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Basic drug calculations

Metric system - Metric basics

The metric system is an international standard of measurement based on decimals. It is used to measure weight, liquids, and length. The modern metric system, introduced in Australia in 1970, is called the International System of Units (SI).

Basic units

The metric system uses a 'basic unit' for each type of measurement, represented by a symbol. Basic units commonly used in nursing and midwifery include:

Type of measurement
Basic unit

weight (mass)


liquids (volume)





* To avoid confusion when the symbol for litres is used alone, it is best to use the upper case L.

Variations to the basic unit (smaller or larger amounts)

Below are some of the common variations to basic units to show smaller or larger multiples of the unit. To indicate these variations, a prefix is added to the basic unit. Note the symbols for each prefix. Examples of everyday objects using these measurements are shown.

Common variations:

one millionth

one thousandth

Basic units:

one thousand times


micro (µ)
milli (m)
kilo (k)


(µg or mcg or microgram)

image of microscope

It is impossible to show a microgram!


milligram (mg)

pinch of salt

A pinch of salt weighs approximately 1 milligram

gram (g)


A paperclip weighs approximately 1 gram

kilogram (kg)

bag of flour

1 kilogram of flour


microlitre (µL)


A syringe can measure microlitres.

millilitre (ml)

drip from a tap

The drip from a tap is approximately 1 millilitre.

litre (L)

litre of milk

1 litre of milk




Bacteria can be measured in micrometres.

millimetre (mm)


A CD is approximately 1 millimetre deep (120 mm across)

metre (m)

a large adult stride

A large adult stride is approximately 1 metre.

kilometre (km)

long road

Roads are measured in kilometres.


If you are new to the metric system it can be helpful to associate real objects with their metric measurements to build up your own awareness of how to estimate using metric measurement. Try these learning activities. (requires Flash player).

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