If want to calculate a percentage increase in Excel (i.e. increase a number by a specified percentage), this can be done by simply multiply the number by 1 + the percentage increase.
For example, if you want to increase the number 50 by 20%, this can be done in Excel by typing the following formula into any Excel cell:
 which gives the result 60.
Note that, in the above formula:
As with any Excel formula, the percentage increase calculation can use values that are stored in your spreadspeed, instead of actual numbers. This is shown below.
Formula:
 
Result:

If want to decrease a number by a specific percentage, simply multiply the number by 1 minus the percentage decrease.
For example, if you want to decrease the number 50 by 20%, this can be done in Excel by typing the following formula into any Excel cell:
 which gives the result 40.
This is shown in the following spreadsheet.
Formula:
 
Result:

Cell B2 of the following spreadsheet shows the addition of sales tax at 22.5%, to an invoice total.
Formula:
 
Result:

A shop has a sale, in which goods are offered at a 15% discount to their original price. The calculation of the new price of several items, after the discount is applied, is shown in column B of the following spreadsheet:
Formulas:
 Results:

For further examples of Calculating Percentage increases or decreases in Excel, see the Microsoft Office website.
Or, for an overview of different types of percentage calculation, see the Percentages In Excel page.