Today in History—April 5: The U.S. Joins the Metric System—Sort Of

Written by

Erik Gregersen

Fact-checked by

Britannica Editors

History

Who gets to say how long an inch is anyway? On this day in 1893, physicist Thomas Corwin Mendenhall did. And it has more to do with the metric system than you might expect.

Mendenhall was the superintendent of the Office of Weights and Measures in the U.S. Treasury Department, so he, in fact, got to say how long an inch was. He published the “Fundamental Standards of Length and Mass,” which came to be called the Mendenhall Order.

Those standards defined the U.S. Customary System—the measurements Americans know and love (inches, yards, pounds, and feet)—in relation to the metric system. The order determined that the fundamental standard of length was to be the meter, and the fundamental standard of mass was to be the kilogram. And the U.S. Customary units of the yard and the pound would be derived from those units.

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This marked a change from the standards that the British used. Instead of 1 inch being 1/36 of a yard, it became 1/39.37 of a meter, and the pound became 1/2.2046 of a kilogram.

Mendenhall didn’t decide this on a whim. A few decades earlier, in 1866, Congress allowed the metric system to be used and defined conversions between U.S. Customary and metric units. Then in 1875 the United States was part of the global convention that founded the International Bureau of Weights and Measures, created to produce new standard meters and kilograms made of a platinum-iridium alloy, which would be distributed among the nations of the convention. Units of measurement were then defined by physical objects because they provided an easy basis of comparison.

Fun Fact

T.C. Mendenhall climbed Mount Fuji in order to measure Earth’s density.

By the time Mendenhall was heading the Treasury’s Office of Weights and Measures, Pres. Benjamin Harrison had accepted this new standard. The standard meter coincided with Congress’s 1866 definition of the inch-meter relationship (39.37 inches = 1 meter), and the standard kilogram was only off by 0.000022 pounds from Congress’s definition of the kilogram-pound conversion (1 kilogram = 2.2046 pounds). The United States already had standards for the yard and the pound, but they were more subject to change than the platinum-iridium measurements, so the new standards were adopted.

And ever since, despite changes in how metric units are defined, U.S. Customary units are still legally defined through conversions from metric. In a strange roundabout way, Mendenhall put America on the metric system.

Today in History—April 5: The U.S. Joins the Metric System—Sort Of

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