Introduction to Percentages

Percentages are used to indicate what proportion of a whole is made up of a specific item and are expressed "out of one hundred".
For example, if 80 % of an ointment is base, you can expect that in 100 grams of that ointment, there will be 80 grams of base. You need to be able to calculate what percentage a component is within a system.




25 g of Zinc and Coal Tar Paste BNF contains 11.25 g of Yellow Soft Paraffin. What percentage of the paste is Yellow Soft Paraffin?

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The percentage of paraffin in the paste is 45 %.

The percentage of paraffin is the PROPORTION OF PARAFFIN
in the paste multiplied by 100.
Percentage of paraffin =      Mass of Paraffin       x 100 %
Total Mass of Paste
=               11.25           x 100 %
25
Percentage of paraffin = 45 %

Further details about the calculation

Percentages are proportions expressed as amounts out of ONE HUNDRED. In the example the proportion of paraffin is expressed as 11.25 g out of 25 g ie

Proportion of Paraffin =       Amount of Paraffin     = 11.25
Total Amount of Paste          25

Whatever the total amount of paste, this proportion will stay the same. Therefore the percentage of paraffin in the paste is the same proportion of 100 that 11.25 is of 25.

The percentage of paraffin is 11.25 x 100 % = 45 % of the total
25

Checking the proportions: 11.25 = 0.45            45 = 0.45
25                         100

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