Without the ability to produce heat from various sources, human civilization would look much, much different than it does today. Although a move toward renewables was well underway by the second decade of the 21st century, most of the world's energy needs are still derived from fossil fuels (petroleum, coal and natural gas).

With fuel being a valuable worldwide commodity, exchange of products often occurs using different pricing systems (such as the British pound and the American dollar) and heat-measurement units (such as the British thermal unit, or Btu, and the therm or Ccf, commonly used in the United States).

One added wrinkle is that the amount of heat released from natural gas can vary both by location and the type of consumer, and it may also change over time. Thus it is not a simple matter of multiplying by a constant factor to get from Btu to therms.

In physics, heat is a form of energy. Energy itself can be somewhat elusive to define, but it is a quantity that has proven useful for quantifying countless processes and allowing scientists a framework to describe some inviolable laws of the cosmos itself.

Heat can be released from the combustion of natural gas and other fossil fuels, from thermonuclear reactions in stars and in nuclear power plants, from friction in mechanical processes (where it is usually considered waste) and from biochemical reactions inside your own body's cells. This is why the harder you work, the more you sweat.

Fossil fuels provide a terrible dilemma for modern society, which has rendered their use all but obligatory for decades to come while concurrently establishing the considerable harm they do to global civilization.

The physical science world includes many different measurements for heat. The SI (metric, or international, system) unit of heat is the Joule (J). An older alternative in the imperial system is the Btu, which is the amount of heat needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit.

A therm is the amount of heat contained in 100 cubic feet (100 cf, or 1 Ccf) of natural gas. As it happens, this amount is very close to 100,000 Btu, or 100 kBtu. Correspondingly, 1 cubic foot (1 cf) holds about 1,000 Btu, or 1 kBtu.

Therms are used to price natural gas in the United States. It is what appears on your natural gas bill, analogous to the kilowatt-hours (kW⋅hr) on your electric bill. (Interestingly, the kW⋅hr is also a unit of energy.)

If not for regional, consumer and time variations, converting from therms to Btu would be a matter of multiplying by 1,000, and 1 therm would be equal to exactly 100,000 Btu or 100 kBtu. But in practice, the situation is a little different.

In 2018, the average heat content of natural gas across all sectors (residential, commercial, industrial and transportation) was about 1,036 Btu per cubic foot. Therefore, 1 Ccf (100 cubic feet) of natural gas translated to 103,600 Btu (1.036 kBtu), or 1.036 therms. This introduces a correction to the previous discussion, in which 100 kBtu (100,000 Btu) was in theory equal to 1 therm.

Since larger quantities of natural gas are typical in commerce, you may be presented with the MMBtu, which is equal to 1 million Btu. If you then use thousands of cubic feet (Mcf) as your volume measurement, you find that 1,000 cubic feet (Mcf) of natural gas equals 1.036 MMBtu. Since a therm is 100,000 Btu and 1 million divided by this number is 10, this also equals 10.36 therms.

• Note that the archaic British "MM" to designate a million and "M" to designate 1,000 can be confusing, since U.S. students most often see M and k in their places. Also, the prefix "C" to mean "times 100" is not used in the metric system.