This page describes how to calculate percentages in Excel in three different ways.
The different percentage calculation types are:
Percentage of a Number 
E.g. What is 25% of 50? 
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 a percentage of a number in Excel, simply multiply the percentage value by the number that you want the percentage of.
For example, if you want to calculate 25% of 50, multiply 25% by 50.
I.e. type the following formula into any Excel cell:
 which gives the result 12.5.
For further examples of calculating a percentage of a number, see the How to Calculate a Percentage of a Number page.
If you want to calculate percentage as a proportion (i.e. calculate the size of a sample as a percentage of a full set), simply divide the sample size by the size of the full set.
For example, if you score 90 out of 120 in an exam, what is your percentage score?
The percentage score is calculated by simply dividing 90 by 120.
I.e. type the following formula into any Excel cell:
 which gives the result 0.75, or 75%.
Note that, to get the result shown as a percentage, it is necessary to set the cell containing the result to have percentage formatting.
As with any Excel formula, the percentage calculation can use references to cells containing numbers, instead of actual numbers. This is shown in the Excel spreadsheet below.
Formulas:

Results:

Note that the result in cell B3 of the above spreadsheet has been formatted as a percentage. The easiest way to apply percentage formatting is to select the cell to be formatted and then click on the percentage button within the 'Number' group on the Home tab of the Excel ribbon (see below):
See percentage formatting section of this page for details of how to select further percentage formatting options.
If you want to calculate the percentage change between value a and value b, this is done using the formula:
For example if a soccer team scores 16 goals in season 1 and 20 goals in season 2, the percentage change in season 2, compared to season 1, is given by the formula:
This gives the result 0.25, or +25%
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 of the above spreadsheet 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 want to change the number of decimal places that are displayed, click on the Dialog Box Launcher (shown above). This opens up the 'Format Cells' Dialog Box, which allows you to specify the number of decimal places to be displayed.
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:
Open up the 'Format Cells' dialog box by either:
or
Within the 'Format Cells' dialog box:
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.