Foot (unit)

A foot is a unit used to measure length. It is a core part of the British imperial and United States customary systems. People use it to measure many things, such as human height or the length of an object. One foot is defined as comprising exactly 12 inches. If you group three feet together, you create one yard.

In writing, the symbol "ft" is common. You might also see a prime symbol, which looks like a small mark, used to represent the foot.

To understand how the foot works today, we must look at international standards. Before 1959, different countries had slightly different definitions. An international agreement in 1959 changed this by linking the foot to the metric system. This agreement defined the international yard as exactly 0.9144 meters. Because a foot is one third of a yard, the international foot is exactly 0.3048 meters.

This precise definition ensures that measurements are consistent across different borders.

Historically, the foot was not a single, fixed length. It was part of many local systems used by different civilizations. The Greeks, Romans, Chinese, and French all had their own versions. The size often changed from one country to another. It could even change from one city to another. Sometimes, the length depended on the specific trade being practiced.

This variety made large-scale measurement very difficult in the past.

Ancient people often used the human body to create measurement units. For an adult European-American male, a foot is typically about 15.3% of his total height. This usually results in a foot length of about 30 centimeters. Archaeologists believe different cultures preferred different units. For example, people in Egypt, India, and Mesopotamia often used the cubit. However, the Romans, Greeks, and Chinese frequently used the foot.

In the Indus cities during the Bronze Age, a foot was about 30.48 centimeters.

Different regions developed unique types of feet throughout history. The Greek foot was about 30.8 centimeters long. The standard Roman foot was normally about 29.6 centimeters. In the province of Germania Inferior, a different version called the "foot of Nero Claudius Drusus" was used. In England, different groups like the Belgic Celts and the Anglo-Saxons introduced different measures. Some English measures were even based on the size of a barleycorn.

These variations show how deeply local culture influenced measurement.

In the United States, a specific version called the survey foot was used for a long time. This was because much of the existing map data used older definitions. The survey foot is slightly different from the international foot. While the difference is tiny, it becomes significant when mapping very large areas. This is because the origin of the coordinate system can be hundreds of miles away. On December 31, 2022, officials recommended moving away from the survey foot. They suggest using either the meter or the international foot instead.

Today, the foot remains significant in several specific ways. The United States is the only industrialized country that prefers the foot for commercial and engineering activities. In the United Kingdom, the foot is legally recognized. British people often use it to describe how tall they are. Even in the world of aviation, the foot is vital. Pilots use the foot to measure altitude, known as the flight level unit. This is one of the few places where the foot is used globally, even outside English-speaking nations.