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Scales in space and time
Scales in space and time

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 Units with prefixes

The SI system also includes standard prefixes that provide another way to write large and small values more efficiently. You will be familiar with a kilo, meaning a thousand, from a kilometre (1000 m), and also centi, meaning a hundredth, from centimetres (0.01 m, or one-hundredth of a metre). So, instead of writing 3000 metres, write 3 kilometres, or 3000 m = 3 km. Similarly, 0.04 m = 4 cm.

Table 2 contains a selection of prefixes, from the large to the small. Note that the case, or capitalisation, of the symbol is important; for example, a mm is a thousand million (i.e. a billion) times smaller than a Mm. Also, there is not a prefix for every power of ten. Some prefixes, such as deca (da), are rarely used.

Table 2  Examples of prefixes
Name Symbol 10n Meaning
giga G 109 1000 000 000
mega M 106 1000 000
kilo k 103 1000
hecto h 102 100
deca da 101 10
    100 1
deci d 10−1 0.1
centi c 10−2 0.01
milli m 10−3 0.001
micro µ 10−6 0.000 001
nano n 10−9 0.000 000 001

Apart from centi, which is commonly used in centimetres, and deci, which is sometimes used to measure volume in decilitres, the major prefixes increase or decrease by three powers of 10: kilo (103), mega (106), giga (109), for example.

  • How would you describe 0.000 003 seconds using a SI prefix?

  • 0.000 003 seconds can be written as 3 microseconds, or 3 µs.

  • How would you describe 0.045 metres using a SI prefix?

  • This can be written as 45 millimetres or 45 mm, or alternatively as 4.5 centimetres or 4.5 cm. Both are correct, but one may be more appropriate than the other in a given context.

Although metres and seconds are the standard SI units for length and time, other units are sometimes used that are appropriate to scales frequently encountered in a particular field. For example, the age of a person might be given in years (y) rather than in seconds as it is more meaningful in that context. However, it can be easily converted from one unit to another, and during this course you will become familiar with that process.

Note that you will encounter both SI units and other commonly used units in the size–time explorer.

Don’t forget, if you need any guidance on the maths content, take a look at the badged open course, Mathematics for science and technology [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .