An anti-metric system poster from 1917 United States of America.

Several myths exist about the metric system and how the United States uses it. Let’s get a few things straight. First, the history of the metric system in the United States goes back to the 1800s, not the 1970s. Second, the SI, the most recent version of the metric system, has been fully adopted by every country or has been given legal approval. This includes countries like the US, Liberia, and Myanmar, which are often called “metric losers.” Last but not least, a country can’t just “turn on” a new weights and measures system. Even France, which is where decimal measurements were first used, slowly switched to the metric system. Also, at least in everyday speech, all countries use both traditional units and metric units.

Even though the country has been using SI units for a long time, measuring in the country is still a mess. Most footraces are measured in metres, but football fields use yards. The power of a car engine is measured in horsepower, which is foot-pounds per second. The size of the same engine is measured in litres. There are different ways to talk about air pressure, such as millibars for altitude and pounds per square inch (or psi) for tyre pressure.

These are just a few examples. The inch-pound system, which is also called the U.S. Customary System, has more than 300 different units for measuring different things. Even though some of these units have the same name, they all mean something completely different. On the website of the United States Metric Association, contributor Dennis Brownridge lists at least nine different ways to define the unit we usually call a “tonne”: short tonne, displacement tonne, refrigeration tonne, nuclear tonne, freight tonne, register tonne, metric tonne, assay tonne, and tonne of coal equivalent.

To understand why the US doesn’t use the European system of measurement in business and everyday life, it helps to look at how it got to the US in the first place.

History of the metric system in the U.S.

On the other hand, the metric system did well over time. By the time the American Civil War ended in 1865, the United States could no longer ignore the decimal-based system of measuring because most of Europe had already switched to it. Because of a law that Congress passed and President Andrew Johnson signed into effect in 1866, weights and measurements from the metric system can be used in all contracts, business transactions, and legal proceedings.

In other words, as of this writing, the United States has been using the metric system as the basis for its standard weights and measures for about 120 years and has officially and legally accepted it for 145 years. But, as we’ll see on the next page, being known doesn’t always mean being useful.

The United States uses the metric system right now.

Mendenhall joined the growing number of academics and politicians who want the United States to use the metric system. But when he died in 1924, the U.S. had not changed. That seemed to change in 1971, when a report called “A Metric America” from the United States National Bureau of Standards suggested that the country switch to the metric system over the course of ten years.