A foot is 12 inches because it was based on the actual length of a human foot.

Americans aren’t fans of the metric system, but they’re not the only folks to eschew decimal-based measurement. The reason a foot is 12 inches, in fact, dates all the way back to ancient Rome, where it was based on the length of a male foot. And this practice wasn’t specific to Rome; many ancient civilizations used feet and other body parts as measurements. 

Friends, Romans, and countrymen initially divided a foot into 16 smaller units of measurement, but later split it into 12 unciae — a word meaning “12th part,” from which we derive the terms “inch” and “ounce.” Measurements weren’t standardized and could vary, but the average foot (as a unit of measurement) in ancient Rome was 11.64 inches. This was slightly larger than the average foot (as a body part) of a Roman man, which was around 10.5 inches. An 11.64-inch human foot would be a size 12. 

Though it’s been said that the measurement is actually based on King Henry I’s foot, which was exactly 12 inches, the concept predates him. That said, Henry did introduce new standards for some measurements, including a yard, which was determined to be the distance from the end of his nose to the end of the thumb of his outstretched hand: about 3 feet. 

Of course, the duodecimal system, which predates the Romans by centuries, is based on the number 12 and shows up in modern timekeeping as well: The number of seconds in a minute (60), minutes in an hour (60), and hours in a day (24) are all divisible by 12. This may have further inspired the Romans to measure a foot as 12 inches. It’s also why there are 12 signs in the Chinese zodiac, which is itself rooted in the fact that Jupiter takes 12 years to complete its orbit around the sun.