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The International System of Units
The International System of Units (abbreviated SI) is a metric system in worldwide use that is based on decimal units of physical quantities.
The International Organisation for Standardisation issues the guidelines for the use of physical quantities, the measurement units and their formulae in ISO31.
| Physical Quantity | Unit (Symbol) |
| length | meter (m) |
| mass | gram (g) |
| time | second (s) |
Prefixes are used for multiples such as mega, giga, tera, peta etc. and submultiples micro, nano, pico, femto etc.
| How much memory is there on a 20GB hard disk? |
| That would be a multiple of the unit for the hardware memory: a GigaByte is 109 Bytes. |
| How small is a nanotube? |
| It is at the molecular level of nanometres, 10-9metres. |
| How fast is a femtosecond laser? |
| It is as fast as 10-15seconds. |
| How powerful is a lightning stroke? |
| The average peak power per lightning stroke is a a terawatt (1012 W) and the average duration is around 30 microseconds (10-6). |
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