When carrying out drug calculations, you will need to be familiar with the units of measurement used. These units form part of the Standard International (SI) system of measurement, also referred to as the Metric System.
| Quantity | Units | Symbol | Relationship |
|---|---|---|---|
| Mass (weight) | kilogram | Kg | |
| gram | g | 1 Kg = 1,000 g | |
| milligram | mg | 1 g = 1,000 mg | |
| microgram | mcg or μg | 1 mg = 1,000 μg | |
| Volume | litre | L | |
| millilitre | ml | 1 L = 1,000 ml | |
| cubic centimetre | cc | 1 ml = 1cc | |
| Amount | mole | mol | |
| millimole | mmol | 1 mol = 1,000 mmol |
You will come across situations where you need to convert a measurement from one unit to another, for example, from micrograms to milligrams or from litres to millilitres.
You will need to do this if quantities given in a calculation are not in the same units. In this case, convert one of the quantities so that they are in the same units.
Tip: Unless the units required are specified, it is a good idea to convert to the smaller units so that you are dealing with whole numbers rather than decimal values.
The main units used are those that are a thousand times bigger or smaller.
We could replace grams in the diagram with litres, metres or moles. The same prefixes would be used.
Express 5mg in micrograms.
SolutionTo convert milligrams to micrograms we need to multiply by 1000.
As we are dealing with a whole number, this would involve adding three zeros on the end.
5 mg = 5 × 1000 = 5000 μg or 5000 mcg
Express 0.5g in milligrams.
To convert grams to milligrams we need to multiply by 1000.
As we are dealing with a decimal value, multiplying by a thousand would involve moving the decimal place three places to the right.
Let’s add some zeros on the end so we have somewhere to move the decimal point to.
The last zero isn’t needed as it comes after the decimal point.
So 0.5 g = 0.5 × 1000 = 500 mg
Express 300ml in litres.
SolutionTo convert millilitres to litres we need to divide by 1000.
This involves moving the decimal place three places to the left.
Let’s add some zeros on the left to make sure we have somewhere to move the decimal point to.
The first and last zeros are not needed as the decimal point ends up after the second zero.
So 300 ml = 300 ÷ 1000 = 0.3 L
A patient is ordered 1.2g of a medicine. The medication is available in 400mg tablets. How many tablets will you administer?
Notice that in this example, the units for the required dose are different to that of the stock dose.
If we convert the required dose from grams to milligrams, then 1.2g = 1200 mg
The number of tablets required is
Express 0.75 grams in milligrams
Express 500 grams in kilograms
Express 56 milligrams in grams
Express 30 micrograms in milligrams
Express 0.7 kilograms in grams
Express 0.2 milligrams in micrograms
Express 300 cc in millilitres
Express 700 millilitres in litres
Express 0.4 litres in millilitres
Express 500 milligrams in grams
Express 1200 mg in grams
Express 0.6 moles in millimoles
Express 3.2 litres in millilitres
Express 50 millilitres in litres.
Express 450 µg in grams.