As a prescriber, you may encounter doses of medicines that are expressed in either grams, milligrams or micrograms or volumes of medicines that are expressed in litres, millilitres or occasionally microlitres when referring to a very small quantity, such as a small number of drops of liquid.
It is important to understand the relationship between these units of mass and volume. The examples in this article provides some practice in converting units. For the purpose of this drug calculation exercise, the standard international (SI) units used will relate to mass and volume:
- Mass – the SI unit used to measure mass is the ‘gram’
- Volume – the SI unit used to measure volume is the ‘litre’.
UNITS OF WEIGHT AND VOLUME
- 1 kilogram = 1000 grams
- 1 gram = 1000 milligrams
- 1 milligram = 1000 micrograms
- 1 litre = 1000 millilitres
To convert from larger to smaller units, multiply by 1000 for each step. To convert from smaller to larger, divide by 1000.
- QUESTION 1 Convert 250 mg to g.
- QUESTION 2 Convert 0.7 g to mg.
- QUESTION 3 Convert 50 ml to litres.
- QUESTION 4 Convert 2 litres to ml.
- QUESTION 5 Convert 568 ml to litres.
- QUESTION 6 Convert 400 micrograms to mg.
- QUESTION 7 Convert 893 micrograms to mg.
- QUESTION 8 Convert 0.63 g to mg.
- QUESTION 9 Convert 2.5 kg to g.
Table 1. Prefixes
| Size | Prefix | Symbol |
|---|---|---|
| One hundredth | centi | c |
| One thousandth | milli | m |
| One millionth | micro | micro, µ |
Table 2. SI units with prefix
| kilogram (kg) | gram (g) | milligram (mg) | microgram (mcg) |
| kilolitre | litre (l) | millilitre (ml) | microlitre (mcl) |