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British thermal unit facts for kids

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The British thermal unit (often called BTU or Btu) is a way to measure heat. Heat is a type of energy. It was first defined as the amount of heat needed to make one pound of water one degree Fahrenheit warmer. This unit is still used in the United States customary units. The main international unit for energy is the joule (J). One BTU is about 1,055 joules.

Even though scientists often use joules, BTUs are still used in some areas. For example, in the United States, the price of natural gas is often given in dollars for one million BTUs. This amount of natural gas would give off one million BTUs of heat if it were burned.

What is a BTU?

A BTU was originally defined as the amount of heat needed to warm up one avoirdupois pound of liquid water by one degree Fahrenheit. This was measured at a steady pressure, like the air pressure around us.

There are a few slightly different definitions of the BTU. This is because the exact amount of heat needed to warm water changes a tiny bit depending on the water's starting temperature. These different definitions vary by less than 0.5%.

Different Ways to Define a BTU

Here are some of the ways a BTU can be defined, and how many joules they equal:

  • Thermochemical BTU: This definition is about 1,054.35 joules. It's based on how much heat is needed to warm water from freezing to boiling.
  • 59 °F BTU: This one is about 1,054.80 joules. It's often used when talking about natural gas prices in America.
  • IT BTU: This definition is about 1,055.06 joules. It was created to link heat units directly to energy units, like the watt.

Big and Small BTUs: Prefixes

Sometimes, people use prefixes with BTU to talk about larger or smaller amounts:

  • kBtu: This means 1,000 BTUs. The "k" stands for "kilo," which means a thousand. You might see kBtu used when tracking how much energy a building uses for heating.
  • MBtu: This can be a bit confusing! In some industries, like natural gas, MBtu means 1,000 BTUs. But in the metric system, "M" usually means "Mega," which is one million.
  • MMBtu: To avoid confusion, people often use "MMBtu" to clearly mean one million (1,000,000) BTUs. This is very common in the oil and gas industry.
  • Therm: A therm is a larger unit, equal to 100,000 BTUs.
  • Quad: A quad is a very, very large unit! It means one quadrillion (1,000,000,000,000,000) BTUs. Quads are used to talk about the total energy used by big countries.

How BTUs Convert to Other Units

One BTU is roughly equal to:

To give you an idea, one BTU is about the amount of heat you get from burning a single wooden kitchen match. It's also the energy it takes to lift something that weighs one pound about 778 feet high.

BTU for Natural Gas

When talking about natural gas, the energy it contains can change a bit depending on what it's made of. So, there isn't one exact conversion for how much energy is in a certain amount of gas.

  • About one cubic foot of natural gas gives off around 1,030 BTUs when it burns.
  • Roughly 1,000 cubic feet of natural gas equals about 1,000,000 BTUs.

BTU per Hour (BTU/h)

When we talk about how fast heat is produced or used, we use BTUs per hour (Btu/h). This is a unit of power. You'll often see this unit used for heating and cooling systems, especially in North America.

  • 1 watt (the international unit for power) is about 3.412 BTU/h.
  • 1,000 BTU/h is about 0.293 kilowatts.
  • 1 horsepower is about 2,544 BTU/h.

Other Related Units

  • Ton of cooling: This is a common unit for air conditioning. One ton of cooling is equal to 12,000 BTU/h. It's the amount of heat that needs to be removed to freeze one short ton of water into ice in 24 hours.
  • R-value: This describes how well insulation works. It's often given in units that include BTUs. A higher R-value means the insulation is better at stopping heat flow.
  • Power Plant Efficiency: BTUs are used to measure how well power plants turn heat into electricity. For example, a typical power plant might need 10,500 BTUs of heat to make 1 kilowatt hour of electricity. This means it's about 32-33% efficient.
  • Centigrade heat unit (CHU): This is an older unit. It's the heat needed to raise one pound of water by one Celsius degree. One CHU is equal to 1.8 BTUs.
  • Calorie: Another old unit for energy in the metric system is the calorie. It's defined as the heat needed to raise one gram of water by one degree Celsius.

See also

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