Metric system

The metric system is a system of measurement that standardises a set of base units and a nomenclature for describing relatively large and small quantities using decimal-based multiplicative unit prefixes. It helps everyone use the same measurements, making trade, science, and everyday life much easier. The modern version of this system is called the International System of Units (SI).

The SI defines seven base units: metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for temperature, mole (mol) for amount of substance, and candela (cd) for light intensity. From these, many other units are derived, like the hertz for frequency and the newton for force.

Unlike older systems that used different units in different places, the metric system is used around the world. It is especially important in science, where consistent measurements are crucial for experiments and discoveries. Today, all base units are defined by physical constants, meaning they are based on unchanging parts of nature rather than physical objects.

Adoption

The SI system is used by most countries around the world as their official way to measure things.

One major exception is the United States, which still uses different measurement systems in many areas even though it sometimes uses the metric system. The process of switching to the metric system is called metrication.

Multiplicative prefixes

Main article: Metric prefix

In the metric system, we can make numbers bigger or smaller by adding special words, called prefixes, to the unit names. These prefixes tell us how many times bigger or smaller the number is compared to the basic unit. For example, the prefix "kilo" means 1000, so a kilometre is 1000 metres. The prefix "milli" means one-thousandth, so a milligram is one-thousandth of a gram. This helps us easily understand and work with very big or very small numbers.

Base units

The metric system started in France in the 1790s with the metre as its main unit of length. Over time, this system was improved and became the International System of Units (SI) in the mid-20th century. Today, the SI uses seven base units to measure different things in nature. These base units help us describe many quantities by combining them in different ways. For example, area is measured in square metres, which comes from multiplying length by length.

There have been many metric systems throughout history, such as the MKS system of units, the centimetre–gram–second (CGS) system, and others. All these systems aim to provide a universal way to measure things accurately as science and technology advance.

Attributes

The metric system is designed to be easy to use and understand. It uses base units that are based on natural phenomena and builds other units from them using logical, decimal-based relationships. This makes calculations straightforward and the system simple to learn.

The metric system can be extended with new units as needed. For example, the katal, a unit for measuring catalytic activity, was added in 1999. The system ensures that its base units can be reliably measured in any well-equipped laboratory, without needing special artifacts. The metre, for instance, was originally defined based on the Earth's size but is now defined by the distance light travels in a specific fraction of a second. Similarly, the kilogram was redefined in 2019 to rely on fundamental constants of nature rather than a physical object. The metric system uses decimal ratios, making it easier to work with large or small measurements by using prefixes like kilo- for 1,000 times larger or milli- for 1/1,000th of a unit. This contrasts with older systems where unit ratios could be irregular and harder to remember.

Everyday notions

The metric system uses basic units that match everyday things. For example, the length around the Earth's equator helped define the metre, and a typical dining table is about 0.75 metres high. A very tall person might be about 2 metres tall.

We measure time in seconds, minutes, and hours based on the Earth's rotation. For mass, everyday items like coins have measurable weights—a 1-euro coin weighs 7.5 g, for instance. Temperature in everyday life is usually measured in Celsius, where water freezes at 0 °C and boils at 100 °C at sea level. The body temperature of a human is about 37 °C.

The metric system also connects different measurements. For example, one litre of water weighs one kilogram, and one millilitre of water occupies one cubic centimetre. These relationships make the metric system easy to use for many common tasks.

History

Main article: History of the metric system

The metric system began as an effort to standardize weights and measures across different regions. For centuries, places like China, Egypt, Greece, and Rome each had their own systems, which made trade difficult.

In the 17th and 18th centuries, a priest named Gabriel Mouton suggested using a single system based on the Earth's size. Later, scientists Antoine and Anne-Marie Lavoisier helped create the first metric system in France. They defined the metre based on the Earth's dimensions and the kilogram based on the weight of water. This new system was adopted in France in 1799, aiming to be used worldwide.

Over time, many metric systems were developed, each with slight differences but all building on early French standards.