What is a British thermal unit? Why do we use BTU to measure heat for Cast Iron Radiators?

Posted by Brian da Silva on

Ye Olde British thermal unit (BTU for short) is a measure of the heat content of fuels or energy sources. It is the quantity of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at the temperature that water has its greatest density (approximately 39 degrees Fahrenheit).

Let’s give you a visual for that one, cause yeah, I’ll admit it sounds confusing:  One British thermal unit (Btu) is approximately equal to the energy released by burning a match.

What is a btu? This burning match depicts how much BTU is produced by radiators but on a smaller scale

A single Btu is very small in terms of the amount of energy a single household or to up the ante, an entire country uses.  Fast Fact For Ya:  In 2018, the United States used about 101.3 quadrillion Btu of energy.

Wait a minute, how many zeroes is a quadrillion? 

Written out, 1 quadrillion is a 1 followed by 15 zeros: 1,000,000,000,000,000.

Yeah, a lot.

Why Do we use British thermal units?

Energy or heat content can be used to compare energy sources or fuels on an equal basis. Fuels can be converted from physical units of measure (such as weight or volume) to a common unit of measurement of the energy or heat content of each fuel.

The U.S. Energy Information Administration (EIA) uses Btu as a unit of energy content, so there you have it.

EIA collects data on the physical amounts (volume or weight) of energy sources produced, imported, exported, and consumed. EIA converts those amounts into Btu equivalents to compare the sources on an equal basis.  So we know how much we’re talkin’ about.

Just for an example, the physical volumes of fossil fuels consumed in the United States in 2018 and Btu equivalents were

  • Petroleum–7.47 billion barrels–36.95 quadrillion Btu
  • Natural gas–29.96 trillion cubic feet–30.97 quadrillion Btu
  • Coal–687.33 million short tons–13.24 quadrillion Btu
Sample Btu conversion factors

Energy source/fuel

Physical units and Btu¹


1 kilowatthour = 3,412 Btu

Natural gas

1 cubic foot = 1,036 Btu
1 therm = 100,000 Btu

Motor gasoline

1 gallon = 120,333 Btu²

Diesel fuel

1 gallon = 137,381 Btu

Heating oil

1 gallon = 138,500 Btu


1 gallon = 91,333 Btu


1 cord = 20,000,000 Btu³

¹ Btu factors are for end-use consumption in 2018 from Monthly Energy Review, May 2019, excluding wood.
² Finished motor gasoline sold at retail in the United States, including fuel ethanol content.
³ This conversion is an estimate. A cord of wood is a volume unit and does not take wood density or moisture content into account. Wood heat content varies significantly with moisture content.
The Btu conversion factors above are approximations. EIA provides more heat contents for fuels and electricity in the
But, we’re here to provide you with Cast Iron Radiators in the USA.  So let’s move on, shall we?
Many types of radiators are now used to heat the rooms of homes and buildings which have central steam or hot water boiler systems but for the first half of this century cast iron radiators were the overwhelming favorite form of radiation. Although there are now very few manufacturers, cast iron radiation is still one of the most comfortable, quiet and durable ways to heat a room. Cast iron radiators heat up slowly and cool down slowly to give a very gentle, even, radiant heating. Since the 50's less expensive copper finned convectors and baseboard have come to predominate but the heating is harsher more of an "all on," "all off" effect.

So, if you’re looking for a Cast Iron Radiator, or you’re replacing an old one, we know our stuff.  Call us: 973-482-4444 or E-mail:  admin@castironradiatorsusa.com Anytime for your project.

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