A Simple Guide to British Thermal Units (BTUs)


If you’ve ever shopped for an air conditioner or a heater, you’ve probably noticed the term BTU in the specifications. But what is a BTU? What does it stand for? How is it relevant?

BTU stands for British Thermal Units. It’s been around since the late 1800s and has become the most common units of measurement for heating and cooling appliances.

This article goes into more depth to help familiarize yourself with BTUs plus help you determine how many BTUs you need to heat or cool your home.



The Meaning of BTU


BTUs in Common Household Products


Build of Your Home



The Meaning of BTU

Basically, a BTU is the amount of heat needed to raise one pound of water 1 degree F.

Sounds like an odd requirement, but knowing the amount of heat and energy is necessary to keep your home comfortable.

The easiest way to figure out the BTUs needed is to take a look at the square footage of the space you want to heat. In warmer climates multiply this number by 10-15. In moderate climates multiply it by 20-30. In colder climates multiply by 30-40.

For example, if you’re trying to heat 1000 square feet in a cold climate, you’re looking at 30,000 to 40,000 BTUs to warm the air in your home.

But to help you out, we’ve also put together this chart:

BTUs for 8-foot Ceilings 

12-ft Length18-ft Length24-ft Length30-ft Length36-ft Length
12-ft Width5,18847,77610,36812,96015,552
18-ft Width7,77611,66415,55219,44023,328
24-ft Width10,36815,55220,73625,92031,104
30-ft Width12,96019,44025,92032,40038,880
36-ft Width15,55223,33831,10438,88046,556

BTUs for 10-foot Ceilings 

12-ft Length18-ft Length24-ft Length30-ft Length36-ft Length
12-ft Width6,4809,72012,96016,20019,400
18-ft Width9,72014,58019,40024,30029,160
24-ft Width12,96019,44025,92032,40038,800
30-ft Width16,20024,30032,40040,50045,600
36-ft Width19,44029,16038,88048,60058,320
These are basic estimates and the actual BTUs you’ll need depend on several other factors: 

  • Number of windows
  • The climate
  • Age of the building
  • Orientation to the south
  • Type of and amount of insulation
  • Construction techniques
  • And more

When you are calculating the BTUs you’ll need for your whole house, you need to include the worst-case scenario. Which means that you should account for the coldest that it will get. 

For example, maybe the coldest it will ever get is -30 degrees F. And maybe that happens once every 3 years. You’ll want to make sure the BTUs you are accounting for considers that huge outlier. 

BTUs in Common Household Products

You’ll most commonly find BTUs used to measure BTUs per hour in heating and cooling appliances, but it’s also used to measure the effectiveness of many other items.

Air Conditioners

When used as a specification for an air conditioner, the BTU number given refers to the amount of heat that the unit can remove from the air per hour (BTU/hr).

Typical BTU ranges air conditioners are as follows:

  • Portable air conditioners: 8,000 – 12,000 BTU
  • Split system air conditioners: 9,000 – 36,000 BTU
  • Window mounted air conditioners: 3,000 – 25,000 BTU

Central air conditioning units (there is one compressor unit for the whole house) generally use tonnage as a measurement of cooling power instead of BTU.

However, to convert between the two is quite simple. 1 air conditioner ton = 12,000 BTU/hr.

So a 2-ton central air conditioning system can remove 24,000 BTUs of heat from the air per hour. It should be noted here that air conditioner tonnage has nothing to do with weight.

Here is a good rule of thumb for air conditioning BTUs required for air conditioning

Heaters & Furnaces

Heaters also use BTUs as a common specification. However, it gets a little more complicated than that because the number can be given in two ways:

  • BTU Input
  • BTU Output

Let’s take a look at the difference between the two.

BTU Input vs. BTU Output

The BTU input is the amount of fuel the heater can consume per hour.

But, it is important to remember that there is no such thing as a 100% efficient heater.

What do we mean by efficiency? Well during the process of consuming the fuel, some of the energy is lost in the process to inefficiencies.

Every heater has an efficiency rating. Sometimes it is given in the specifications, other times it’s not and you will have to ask the manufacturer to provide it.

So a heater that has a BTU input rating of 80,000 BTU and an efficiency rating of 85% puts out about 68,000 BTU per hour.

This is calculated by the following formula:

(Input BTU x efficiency %)/100

This number is the BTU output specification.

And as you can see the two numbers can be quite different. It is very important when choosing a heater that you are aware of the efficiency rating of the product.

For example, a pellet stove with a BTU input rating of 50,000 and an efficiency rating of 70% actually has a BTU output of 35,000 per hour.

Compare this with a different model pellet stove that has an input rating of 45,000 BTU and an efficiency of 85%. This means that the BTU output is 38,250. Which is actually higher, despite having the lower BTU input rating because of the lower efficiency.

Solar Hot Water Panels

Another device that uses BTU’s as a measurement is solar hot water panels.

The specific measurement used for solar hot water panels is BTU per square foot per day. It may be referred to simply as BTU per day in specifications – but both the numbers are the same measurement.

The higher the BTU number specified, the higher the water heating efficiency.

Gas Stoves & Grills

BTU is used to measure the output of each burner on a gas stove/grill.

A home gas stove will typically put out about 7,000 BTU per hour, but some models will put out as low as 3,000 BTU and as high as 12,000 BTU.

You may find that the specific gas stove or grill you are after advertises either a per burner BTU number or a product-wide BTU.

If the number is product wide, then simply divide the BTU by the number of burners to get the per burner BTU

How Much BTUs Do You Need?

The number of BTUs you’ll need to heat your home will depend on a few different factors, like the size of your home and the build of your home.

You already know the BTU requirements for the size of the home. This section will tell you about the build of your home.

Build of Your Home

Before picking out the gas stove, you’ll need to do a walk-through of your home to take some notes on a few particulars, such as the layout and build of your home.

These notes will be key parts on which gas stove you choose. Here’s what you need to think of.


How well is your home insulated should be one of the first things you consider.

This is because a home that is poorly insulated will need a more powerful gas stove with a higher BTU range to compensate for the lack of insulation.

If your home is well insulated you will not need as powerful of a stove.

You can make an educated guess to determine how well your home is insulated. Basically, newer homes are well insulated, while it’s safe to assume older homes aren’t as well insulated.


A lot of heat is lost through windows. Take a note of how many windows you have in your home. Also pay close attention to the condition the windows are in.

If there are a lot of windows or windows are in bad shape, then you’ll need to increase the BTU range of your new gas stove.

Ceiling Height

If you have high ceilings, like a cathedral ceiling, you’ll require a more powerful stove with a higher BTU range.

This is because hot air rises and heat can collect above the living area and get trapped where it is of little use.

Climate & Geographical Location

There’s not much you can do here. Unless you want to move.

If you live in an area that is usually warm and maybe it reaches 30-40 degrees a few times a year, you don’t need a powerful gas stove.

But if you live in a cold area, then you’ll want to increase that BTUs.


When you’re shopping for a new fireplace or an air conditioner, now you’ll know exactly what you need to look for to keep the temperature in your home comfortable.

But if you have any questions, feel free to reach out and we’ll be happy to help you figure out the best BTUs

It doesn’t matter if you are in Roanoke, Lynchburg, Blacksburg, or some other city anywhere in the USA — if you have any questions about BTUs, please don’t hesitate to email me at mitchell@patriotchimney.net or call me at 540-225-2626.

I’m happy to help!


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