BTU input, which measures the BTUs used by the fuel, and the BTU output number refers to the efficiency of the fuel. The way to measure the right amount of BTUs per hour your heater or furnace produces you need to calculate it by multiplying the input BTU by the efficiency percentage and dividing it by 100.

For example, if you have a 40000 BTU input heater with 75% efficiency the BTU output would be in fact 30000 BTUs. Take to mind that there is no such thing as a 100% efficient heater.

With this in mind, again when choosing your heater you need to know the details of what space you are going to heat up, and most importantly what fuel your heater will use so you can more accurately calculate the energy efficiency output of the unit.

Just as with the air conditioners, we have a BTU calculator you can use, which will make your life easier and show you all you need to know and how to go about buying your next heater. 

Simple Heater BTU Chart

Area To Be Heated (square feet)Capacity Needed (BTUs per hour)
100 up to 1505,000
150 up to 2506,000
250 up to 3007,000
300 up to 3508,000
350 up to 4009,000
400 up to 45010,000
450 up to 55012,000
550 up to 70014,000
700 up to 1,00018,000
1,000 up to 1,20021,000
1,200 up to 1,40023,000
1,400 up to 1,50024,000
1,500 up to 2,00030,000
2,000 up to 2,50034,000


What Does BTU Stand For?

BTU stands for British Thermal Unit and is used to measure thermal energy. 1 BTU is the energy needed to change the temperature of a pound of water by one degree Fahrenheit warmer or colder at sea level. 

What is BTU Rating?

BTU rating is used as a measurement of efficiency and power shown on heaters air conditioners and furnaces to inform users how much BTUs is the energy input or output of the utility.

What is BTU Hour?

BTU per hour is the amount of energy a certain unit can produce. For example, an air conditioner with 9000 BTU means that the air conditioner will cool 9000 heat units in an hour.

On the other end, as discussed, the way to measure the right amount of BTUs per hour your heater or furnace produces you need to calculate it by multiplying the input BTU by the efficiency percentage and dividing it by 100.

What is a BTU Meter?

BTU meter measures thermal energy and is usually used in chilled water systems for industrial and commercial use. It is used to measure cooling and heat consumptions and these meters are usually used to bill customers and users for their energy use.

How to Calculate BTU

Being able to calculate BTUs by yourself when you do not have access to a BTU calculator and want to know exactly how many BTUs you need for your space – there are a few calculations, which will lead you to the answer.

First, you would need to measure the height, width, and length of your room. After that, you need to take into consideration the number of doors and windows, which can lead to certain heat or cold to leave the room at any point.

Based on the measures multiply the height, width, and length to get the cubic volume of your space. Lastly, consider what areas you are measuring then multiply the number by 5 for dining rooms, by 4 for your bedroom and by 3 for your kitchen or more common area for example.

Combined this will give you a rough number of what BTUs you would need to optimally heat or cool your space. If you are looking for a more precise number, we suggest you turn to a professional who can do all the calculations for you.


To sum it up BTUs measure energy output and input of our heating and air conditioning units and give us a better idea of how energy effective they are.

By considering the BTU number on any unit, we can make a more educated decision the next time we are looking to buy a utility for our house and save some money on electric bills while enjoying a warm or cold living space.