SI units of measurement must be able to work across a very wide range. Look at the example given here showing the diameter of a human skin cell when compared with that of the planet Earth in metres. Drag the measurement box along the slider to see the size of the cell and the Earth.
It is obviously difficult to work with such large and small numbers, so SI units have a series of standard prefixes that act as shorthand for larger and small values.
When a quantity reaches 1000 X its units it is shortened to kilo units. Think of weight. If someone weighs 70,000 grams we write this as 70 kilograms. When a quantity reaches 1000000 X its units we write this as mega units. When we think of the output of power stations we think in terms of megawatts of output. When a quantity is only one thousandth of its unit, we write this as milli units. We often measure small lengths in millimetres. When a quantity is only one millionth of its unit, we write this as micro (µ) units. Some drugs are given in microgram quantities, for example. A full chart of prefixes can be seen by clicking the button at the bottom of the page.
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