What is BTU? Understanding British Thermal Units

We may be paid a commission if you purchase through links on this page. More info.

If you have ever looked at the specifications of an air conditioner or heater then you have probably noticed the term BTU in the specifications. But what is BTU? What does it stand for? What does it mean?

Well this is the article you need. It it we will cover all those questions and provide you with valuable conversion charts, calculators and cheatsheets for everything BTU.

what does btu stand for

The Meaning of BTU

BTU stands for British Thermal Units and is a measurement of thermal energy. It was first used in the late 1800s and has since become one of the most common units of classification in heating/cooling appliances.

1 BTU is the energy needed to heat or cool one pound of water 1 degree Fahrenheit at sea level.

what is btu

BTU In Common Household Products

You will most commonly find it used to measure BTU/hr (BTUs per hour) in heating and cooling appliances but it is also used to measure the effectiveness of many other items.

BTU is used as a unit of classification in:

In the below sections I will go over how BTU is used as a unit of specification in these common household items.

What Does BTU Mean In 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

air conditioner tonnage

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.

Bigger Is Not Better

Choosing the right size air conditioner is very important, and bigger is certainly not better.

An air conditioner which is too powerful for the area that needs cooling actually has quite a few downsides:

Too large an air conditioner = More Costs

The most obvious downside to picking an air conditioner that is too large is that you will need to spend more to purchase the unit in the first place.

If the air con unit needs installing, then it is likely that installation costs will also be higher for a more powerful system.

More Wear and Tear

When you use an air conditioner that is too large for the area it is cooling, you will find that the compressor will need to turn on and off much more than it is designed to.

This is due to the fact that a larger unit will indeed cool down an area very quickly, but it will then turn the compressor off. When the temperature rises above the set temperature again it will re-activate. This on-off cycle happens much more frequently with a larger unit and can result in parts wearing out quicker than they otherwise should.

This then leads to increased maintenance costs.

Higher Humidity

If you ask me, the worst type of heat is humid heat. It makes you sticky, sweaty and just feeling yuck overall.

If you choose an air con unit that is too powerful, then you will likely find that the area being cooled is more humid than you may otherwise like.

This is due to the fact that because the compressor is actually off a lot of the time, it doesn’t remove as much moisture from the air as it is designed to do.

Choose The Right Size Air Conditioner

For a single room, use the table below to give you an estimation of the size of the air conditioner required.

These figures are based on a room with 8-foot ceilings, 2 windows and 1 door. You should consult a professional before deciding exactly what sized airconditioner you need.
Room Size (Sq Ft)Recommended BTUs

If you are calculating the size of a central air conditioner, then you can use the following tools to help you calculate the right sized unit for your home:

Heaters and Furnaces

Heaters also often use the BTU measurement as a common specification. However, it gets a little more confusing here as 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 (Heating Efficiency)

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: (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.

E.g. 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.

BTU Input – Output Efficiency Calculator

BTU Efficiency Calculator

Click the "Next" button to move to the next step of the form.

You must select one of the options above to load the next step of the BTU calculator.

BTU Output Calculation

Enter the percentage efficiency WITHOUT the % symbol
The answer!
The answer!

BTU Input – Output Conversion Chart

btu efficiency conversion chart

Choose The Right Size Heater

It is important when choosing a heater, you choose a model that has the right output for your space.

Heaters are most efficient when running on their maximum setting. This means that if you have a heater that is too big for the space, it will need to be turned to a lower setting – resulting in lower heating efficiency.

And visa versa, when a heater is too small it will not be able to heat the space properly and will result in inadequate performance.

Heating Fuels BTU Chart – Cost Comparison Table

So the BTU input for heaters has to come from somewhere right?

Each heater type has a fuel it requires – and that fuel has a BTU rating.

The table below shows the approximate BTU input, output, heating efficiency and heating cost per million BTU.

FuelUnitBTU Per UnitCost Per UnitCost per million BTU InputAverage Heater Efficiency %BTU Output per 1M BTU InputCost Per Million BTU Output
Electricity1 Kilowatt-hour3,412.00$0.13$37.9598980,000$39
Natural Gas1,000 Cubic Foot (cu.ft.)1,050,000.00$10.91$10.3985850,000$12
Propane1 Gallon91,600.00$2.65$28.9385850,000$34
Heating Oil1 Gallon139,000.00$3.14$22.6080800,000$28
Kerosene1 Gallon135,000.00$2.15$15.8985850,000$19
Pellets1 Ton16,500,000.00$280.00$16.9780800,000$21
Wood - air dried1 Cord20,000,000.00$200.00$10.0060600,000$17

Pricing based on data from https://www.eia.gov

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 and 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.

Do you have any questions relating to this article? Email us at [email protected] or call us on +1 (310) 961-4908

This content may contain affiliate links. If you purchase through these links we may be compensated. More info.

Photo of author
Aaron is the founder of and Essential Home and Garden. With over 15 years of hands-on experience in home ownership, lawn care, and gardening, Aaron is a seasoned expert in areas like lawn care, DIY, HVAC, and pest control.

3 thoughts on “What is BTU? Understanding British Thermal Units”

  1. Avatar photo

    I have a Coleman 2 burner classic camp cook stove that advertised 20,000 btu max for both burners on is there a way to calculate 1 burner on it’s lowest possible setting?
    I do this in my small RV with a wood stove fan an a hot plate over the burner.
    Works very well compared to my infra red heater on low at 3200 btu which is too much after a while that uses no fan.
    Yes i provide proper ventilation and have a CO2 detector.

  2. Avatar photo

    Would you know what Heating Input 12000BTU/HR*4 means? I’m looking at a gas grill that has four burners but I don’t know if that means each burner individually has 12000BTU or combined. I called for clarification but no ones knows.

    • Aaron Green

      I would guess that it means each burner has 12,000 btu.

Leave a Comment