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Basic drug calculations

Metric system - Metric conversion

To calculate medication doses, the measurements for the dose required and the stock available must be expressed in the same units. If they are not, you must 'convert' a measurement from one unit to another. For example, if the dose required is in milligrams and the stock available is in grams, convert the grams to milligrams. The calculation will be easier if you always convert the larger unit (e.g. grams) so that it is the same as the smaller unit (e.g. milligrams).

To convert a unit, you need to know:

  1. the common metric equivalents (equivalent means 'of the same value', but expressed as a different unit), then

  2. how to convert from one unit to another.

1. Common metric equivalents

Units of weight
1000 micrograms = 1 milligram
1,000,000 micrograms = 1 gram
1000 milligrams = 1 gram
1000 grams = 1 kilogram

Units of volume
1000 millilitres = 1 litre

Units of length / height
1000 millimetres = 1 metre
10 millimetres = 1 centimetre
100 centimetres = 1 metre
1000 metres = 1 kilometre


Test your understanding - Metric equivalents matching activity

 

2. How to convert from one unit to another

Problem: To convert any unit to a smaller unit Example: Convert 2.85 grams to milligrams

Step 1: Write down the equivalent.

Step 2: Multiply the value of the larger unit by the equivalent.

How: Move the decimal point 3 places to the right.

Equivalent: 1 g = 1000 mg

2.85 x 1000


2.8 x 1000 = 2850 mg

See this happen.
(requires Flash player)

 

Problem: To convert any unit to a larger unit Example: Convert 600 milligrams to grams

Step 1: Write down the equivalent.

Step 2: Divide the value of the smaller unit by the equivalent.

How: Move the decimal point 3 places to the left.

Equivalent: 1 g = 1000 mg

600 ÷ 1000


600 ÷ 1000 = 0.6 g

See this happen.
(requires Flash player)

In summary:

  • To convert from larger to smaller units - multiply by the equivalent.

  • To convert from smaller to larger units - divide by the equivalent.

To avoid confusion with calculations:

  • Always place a '0' in front of a decimal point, eg 0.4.

  • Don't add a decimal point at the end unless necessary, eg write 2 mg, not 2.0 mg.

  • Avoid working with decimals if possible, eg write 250 mg rather than 0.25 g.

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