Why metric units won everywhere except the US (and what it costs)

Of the world’s 195 sovereign states, three have not officially adopted the metric system: the United States, Liberia, and Myanmar. Everywhere else uses metric for government, commerce, science, and daily life. The US is the anomaly that matters because it’s 14% of global GDP, accounts for 80%+ of internal commerce among non-metric usage, and runs the most-cited scientific journals on a slightly schizophrenic units footing.

How metric won the rest of the world

The metric system was created during the French Revolution (1790s) as a decimal alternative to the patchwork of local units used across pre-modern Europe. The Système International d’Unités (SI) was formalised in 1960 and is now the legal basis for measurement in 193 of 195 UN member states.

Major adoption dates:

• France — 1795 (compulsory 1840).
• Germany — 1872 (unified empire-wide).
• UK — partial 1965, retail mandatory 2000 (though pints, miles, and feet still survive in road signs, beer, and personal height).
• Australia — completed 1988 (one of the most successful national transitions).
• India — completed 1962 (despite the old Indian system being intricate, the transition was rapid).
• Japan — 1959 (legally). Traditional units (shaku, koku) survive in agriculture and crafts.
• China — completed mid-20th century; traditional units retained colloquially (e.g., jin = 0.5 kg).

The US almost-transitions

Three serious attempts:

1. The Metric Act of 1866

Congress authorised the use of metric units throughout the US. Didn’t require it. Result: metric adopted in some scientific contexts, never in commerce.

2. The Metric Conversion Act of 1975

Declared metric “the preferred system of weights and measures”. Created a US Metric Board to coordinate voluntary conversion. The board was abolished in 1982 with almost no progress made. Voluntary adoption was the legal framework; nobody volunteered.

3. The Omnibus Trade Act of 1988

Required federal agencies to use metric by 1992. Many did (NASA, NIST, the military). Civilian commerce continued using customary units. The grocery store, the construction site, and the gas station never made the switch.

What customary units cost the US

Concrete examples where the dual-system cost real money or lives:

• Mars Climate Orbiter (1999).Lost on arrival because the spacecraft’s onboard software expected metric Newton-seconds and Lockheed Martin’s ground software output customary pound-seconds. Cost: $125 million plus the mission objective.
• Manufacturing for export. US manufacturers targeting EU customers either maintain dual production lines or accept ~5% friction on every component spec conversion. Roughly $40 billion of annual cost is estimated by the National Institute of Standards and Technology (NIST, 2018 estimate).
• Pharmaceuticals and healthcare. US hospitals chart weight in pounds for adults, kg for paediatrics. Medication dosing errors caused by unit confusion happen; the Institute for Safe Medication Practices documents thousands annually.
• Construction materials.Lumber is sold in nominal inches (a “2×4” is 1.5×3.5 actual), pipes in nominal inches that don’t match metric pipe standards, fasteners in fractions vs metric coarse threads. Cross-border construction projects double the parts list.

Where the US has quietly gone metric

Despite the customary-system reputation:

• Soft drinks are sold in litres (2L bottles) since the 1970s.
• Wine and spirits are in 750 ml and 1.75 L bottles by federal regulation (since 1979).
• Skis, bicycles, firearms are routinely metric. The military uses metric exclusively for combat operations.
• Pharmaceuticals and lab work are essentially all metric.
• Cameras, lenses, electronics are metric.

The customary system holds in groceries (pounds and ounces), road distances (miles), gasoline (gallons), home construction (feet and inches), and casual personal measurement (height in feet, weight in pounds).

The cost-benefit of the holdout

Pure costs to staying customary:

• Dual production lines for export.
• Education time on conversion (US schoolchildren spend significant time learning both systems).
• Errors at boundaries (medical, engineering, aerospace).

Pure benefits:

• No transition cost (the cost of switching is real and front-loaded; Britain’s 1960s-2000s metrication cost billions).
• Familiar units for the consumer base.
• Body-scaled units (foot, inch, gallon) feel intuitive to humans in a way millimetres and litres don’t.

Most economists estimate net cost to the US is real but modest as a share of GDP — maybe 0.1-0.3% annually, comparable to other minor regulatory frictions.

Practical takeaways for travellers and traders

1. Speed limits in miles per hour only in the US, UK, and a few small territories. Everywhere else, km/h. Read the units before you read the number.
2. Temperature in Fahrenheit only in the US, the Bahamas, Liberia, and a handful of Caribbean islands. Celsius everywhere else. See our Celsius vs Fahrenheit comparison.
3. Body weight in pounds (US, UK informally), kilograms (continental Europe, most of Asia and Latin America), or stones+pounds (UK formal). Three completely different scales for the same measurement.
4. Cooking is the highest-friction zone. US recipes use cups (volume) for dry ingredients; the rest of the world uses grams (weight). See our cups to grams baking guide.

Use our converters for length, weight, temperature, and more. The methodology page for unitscovers the SI definitions and why metric’s decimal structure won the rest of the world.

Sources: NIST “The United States and the Metric System” (publication SP1138, 2024 edition); BIPM SI Brochure 9th edition; UK Metric Association archive of national metrication transitions; House report on the Mars Climate Orbiter loss (NASA Mishap Investigation Board, 2000).