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. A basic dimensional analysis check on the interface — making sure both sides emitted the same units, not just the same numbers — would have caught it. 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.

Walkthrough: a real cross-border supply chain

US manufacturer ships steel fittings to a German construction firm. The fitting is specified as 1/2" NPT(a US National Pipe Thread, sized in inches). The German firm’s drawings call for ISO 228 G1/2 — a metric pipe thread that’s nominally the same size (around 21 mm OD) but structurally incompatible: NPT is tapered, G is parallel; the threads-per-inch differ; the sealing surface works differently.

Despite both being labelled “1/2 inch,” an NPT fitting cannot be installed into a G1/2 socket reliably. Either an adapter ($5-20 per joint plus labour), a complete redesign at the US factory ($10k-100k tooling change), or a different supplier. Total friction cost on a typical industrial order: 3-8% of order value in adapters, rework, and delayed schedules.

Multiply by the millions of US-to-EU industrial components shipped annually and the “modest” 0.1-0.3% GDP-drag estimate starts to look conservative at the firm level even if it’s accurate in aggregate. Some firms absorb it as cost of doing business; others ship dual-spec product lines; a few have given up on the foreign market entirely.

Common mistakes

• Confusing US customary with imperial. They’re different systems. A US gallon (3.785 L) is not a UK imperial gallon (4.546 L). A US fluid ounce (29.57 mL) is not a UK fluid ounce (28.41 mL). Recipes and fuel-economy figures don’t cross-convert directly. See our US vs UK cup comparison.
• Assuming metric is decimal everywhere. Time (60 seconds, 60 minutes, 24 hours) is metric by adoption but not decimal. Angle (degrees, minutes, seconds) is similarly mixed. Pure-decimal alternatives (decimal time, grad angle) exist and are essentially unused.
• Reading temperature without checking the scale.“It’s 30 degrees out” is shorts weather in Celsius (~86°F) and parka weather in Fahrenheit (~-1°C). Most international weather reports default to Celsius; US domestic to Fahrenheit. Travel apps default to the device locale.
• Trusting unit prefixes loosely.The SI prefix “k” means exactly 1000. The computing prefix “K” (in KB, KiB) means 1024 in some contexts and 1000 in others. Storage vendors use 1000; OS reporters use 1024 — which is why a “1 TB” drive shows as 931 GB. The IEC binary prefixes (KiB, MiB, GiB) exist specifically to remove the ambiguity but are unevenly adopted.
• Quoting fuel economy across systems. US miles-per-gallon (higher is better) and EU litres-per-100km (lower is better) move in opposite directions. 30 mpg US ≈ 7.8 L/100km. Direct comparisons require conversion in both axis and direction.

For the related fuel-economy comparison, see our fuel economy guide. For temperature-scale handling, the Celsius vs Fahrenheit comparison covers the math.

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); IEC 80000-13 on binary unit prefixes.