Measurement is a process that uses numbers to describe things based on what we can observe about them. This is done to be able to compare them to each other. We can measure how big things are, how warm they are, how heavy they are, and lots of other features as well. Units of Measurement provide standards for our comparisons, so that the numbers from our measurements refer to the same thing. For example, the metre is a standard unit to measure length. Before 1982, it was defined as the distance between two markers on a special rod. Now scientists define the metre as a fraction of the speed of light.
Saying something has a length of 2 metres means that it is exactly twice as long as that rod used to define the metre, or that light takes twice the time defined for a metre to travel that distance.
Today, most units of measure fall into one of three systems. The older two, the British imperial system and the closely related US customary system use the foot as a measure of length, the pound as a measure for weight and the second as a measure for time. There are other units as well. In these systems the number of smaller units that make the bigger units varies. For example, there are 12 inches in a foot and 16 ounces in a pound. A newer system is the metric system or SI system which usually use 10 of the smaller unit to make the bigger one. It uses the metre for length, the kilogram for weight, and, as with the others, the second for time.
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Number and Unit of measure
The property of the thing being measured is given as a number of units of measure. The number only has sense when the unit of measurement is also given.
For example, The Eiffel Tower in Paris, France is 300 meters tall. That is, the distance from the top to the bottom of the Eiffel Tower is 324 meters. The property of the Eiffel Tower being measured is a distance. The number measured is 324. This number does not make sense without the unit of measure. The unit of measure is the meter.
Measurement Standards
Standards are special objects that are used to make measurements in terms of fixed units of measurement. A metre stick is an example of a standard. When you measure something with a metre stick, you can compare that measurement to anything else that is also measured with a metre stick. This makes measurement easier and comparisons between measurements easier.
Size of Units of Measurement
There are units of measurement of different sizes. There are small units of measurement to measure small things. There are big units of measurement to measure big things.
Science, medicine and engineering use smaller units of measurement to measure small things with less error. It is easier to measure large things using larger units of measurement. Large measurements like the width of a galaxy and small measurements like the mass of an atom use special units of measurement.
Systems of Units of Measurement
There are many different standards and units used all over the world.
Metric System
 see SI and Metric system
The metric system is a system of measurement used in most of the world. It is also called the International System of Units, or SI.
Units of measure in the metric system include:
 The units of length or linear size are based on the metre. They include the kilometre (km) which is 1000 meters, the centimetre (cm), and the millimetre (mm) which is 1/1000th of a meter.
 The unit of volume is the litre. It is used for measuring an amount of liquid. A millilitre (abbreviated as ml) is the amount of liquid that would fill up a cube that measures 1 centimetre on each side. One l litre of liquid would fill up a cube that is 10 cm on each side.
 The unit of mass is the kilogram. A kilogram (kg) weighs the same as a litre of water (at normal temperature, and pressure). 1 gram (g) is the weight of 1 millilitre of water at 0 degrees Celsius. The metric tonne is 1000 kilograms or a million grams.
British imperial Units
Imperial units were defined in the United Kingdom in 1825. These units sometimes based on similar units that were in use before 1825. Imperial units were used in countries that were part of the British Empire. While many of these countries, including the United Kingdom, have officially adopted SI, the older system of units are still used.
US customary units
US customary units are the official units used in the US. These are similar to the British imperial units and also based on the units used in the United Kingdom from before American Independence. But some of the units are different to the British ones. For example, there are 20 imperial fluid ounces in an imperial pint, but 16 US fluid ounces in a US pint. Additionally, the US fluid ounce is slightly bigger than the imperial fluid ounce. The result is that US pints and gallons are smaller than imperial pints and gallons. In the United States, the metric system has been legal for trade since 1866 but other measurements such as the gallon, inch, and the pound are still widely used.
Imperial and US units of measurement include:
 Length  inch (in), foot (ft), yard (yd), and mile.
 1 foot = 12 inches
 1 yard = 3 feet (plural of foot) = 36 inches
 1 mile = 1760 yards = 5280 feet
 Imperial volume  imperial fluid ounce (fl oz), imperial pint (pt), and imperial gallon (gal).
 1 imperial pint = 20 imperial fluid ounces
 1 imperial gallon = 8 imperial pints
 US volume  US fluid ounces (fl oz), US cup (cp), US pint (pt), US quart (qt), and US gallon (gal).
 1 US cup = 8 US fluid ounces
 1 US pint = 2 US cups = 16 US fluid ounces
 1 US quart = 2 US pints
 1 US gallon = 4 US quarts = 8 US pints
 Weight and mass are measured in ounces (oz) and pounds (lb), and stone (st) in imperial only.
 1 pound = 16 ounces
 1 stone = 14 pounds
The ounces for weight and volume are different. Even when measuring water, the number of ounces of weight is not the same as the number of fluid ounces.
Converting Between Systems
 Metric to US
 1 meter = 1.09 yards = 39.37 inches.
 1 liter = 33.3 fluid ounces = 1.76 pints = .26 US gallons.
 1 kilogram = 35.32 ounces = 2.2 pounds
 US to metric
 Length
 1 inch = 2.54 centimetres
 1 foot = 30.48 centimetres
 1 yard = .914 metres
 1 mile = 1.61 kilometres
 Volume
 1 fluid ounce = 29.6 millilitres
 1 pint = 473.1 millilitres
 1 gallon = 3.79 litres
 1 cup = 16 ounces
 Mass
 1 ounce = 28.35 grams
 1 pound = .45 kilograms
Other Units of Measure
Time
The unit of time is the second. The minute (60 seconds) and hour (60 minutes or 3600 seconds) are larger units. A day is usually said to be 24 hours, but is actually a little bit longer than that. This difference is corrected at the end of some years with what is called a leap second. A week (7 days) and month are also standard units.
Money
A unit of measurement that applies to money is called a unit of account. This is normally a currency issued by a country. For instance, the United States use dollars. Each dollar is 100 cents. The United Kingdom uses pounds. Each pound is 100 pennies or pence. Other countries in Europe use the Euro which is 100 cent for the Euro (they are not called "cents").
Images for kids

Units of measurement, Palazzo della Ragione, Padua

The former Weights and Measures office in Seven Sisters, London