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Wikipedia's page for Imperial Units does not list Fahrenheit. The corresponding page for SI Units lists Kelvin as an SI unit, and Celsius as a derived SI unit. This leads me to believe that Fahrenheit does not belong to the Imperial System.

Fahrenheit is listed as a page belonging to the Imperial Units category, also on Wikipedia.

I know that the US mostly uses the Imperial system and also uses Fahrenheit for temperature. Meanwhile, the UK mostly uses the metric system and also uses Celsius for temperature. Is this just a coincidence, or is Fahrenheit actually a part of the Imperial system?

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The Weights and Measures Act (the origin of the Imperial Units) does not speak of temperature. It was intended to create a uniform system for trade. You don't sell temperature, in the way you sell a pint of milk or a yard of cloth. And frankly, when it was first conceived (before Magna Carta, which already stated:

"There shall be but one Measure throughout the Realm"
"One measure of Wine shall be through our Realm, and one measure of Ale, and one measure of Corn, that is to say, the Quarter of London; and one breadth of dyed Cloth, Russets, and Haberjects, that is to say, two Yards within the lists. (2) and it shall be of Weights as it is of Measures."

nobody had a sense of temperature - let alone a reliable way to measure it.

This is why Fahrenheit is an "orphan" measure, and not always considered part of the Imperial system. It didn't come into being until 1724 - about 500 years after Magna Carta.

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  • $\begingroup$ This shows why it was not part of the imperial system at the time it was created. There were many Weights and Measures Acts throughout history, though. The 1824 Act (a major overhaul) used Fahrenheit as a "natural phenomenon" to define the pound, at the least. $\endgroup$
    – Geobits
    Commented Apr 14, 2015 at 18:52
  • $\begingroup$ I thought the Fahrenheit was a crutch to define the gallon (10 pounds of water at 62F = 1 gallon) in the 1824 Act. The primary definition of the pound in 1824 was by reference to the standard (which was destroyed in a fire in 1834). $\endgroup$
    – Floris
    Commented Apr 14, 2015 at 19:32
  • $\begingroup$ Ah, you're right. I misread the source, which had the ratio of water weight at 62F to the standard pound, "in case the original were lost" and they needed to make a new one. So it was a definition, but only a secondary one. $\endgroup$
    – Geobits
    Commented Apr 14, 2015 at 19:47
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According to the wiki page on Imperial and US customary units Fahrenheit is part of both the Imperial and US customary system. I can't think of any reason it wouldn't be included in the Imperial system.

Note that in the wiki page on Imperial units it is mentioned that the weight's and measures act (which defined the Imperial system) explicitly used the Fahrenheit scale to define the pound.

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