It is important when prescribing medicines to have an understanding of how different units of measurement relate to each other.
Tables 1 and 2 describe the Standard International (SI) units that measure mass or volume that we are familiar in the context of prescribing:
Table 1. Prefixes
| Size | Prefx | Symbol |
|---|---|---|
| One hundredth | Centi | C |
| One thousandth | Mill | 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) |
- 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
The units of kilolitre and microlitre are rarely used in medicine. To convert from larger to smaller units we multiply by 1000 for each step. To convert from smaller to larger we divide by 1000.
QUESTIONS
Now try these exercises. Convert:
- micrograms to mg
- 250 mg to g
- 250 g to kg
- 300 ml to litres
- 30 ml to litres
- 0.8 litres to ml
- 0.08 litres to ml
- 4 kg to mg
- 4 mg to micrograms.
Check your answers on page 394