System of measurement Facts for Kids

A baby bottle that measures in three measurement systems—imperial (UK), US customary, and metric.

A system of measurement is a set of units and rules for how they relate to each other. These systems have always been important for science and commerce (buying and selling things). Today, the main systems used are the International System of Units (also called SI or the metric system), the British imperial system, and the United States customary system.

History of Measurement Systems
- Where Systems Are Used Today
The Metric System
Imperial and US Customary Units
- Area Examples
Units of Currency
See also

History of Measurement Systems

Long ago, people made up their own ways to measure things. For example, they might use the length of a king's thumb or foot. They also used the length of a stride (a step) or an arm. These early units were often different from one place to another.

Over time, people needed more standard ways to measure. This was important for merchants who traded goods and for scientists doing experiments. The French Revolution in the late 1700s led to the creation of the metric system. This system was designed to be more logical and spread around the world. It slowly replaced many of the older, local units.

In most measurement systems, three basic things are measured:

length (how long something is)
mass (how much "stuff" is in something)
time (how long an event lasts)

Other measurements, like speed (how fast something moves), are made using these basic units. For example, speed is distance (length) divided by time. In the past, many different units were used for the same thing. For length, people used inches, feet, yards, miles, and more. Converting between these units was often tricky because they didn't use simple numbers like ten.

Scientists and engineers wanted a system that was the same everywhere and easier to use. Changing a whole measurement system costs a lot of money and effort. But the benefits of a clear, worldwide system eventually won out.

In the metric system, there is one main unit for each basic measurement. For example, the basic unit of length is the metre. To get smaller or larger units, you just multiply or divide by powers of ten. This means you simply move the decimal point. So, 1 metre is 1,000 millimetres, or 0.001 kilometres. This makes calculations much simpler.

Where Systems Are Used Today

Most countries around the world now use the metric system. However, the United States still mostly uses US customary units. Liberia and Burma also use their traditional units.

In Canada, people mainly use the metric system. But because they trade so much with the US, some US units are still used. Also, some older Imperial units are still found there.

The United Kingdom uses the metric system for most things. But for some uses, like road signs, they still use imperial units (miles and yards). In the US, scientists and the military use metric units a lot. But for everyday things at home, people mostly use customary units. For example, drinks are often sold in litres, and medicines are measured in milligrams.

Some special units are still used internationally. For example, nautical miles and knots are used for planes and ships.

The Metric System

The metric system started in France in 1795. Since then, it has grown and changed. It is now used almost everywhere in the world.

A great thing about metric units is how they are related. Smaller and larger units are made by multiplying or dividing by ten. Their names use prefixes like "kilo-" (meaning 1,000) or "milli-" (meaning 1/1,000). This makes it easy to switch between units.

The first metric system had two main units: the metre for length and the gram for mass. All other units, like those for area, volume, or density, were made from these two.

Today, the international standard for the metric system is called the International System of Units (SI). This system uses seven basic units. All other units can be made from these seven. The SI base units are:

the metre (for length)
the kilogram (for mass)
the second (for time)
the ampere (for electric current)
the kelvin (for temperature)
the mole (for amount of substance)
the candela (for light intensity)

Imperial and US Customary Units

Both imperial units and US customary units came from older English units. Imperial units were once used across the British Empire. Most of these countries have now switched to the metric system. The United States still uses its customary units as its main system. This is because it would be very expensive and difficult to change all their industries and businesses to metric.

Even though imperial and US customary systems are similar, they have some differences.

Length and Area: Units like the inch, foot, yard, and mile are the same in both systems.
Mass and Weight: Units larger than a pound (lb) are different. The imperial system uses a "stone" (14 lb) and different "hundredweights" and "tons." The US system does not use the stone. Its hundredweights and tons are lighter.
Volume: This is where the biggest differences are. A US fluid ounce is slightly larger than an imperial fluid ounce. However, there are 16 US fluid ounces in a US pint, but 20 imperial fluid ounces in an imperial pint. This means an imperial pint is about 20% larger than a US pint. The same difference applies to quarts and gallons. Six US gallons are a little less than five imperial gallons.

Besides the common Avoirdupois system for mass and weight, there are other older systems:

Troy weight: Used for precious metals like gold and silver. The troy ounce is still used today for these metals.
Apothecaries' system: Used in pharmacology (medicine) in the past, but now replaced by the metric system.

Area Examples

Sometimes, people describe large areas by comparing them to well-known places:

In the US, people might say a building is "the size of an American football field". A football field is about 100 yards long and 160 feet wide.
In the UK, people often compare areas to a football pitch. These pitches can vary in size, but a typical one for the UEFA Champions League is 105 metres by 68 metres.

Units of Currency

A unit of measurement for money is called a unit of account. This is usually the currency of a country, like the US dollar or the euro. The ISO 4217 is an international standard that gives three-letter codes for different currencies, like USD for US Dollar.

System of measurement facts for kids

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