This page describes how to calculate percentages in Excel in two different ways, either as a percentage proportion or as a percentage change.
Examples of each of these two percentage types are:
Percentage as a Proportion  E.g. if, out of 120 questions, you answer 90 correctly, how is this expressed as a percentage score? 
Percentage Change  E.g. if your sales figures are $1,250 in January and $1,750 in February, what is the percentage increase in February, compared to January? 
If you want to calculate percentage as a proportion (i.e. calculate the size of a sample, as a proportion of a full set), this is done by dividing the sample size by the size of the full set.
For example, if you take an exam and answer 90 out of 120 questions correctly, this can be expressed as a percentage by simply dividing 90 by 120. This gives the result 0.75, or 75%
This percentage calculation is shown in the Excel spreadsheet below.
Formulas:
 Results:

Note that, to get the result shown as a percentage, as in the above 'Results' spreadsheet, it is necessary to set cell B3 to have percentage formatting.
If you want to calculate the percentage change between value a and value b, this is done using the formula:
For example if your sales figures are $1,250 in January and $1,750 in February, the percentage change in February, compared to January, is given by:
This gives the result 0.4, or +40%
This percentage calculation is shown in the Excel spreadsheet below.
Formulas:
 Results:

As in the previous example, in order to get the result displayed as a percentage, it is necessary to set cell B3 to have percentage formatting.
Percentages in Excel are stored as decimal values. For example, 25% is stored as the value 0.25, 50% is stored as the value 0.5, etc.
It is the formatting of a cell that makes the underlying decimal value appear as a percentage.
You can change a cell's formatting to display percentages in one of the following ways:
The easiest way to change cell formatting is to select the cell(s) to be formatted and then select the percentage button from the 'Number' group on the Home tab of the Excel ribbon (see below):
If you require more formatting options or if you have an older version of Excel, that does not have the ribbon, you can change the formatting of an Excel cell via the 'Format Cells' Dialog Box. To do this:
The result of formatting (with zero decimal places) is shown below.
Values Displayed as Decimals:
 After Formatting as a Percentage:

Further examples of Calculating Percentages in Excel can be found on the Microsoft Office website.