The metric system was first made fully legal (but not mandatory) across the United States in 1866, and later named the “preferred” system for trade and commerce in 1975–1988, still on a largely voluntary basis.

Quick Scoop

  • 1866: Congress passes the Metric Act, making it legal to use metric measurements anywhere in the U.S.
  • 1975: Congress passes the Metric Conversion Act, saying the metric system is the “preferred system of weights and measures for United States trade and commerce,” but keeps U.S. customary units legal.
  • 1988: An amendment reinforces metric as the preferred system for federal agencies and trade, yet still does not force the entire country to abandon customary units.
  • Today: Metric is widely used in science, medicine, the military, and some industries, while everyday life (road signs, many products) still uses customary units—by choice, not because metric is illegal.

So: it was made legal in 1866 and later strongly encouraged, but never made universally mandatory for all uses in the U.S.

A bit of story-style context

Imagine the U.S. in the 1800s: trade is expanding, and the metric system is spreading in Europe. To keep up, Congress passes the Metric Act of 1866, which doesn’t force anyone to switch but says, in effect, “You’re allowed to use metric here, and courts will recognize it.”

Jump ahead to the 1970s, when many countries are completing metrication. The U.S. worries about being out of step, so the Metric Conversion Act of 1975 sets a national policy to coordinate a gradual, mostly voluntary shift, established via the United States Metric Board to help guide this process. The board is later dissolved, and the big push fades, but the legal framework stays in place.

In 1988, Congress updates the law to say that metric is the preferred system for U.S. trade and commerce, especially for federal agencies, but again, it doesn’t outlaw customary units. That’s why you can still buy a gallon of milk, even though the underlying standards are traceable to metric definitions.

Key dates and “mandatory?” status (HTML table)

[7][1] [1][7] [3][5] [3] [5][9] [5][9] [9][7] [7][9] [3][9] [9][3]
Year / Law What it did Was metric mandatory?
1866 – Metric Act (Kasson Act) Legalized the use of the metric system in the U.S. and provided official conversion tables.No. It simply made metric lawful; people and businesses could still use customary units.
1975 – Metric Conversion Act Declared metric the “preferred system of weights and measures for United States trade and commerce” and created the United States Metric Board to coordinate a voluntary conversion.No. It explicitly allowed continued use of U.S. customary units in all activities.
1982 – Dissolution of U.S. Metric Board The board created to guide metrication was abolished, reducing federal pressure for conversion.No change; metric remained legal and preferred in policy, but not compulsory.
1988 – Omnibus Trade and Competitiveness Act amendment Amended the Metric Conversion Act so federal agencies must use metric “to the extent economically feasible,” confirming it as the preferred system for trade and commerce.Partly, for federal agencies and contracts, but not a blanket mandate for all businesses, states, or the public.
1991 – Executive Order 12770 Directed executive-branch agencies to take “all appropriate measures” to use metric as the preferred system and set up an interagency council on metric policy.More pressure on federal operations, but still not full nationwide compulsion.

So, was it ever fully mandatory?

In practice, the U.S. never passed a law that said, “From now on, everyone must use the metric system for everything.”

  • Federal rules and contracts often require metric (for example, many defense, scientific, and technical standards), but that’s a condition of doing business with the government, not a criminal law banning inches and pounds.
  • States and industries adopt metric at different speeds—medicine, science, and the military are heavily metric; road signs and most consumers still see miles and gallons.
  • Even when metric is the “preferred” system by statute, U.S. customary units remain legal for general use.

A useful way to think of it: metric is legally allowed since 1866, officially preferred since the 1970s–1980s, and partially required in specific government and technical contexts—but never imposed as a universal, everyday mandate on the entire country.

TL;DR:

  • Legal in the U.S. since 1866.
  • Declared “preferred” in 1975 (strengthened in 1988).
  • Still not universally mandatory; customary units remain legal and common in daily life.

Information gathered from public forums or data available on the internet and portrayed here.