Why Most Countries Use Kilograms

The metric system was developed in France in the 1790s, during the French Revolution. Scientists and reformers wanted a rational, decimal system based on natural constants rather than arbitrary historical artifacts — like the length of a king's foot or the weight of a specific grain of barley. The meter was defined as one ten-millionth of the distance from the North Pole to the equator; the kilogram as the mass of one liter of water.

France adopted the system in 1795. Napoleon spread it across conquered Europe. Over the 19th century, most European nations adopted metric for science and trade. The 1875 Metre Convention established an international bureau (BIPM) to maintain metric standards, and signatory nations committed to using the system for international trade.

International trade was the primary driver. As the global economy integrated, incompatible measurement systems created friction — different countries' 'pounds' and 'feet' were not identical, causing errors in manufacturing, shipping, and commerce. A single universal system reduced that friction. By the mid-20th century, adopting metric was also tied to joining international scientific, military, and trade organizations.

Colonial history also played a role. Countries that were former French, Spanish, or Portuguese colonies often inherited metric systems. Former British colonies received imperial units but many switched to metric after independence — partly because their primary trade partners were moving metric, and partly as a deliberate break from colonial legacy.

The US has been officially metric-friendly for longer than most people realize. The Metric Conversion Act of 1975 designated metric as the 'preferred system' for trade and commerce, and the Omnibus Trade and Competitiveness Act of 1988 required federal agencies to use metric for procurement and grants. But both laws were voluntary — no consumer mandate, no timeline for everyday adoption.

The real barriers have been economic and cultural inertia. Replacing every road sign, consumer product, industrial machine, and measurement standard requires enormous upfront investment. The US also has a uniquely large domestic market — American businesses can afford to ignore metric because they can sell domestically at scale without it. Countries with smaller domestic markets needed metric to export.

Even in the US, kilograms dominate in professional contexts. The pharmaceutical industry doses all medications in milligrams per kilogram of body weight. The military uses metric for ammunition and equipment specifications. NASA and all scientific research use metric. Food nutrition labels show serving sizes in grams. International shipping rates are calculated in kilograms.

American consumers encounter kilograms on imported food packaging, at some gyms (bumper plates are marked in kg), on medical scales in hospitals (which show both lb and kg), and whenever they travel internationally.