Interesting History of Units

Where some familiar units came from.

The meter

After the French Revolution, France wanted a rational, universal measure. The meter was defined in the 1790s as one ten-millionth of the distance from the equator to the North Pole along the Paris meridian. Surveyors measured a portion of that arc; the rest was derived. Today the meter is defined by the speed of light in a vacuum—no physical bar needed.

The kilogram

The kilogram was originally the mass of one liter of water at 4 °C. For practical use, a platinum–iridium cylinder called the “international prototype kilogram” (IPK) was made and kept near Paris. For over a century it was the world’s definition of the kilogram. Since 2019 the kilogram has been defined in terms of the Planck constant, so the IPK is no longer the definition—though it’s still carefully preserved.

The inch and the foot

“Foot” likely came from the length of a human foot; “inch” from the Latin uncia (one-twelfth), as in 12 inches to a foot. Over time, different regions had different “feet.” The US and UK eventually fixed the inch to exactly 2.54 cm in 1959, so the “international foot” is now precisely 0.3048 m.

Fahrenheit and Celsius

Daniel Fahrenheit (early 1700s) set 0 °F to a cold brine mixture and 96 °F to human body temperature (later adjusted so water freezes at 32 °F and boils at 212 °F). Anders Celsius (1742) originally had 0 for boiling and 100 for freezing; it was later flipped to 0 °C = freezing, 100 °C = boiling. Both scales are still in daily use alongside the scientific Kelvin scale.

The mile

“Mile” comes from the Latin mille passus—1,000 paces (each pace being two steps). Roman soldiers counted steps to measure distance. The modern statute mile (5,280 feet) was set in England; the number 5,280 came from 8 furlongs (furlong = “furrow length”), each 660 feet. Nautical miles are different: one nautical mile = one minute of latitude, tying distance to the Earth’s geometry.

The gallon

Gallons started as a volume for measuring wine and beer. Many regional “gallons” existed. The US gallon (231 cubic inches) is tied to the old wine gallon. The UK imperial gallon (1824) was defined as the volume of 10 pounds of water at 62 °F—about 4.546 L. That’s why US and UK gallons are not the same.

Want to try converting between these units? Use the tounits.com converter.