Why the US Never Went Metric (And What It's Cost)

The Country That Chose Inches Over the World

When the United States declared independence in 1776, almost every country on earth measured in ad hoc, locally defined units — chains, furlongs, hogsheads, drams. Then France invented the metric system in the 1790s, and within two centuries, virtually the entire world converted. Today, 195 of 198 countries use the metric system as their official standard of measurement. The three that do not are Liberia, Myanmar, and the United States — and even Myanmar began a formal transition in 2011.

The US is not metric by choice, inertia, and a series of political decisions that stretch back 50 years. The story involves a toothless law, a defunded federal board, a frustrated chair's resignation letter, and two catastrophic accidents that killed spacecraft and terrified passengers — all because someone used the wrong units.

A Law Without Teeth: The Metric Conversion Act of 1975

In 1975, Congress passed the Metric Conversion Act, declaring the metric system "the preferred system of weights and measures for United States trade and commerce." It sounded decisive. It was not. The critical word buried in the legislation was voluntary. No industry was required to switch. No deadline was set. No penalties existed for non-compliance.

To oversee the transition, the act created the US Metric Board — a 17-member advisory body charged with coordinating and planning America's move to metric. The board had authority to educate and recommend. It had no authority to mandate anything.

By 1981, the board's chair, Louise Branscomb, submitted a resignation letter that became a quiet monument to bureaucratic futility. She wrote that without any enforcement mechanism, the board was powerless to effect real change and that voluntary conversion had produced almost no measurable progress. President Reagan defunded the US Metric Board entirely in 1982. The American metrication experiment was over — at least officially.

1988: "Preferred," Again

Six years later, Congress tried again. The Omnibus Trade and Competitiveness Act of 1988 re-declared metric the preferred system for federal government trade and directed federal agencies to use metric units in procurement, grants, and business activities. Federal agencies were given until 1992 to comply. Most did not. No enforcement followed. The pattern was identical to 1975: a statement of preference with no consequence for ignoring it.

Today, the US remains legally committed to the metric system — in the same way someone can be committed to going to the gym while never actually going.

Timeline of US Metric Conversion Attempts

When Unit Confusion Becomes a Disaster

Measurement confusion is not merely inconvenient. In at least three well-documented cases, the collision between imperial and metric units produced accidents that destroyed hardware, cost hundreds of millions of dollars, and put human lives at risk.

The Gimli Glider — Air Canada Flight 143 (1983)

On July 23, 1983, a brand-new Boeing 767 — Air Canada Flight 143 — ran completely out of fuel at 41,000 feet over central Canada. The cause was a unit error so simple it is almost unbelievable: ground crew at Montreal calculated the required fuel load in pounds, but the 767 was a metric aircraft and the fuel computer expected kilograms. A kilogram weighs roughly 2.2 times more than a pound, so the plane was loaded with less than half the fuel it needed.

Both engines flamed out. Captain Robert Pearson, a glider pilot in his spare time, dead-stick landed the unpowered 767 on a closed dragstrip in Gimli, Manitoba. The plane became known as the "Gimli Glider." All 69 people on board survived — an extraordinary outcome that had more to do with luck and skill than system design.

NASA Mars Climate Orbiter — $327.6 Million Lost (1999)

On September 23, 1999, NASA's Mars Climate Orbiter — a spacecraft that had traveled 416 million miles over nine months — was destroyed as it attempted to enter Mars orbit. The cause: Lockheed Martin, which built the spacecraft, had been sending thruster data in imperial units (pound-force seconds), while NASA's navigation software expected metric units (newton-seconds). One pound-force second equals approximately 4.45 newton-seconds, and the accumulated error over months of flight pushed the orbiter onto a trajectory that took it too deep into the Martian atmosphere.

The mission cost $327.6 million. The error was a single software interface file that had never been checked for unit consistency. NASA's own review board called the failure "a process failure" — a systemic assumption that both parties were using the same units, without verification.

Korean Air Cargo Flight 6316 (1999)

Also in 1999, Korean Air Cargo Flight 6316 crashed near Shanghai's Hongqiao Airport after the crew descended to 1,500 feet when air traffic control had assigned an altitude of 1,500 meters. The crew interpreted the metric altitude instruction in feet — a factor-of-three error. Three crew members and five people on the ground were killed. International aviation now mandates that all altitude communications be in feet under ICAO rules, but the incident demonstrated how fatally ambiguous unit systems can be in high-stakes environments.

Famous Unit Conversion Disasters

The Economic Cost of Staying Imperial

The accidents above are dramatic, but the larger cost of the US measurement divide is quieter and more pervasive. A 2015 study estimated that the US loses more than $50 billion annually in trade friction attributable to its non-metric status. These costs include:

• Dual manufacturing runs — producing the same product in imperial sizes for the US and metric sizes for export
• Product relabeling and dual-unit packaging for goods sold in both US and international markets
• Engineering conversion errors in multinational projects
• Lost export contracts with buyers who require metric specifications and won't accept dual-standard products
• Extra quality-control steps to verify unit consistency across supply chains

American businesses that export heavily — particularly in aerospace, pharmaceuticals, and automotive — largely operate in metric internally. Boeing aircraft are designed and documented in metric units. US pharmaceutical companies use metric for all drug calculations. The military adopted metric in the 1950s. In practice, America already has two parallel measurement cultures: a scientific and industrial one that is metric, and a consumer one that stubbornly remains imperial.

The Great Irony: Imperial Is Defined in Metric

Here is perhaps the sharpest irony in this entire story: the US imperial system is not actually self-defining. Since the International Yard and Pound Agreement of 1959, the US inch has been officially defined as exactly 25.4 millimeters. The pound is defined as exactly 0.45359237 kilograms. In other words, the "American" system of measurement is legally a derivative of the metric system. Imperial units have no independent physical definition — they are approximations of metric units, frozen in place by international treaty.

The US uses metric to define its non-metric units, and then uses those non-metric units for everyday life.

Why Americans Resist

Surveys consistently show that most Americans are aware of the metric system and most could not tell you why the US hasn't adopted it. The resistance operates at several levels:

• Cultural identity: Miles, feet, and Fahrenheit are woven into American culture in ways that feel identity-adjacent. Asking someone to switch from 72°F to 22°C for their ideal room temperature feels like asking them to stop feeling American.
• Industry retooling costs: Replacing every road sign, every speedometer, every ruler in every classroom, every product label, every piece of manufacturing tooling would cost an estimated hundreds of billions of dollars in transition costs — a number that makes $50 billion in annual friction look manageable by comparison.
• Sports culture: NFL fields are 100 yards. NASCAR tracks are measured in miles. A four-minute mile is a milestone; a 3:42.88-kilometer run is not. These cultural reference points have enormous staying power.
• Political toxicity: After the 1982 Metric Board defunding, metrication became associated with government overreach. Any politician who campaigned for mandatory metrication would face significant backlash — and no politician wants to be the person who changed the road signs.

The Science Exception

There is one domain where the US is fully, unambiguously metric: science. Every research paper published in the United States uses SI units. Every university physics and chemistry course teaches metric. The Centers for Disease Control reports disease statistics in metric units. Drug labels specify dosages in milligrams and milliliters. NASA (when it isn't accidentally receiving imperial data) designs missions in metric. The irony is complete: America's scientific establishment operates in a different unit system than America's grocery stores.

This two-system reality means that every American science student must, at some point, learn to fluently navigate both. It is a redundant cognitive burden that students in every other developed country do not carry.

Convert Between the Systems

Until the US finishes a conversion that has been "in progress" for 50 years, knowing how to move between imperial and metric is a practical life skill. Use these tools:

• Miles to kilometers converter — the conversion that would change every American road sign (1 mile = 1.609 km)
• Weight converter — pounds to kilograms and back, with the precision the Gimli Glider crew needed
• Temperature converter — Fahrenheit to Celsius, the conversion that summarizes the entire US metric divide in a single daily act

Frequently Asked Questions

Did the US ever try to adopt the metric system?

Yes — twice. The Metric Conversion Act of 1975 made metric the preferred system but kept conversion entirely voluntary. A US Metric Board was created but defunded by Reagan in 1982 after almost no progress. The Omnibus Trade and Competitiveness Act of 1988 again declared metric "preferred" for federal trade, also without enforcement. Neither law produced meaningful adoption.

What disasters have been caused by metric vs imperial confusion?

The most famous are the Gimli Glider (Air Canada Flight 143, 1983), where a Boeing 767 ran out of fuel because ground crew used pounds instead of kilograms, forcing a powerless emergency landing; and NASA's Mars Climate Orbiter (1999), destroyed when Lockheed Martin sent thruster data in imperial units while NASA's software expected metric, costing $327.6 million.

Is the US inch actually defined in metric units?

Yes. Since 1959, 1 inch = exactly 25.4 mm, and the pound is defined as exactly 0.45359237 kg. The US imperial system has no independent physical definition — it is a set of metric-defined quantities used by a country that officially prefers metric but practically does not use it.

How much does the US lose annually by not using metric?

A 2015 study estimated over $50 billion per year in trade friction from dual measurement systems — including dual manufacturing runs, relabeling, engineering errors, and lost export contracts.