Metrication facts for kids

Cars in the US and UK often show speed in both miles per hour (MPH) and kilometres per hour (km/h). In Canada, it's usually km/h only, or km/h first.

Most countries, like Australia, use only kilometres per hour (km/h) for car speed.

Metrication is the process of changing from older ways of measuring things to the metric system. This system uses units like metres for length, kilograms for weight, and litres for volume. Many countries around the world have switched to the metric system. This big change started in France in the 1790s. Even today, over 200 years later, some countries and industries are still working on fully adopting the modern SI system, which is the official name for the metric system.

Why Countries Go Metric

Most countries now use the metric system as their official way to measure things. Some countries have made it official but are still working on using it everywhere. A few countries have not yet decided to switch. It's hard to get an exact number of countries in each group.

As of 2010, a study showed that seven countries had not fully adopted the metric system. These were the Federated States of Micronesia, Liberia, the Marshall Islands, Myanmar, Palau, Samoa, and the United States. However, other sources, like the U.S. Central Intelligence Agency (2016), say only Myanmar, Liberia, and the United States had not adopted it.

Some countries, like Guyana and Antigua and Barbuda, officially adopted the metric system but found it hard to use everywhere. The United Kingdom also uses the metric system for most official trade. But it still uses some older units, like miles for road distances and pints for beer.

Some countries, like the United States, the United Kingdom, and Canada, have people who don't want to switch to the metric system. They prefer their older ways of measuring.

Measuring Before the Metric System

Before the metric system, people used many different ways to measure. The Romans used a "foot" (pes) that had 12 "inches" (unciae). They also used a "pound" (libra). These units changed a lot over time and from place to place.

In medieval Europe, each city or trade group had its own rules for weights and measures. For example, the "ell" was a unit of length. But its length could be 40.2 centimetres in one part of Germany and 94.5 centimetres in Edinburgh! In 1838, a study in Switzerland found 37 different versions of the "foot."

Even in Imperial China, where units were once standardized, things changed. By 1936, there were 53 different values for the "chi" (a length unit).

The idea for a single international system came from the growth of global trade. Companies needed to know that products from other countries would be the same size or weight as described. The metric system helps with this because it is used worldwide.

The metric system also makes learning and using measurements easier. All its units are based on a few main units, like the metre, kilogram, and second. They use decimal prefixes (like kilo-, centi-, milli-) to show bigger or smaller amounts. This is different from older units like inches, feet, and miles, which have different names and confusing conversion rates (like 12 inches in a foot, 3 feet in a yard, 1760 yards in a mile).

With the metric system, measurements are always in decimals. This makes adding and multiplying them much simpler. It also helps science and technology because units for things like electricity and force are all connected in the SI system.

Early Ideas for a Metric System

Decimal numbers are key to the metric system. They allow you to use one main unit and easily make it bigger or smaller by multiplying or dividing by 10.

In 1585, a man named Simon Stevin first suggested using decimal numbers for everyday things. He thought they would be great for money and measurements. Later, the decimal point was invented, making it even easier.

In 1670, Gabriel Mouton suggested a system where the main unit of length would be based on the Earth's size. He also proposed using prefixes, similar to what we have in the SI system today.

In 1790, Thomas Jefferson in the United States suggested a decimal system for money and measurements. He wanted a "foot" unit based on a pendulum's swing. His ideas were similar to earlier ones, but they didn't become the official system in the US.

How the Metric System Spread

This decree from April 1795 made the metric system official in France.

France officially adopted the metre in 1801. However, the change was difficult for people. There weren't enough metre sticks, and many people couldn't read. This made the metric system unpopular at first.

Napoleon, who ruled France, even made a new system called mesures usuelles (customary measures) in 1812. This system used old unit names but redefined them using metric values. For example, a "pound" (livre) became exactly 500 grams. This helped people get used to the new system. In 1840, France went back to using only the metric system.

Germany's Switch to Metric

Before the French Revolution, Germany was made up of many small states. Each had its own measurement system. During Napoleon's time, some German states started using metric-based units. For example, the "pound" (Pfund) in Baden was set to 500 grams.

In 1832, Carl Friedrich Gauss suggested adding the second to the metre and kilogram to create a more complete system. This led to the CGS system. Scientists worked together across Europe to standardize measurements for studying the Earth.

In 1872, the metric system became the official system in the new German Empire.

Italy's Metric Journey

When Napoleon controlled parts of Italy, the metric system was introduced. After Napoleon's defeat, Italian states went back to their old systems. But by 1860, most of Italy was united, and the metric system became official across the kingdom in 1861. Conversion tables were displayed to help people switch.

Netherlands Adopts Metric

The Netherlands started using the metric system in 1799. Like France, they had some problems with the change. In 1816, the Netherlands created its own "Dutch" metric system. It used metric units but gave them traditional Dutch names. For example, an "ounce" (ons) was defined as 100 grams.

Norway's Quick Adoption

In 1875, Norway was the first country to sign the Metre Convention, which created international standards for the metric system. It was a very fast decision by the Norwegian Parliament.

Portugal's Metric Change

Portugal officially adopted the metric system in 1814. They also used traditional Portuguese names for the new metric units. For example, a "pound" (libra) became 1 kilogram.

Spain's Switch to Metric

Before 1700, each region in Spain had its own measurement system. The new rulers tried to centralize things. Spain helped French scientists measure the Earth, but they were also worried about the French Revolution. So, they kept their old units for a while.

However, by 1849, it was too hard to keep the old system. The metric system became the legal system in Spain. Spanish scientists worked with others in Europe to create accurate metric standards. Spain was a key player in setting up the Metre Convention in 1875, which created the International Bureau of Weights and Measures.

Switzerland's Path to Metric

Switzerland tried to introduce the metric system in 1801, but it wasn't fully used. Different regions (cantons) used different systems. In 1835, some cantons adopted a system based on a "federal foot" (0.3 metres).

In 1838, Guillaume-Henri Dufour created the first official map of Switzerland using the metre as the unit of length. In 1868, the metric system became legal alongside the federal foot. Finally, in 1875, Switzerland joined the Metre Convention, and the metric system became compulsory from 1877.

United Kingdom's Metric Journey

In 1824, the UK created one standard "imperial" system for its empire. This system standardized existing British units but didn't switch to metric.

Over the next 80 years, many groups suggested adopting the metric system. In 1864, metric units became legal for "contracts and dealings." The UK signed the Treaty of the Metre in 1883. British scientists were very involved in promoting the metric system.

In 1965, the UK started an official plan to go metric. This plan is still not fully finished. The metric system is used for most businesses and trade. But imperial units are still common for things like road signs (miles) and beer (pints). People also often use imperial units for their height and weight.

United States' Metric Journey

In 1805, Ferdinand Rudolph Hassler brought copies of the French metre and kilogram to the United States. In 1830, Congress decided to create standard measurements. Hassler suggested the metric system, but Congress chose British standards instead. However, the US Coast Survey used the metre for its measurements.

In 1866, the United States Congress made it legal to use the metric system in the US. But it wasn't mandatory. In 1893, the US officially defined its traditional units (like the foot and pound) based on the international metric standards.

Attempts to make the metric system mandatory in the US failed. The main reasons were the cost and inconvenience of changing. The US officially adopted the metric system for government use in 1975. However, it remains voluntary for the public and most businesses.

Today, the American public and many businesses still use US customary units. But metric use has grown, especially in manufacturing and education. For example, NASA decided to use metric units for all future moon missions.

Metrication in Other English-Speaking Countries

Many countries in the Commonwealth started metrication after the UK. India began in 1959. South Africa, New Zealand, Australia, and Canada all started their programs in the late 1960s and early 1970s.

Australia, New Zealand, and South Africa mostly completed their switch within ten years. In Canada, the process stopped in the 1970s. Canadians still use imperial units for some things, like square feet for housing and miles for railways that cross into the US. Many food labels show both metric and imperial units.

Most other Commonwealth countries adopted the metric system in the 1970s. Ireland finished its metric conversion in 2005. Hong Kong uses three systems: traditional Chinese, British imperial, and metric.

When Countries Went Metric

The metric system officially started in France in December 1799. In the 1800s, almost all European countries adopted it:

Portugal (1814)
Netherlands, Belgium, Luxembourg (1820)
Switzerland (1835)
Spain (1850s)
Italy (1861)
Romania (1864)
Germany (1870, legal from 1872)
Austria-Hungary (1876, law adopted 1871)

Later, Thailand adopted it in 1923, and Denmark and Iceland in 1907.

How Countries Switch Systems

There are three main ways countries change to the metric system:

The "Big-Bang" Way: This is a fast switch. The country makes older measurements illegal, changes all government documents, and teaches only metric in schools. India used this method. Its changeover took only two years (1960-1962). Australia and New Zealand also did a quick change.

The Phase-In Way: This is a slower process. A law is passed to allow metric units alongside old ones. Then, people are educated about metric, and slowly, older measures are banned. The UK and Canada used this method, and the process is still not complete in those countries. Japan also took a long time (70 years) with this method.

Redefining Old Units: This method changes old units to have exact metric values. For example, in the Netherlands, 500 grams is often called a pond (pound). In Sweden and Norway, a mil (Scandinavian mile) is now 10 kilometres. This helps people get used to metric values while still using familiar names.

It can be hard to know how much ordinary people actually use metric in their daily lives. Older people often stick to the older units. In Canada, people are comfortable with both Fahrenheit and Celsius for cooking. In the UK, Fahrenheit is rarely used, but miles are still common for road signs.

Things Still Measured Differently

Even in countries that mostly use the metric system, some older units are still common in specific areas:

Camera parts: Tripod screws are often measured in inches.
Tyre pressure: Often measured in "psi" (pounds per square inch) in many countries.
Engine power: Often measured in "horsepower" instead of kilowatts.
Construction: Workers might use old inch-based names for planks and nails.
Boat length: Small sailboats are often measured in feet.
Housing space: Often sold or rented in square feet or square yards in many countries.
Plumbing: Some pipes and threads are still in inch sizes.
Oven temperatures: Some ovens use "gas marks" or Fahrenheit-based scales.
Wheel diameters: Car and bicycle wheels are usually measured in inches.
Computer screens: TV, computer, and phone screens are measured diagonally in inches.
Precious metals: Gold and silver are often sold by the troy ounce.
Golf courses: Measured in yards in many countries.
Fuel: Some countries still use gallons (imperial or US) for petrol.
Road signs: Distances and speed limits are in miles in the USA, UK, and some Caribbean nations.

In countries like Antigua and Barbuda and Saint Lucia, the switch to metric is still happening.

United Kingdom Exceptions

In the UK, many people still prefer imperial measures for some things. While businesses mostly use metric, imperial units can be displayed alongside them. For example, road signs use miles, and beer is sold in pints. People also commonly use stones and pounds for weight, and feet and inches for height.

The UK government started a big metrication program in 1965, aiming to finish in 10 years. But it wasn't made compulsory, and the program stopped in 1980. The UK negotiated to keep using miles on road signs and pints for beer and milk.

United States Exceptions

The metric system has slowly influenced the US through trade. It became legal to use in 1866. The US government officially adopted it in 1975 for military and government agencies. But it's still voluntary for the public.

Labels on many products must show both metric and US customary units. Most states now allow metric-only labels for products they regulate. However, the American public and many businesses still use US customary units.

Canada Exceptions

Canada allows both metric and imperial units on goods. The metric unit must be listed first. For liquids, it must say if it's a US or Canadian (Imperial) unit.

Air and Sea Travel Measurements

Air and sea travel often use the nautical mile. This unit is about 1,852 metres (1.151 miles). Speed is measured in "knots" (nautical miles per hour).

Aviation has moved towards metric, but altitude is still often given in feet in many countries. Runway lengths are usually in metres. The International Civil Aviation Organization (ICAO) wants a single metric system worldwide, but allows feet for altitude.

The International Maritime Organization (IMO) decided that future shipping rules should use only SI units.

Mistakes from Unit Confusion

Sometimes, confusion between measurement systems can cause problems.

In 1983, an Air Canada plane ran out of fuel mid-flight. This happened because of a mistake in converting between litres, kilograms, and pounds. The plane got 22,300 pounds of fuel instead of the needed 22,300 kilograms.

In 1999, the Mars Climate Orbiter spacecraft was lost. One team used metric units, but a subcontractor provided data in pound-force-seconds instead of newton-seconds (metric). This error caused the spacecraft to fly too low and burn up in the Martian atmosphere.

Images for kids

Conversion table in an 1848 German schoolbook showing the metric equivalent of the foot in use in over two dozen countries, including several German states
Johann Jacob Baeyer, founder of the Europäische Gradmessung
Stone marking the Austro-Hungarian/Italian border at Pontebba displaying myriametres (10 km), a unit used in Central Europe in the 19th century
Tablet showing conversions of legacy units of weights and measures to metric units, Vicopisano, Tuscany
Don Carlos Ibáñez e Ibáñez de Ibero, first president of the International Geodetic Association and of the International Committee for Weights and Measures
Adolphe Hirsch, secretary of the International Geodetic Association and of the International Committee for Weights and Measures
Guillaume Henri Dufour, founder of Swisstopo
Heinrich von Wild, president of the International Meteorological Organization and member of the International Committee for Weights and Measures
The metric units of measurement on Canadian canned food labels are merely the equivalent of the still widely used imperial units such as the ounce
Countries using the metric, imperial and US customary systems as of 2019 (based on popular and official usage).
Precious metals, such as gold, are often measured in troy ounces.
A shower hose sold in Germany with metric length (150 cm) and imperial (1⁄2-inch) fittings.
A measuring cup, manufactured and sold in the United States (circa 1980) features graduations in both metric and US customary systems. Held in the right hand, a person would have the metric graduations in front, facing them. But in filling the measuring cup from another container, right-handed people would hold it in their left hand, and read from the customary graduations.
Ferdinand Rudolph Hassler, first superintendent of the United States Coast Survey