The metric system was made legal in the United States by the Metric Act of 1866, which was enacted on July 28, 1866. This law legally recognized the metric system of measurement and made it lawful to use metric units throughout the U.S. However, it did not make the use of the metric system mandatory; it simply allowed and protected its use by stating that no contract or court pleading would be invalid or objected to on the basis of using metric weights or measures.
In summary:
- The metric system became legal in the U.S. in 1866.
- Its use was authorized but voluntary, not mandatory.
