Basic
drug
calculations
Metric system  Metric conversionTo 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:

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

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

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 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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