In Microsoft Excel, a formula is an expression that operates on values in a range of cells or a cell. Formulas enable you to perform calculations such as addition, subtraction, multiplication, and division, as well as find out averages and calculate percentages for a range of cells. They can also manipulate date and time values and perform more complex calculations using functions. A formula can contain functions, mathematical operations, cell references, and constants. Formulas always start with the equal sign (=) typed in the cell, followed by your calculation. Excel has pre-built functions, such as SUM, AVERAGE, COUNT, and PRODUCT, which are commonly used in formulas. Using formulas, you can perform calculations and data analysis on the contents of the cells.