Si Units

Dictionary of Biological Psychology

Si stands for Système International (d’Unités)—the system of units used in measurement. The measures of main interest to biological psychologists are: length, measured in metres (m) and area, measured in square metres (m²); volume, measured in litres (l); and mass (weight), measured in grams (g). The SI unit of time is the second (s) and of temperature, the kelvin (K), which has absolute zero for zero. In practice, for temperature centigrade is used (degrees C: 0 °C is the freezing point of water, 100 °C the boiling point at standard pressure).

Units can all have prefixes placed in front of them to indicate multiplying factors. For example, a kilogram (kg) is 1000 g, a millilitre (ml) one thousandth of a litre. The prefixes used are as shown in the adjacent table.

These prefixes—either the full term or the abbreviation—are simply added to the unit being used, e.g. g, m, l or s. It is important to use the abbreviations in the case given here: for example, m is a milli-, a millionth; M is mega-, one million times. One would not wish to get these confused: one might, for example, order thousands times more of a chemical than one required, and have to pay for it.