You can create a simple formula to add, subtract, multiply or divide values in your worksheet. Simple formulas always start with an equal sign (=), followed by constants that are numeric values and calculation operators such as plus (+), minus (), asterisk(*), or forward slash (/) signs.
For example, when you enter the formula =5+2*3, Excel multiplies the last two numbers and adds the first number to the result. Following the standard order of mathematical operations, multiplication is performed before addition.

On the worksheet, click the cell in which you want to enter the formula.

Type the = (equal sign) followed by the constants and operators that you want to use in the calculation.
You can enter as many constants and operators in a formula as you need, up to 8192 characters.
Tip: Instead of typing the constants into your formula, you can select the cells that contain the values that you want to use and enter the operators in between selecting cells.

Press Enter.

To quickly add values, you can use AutoSum instead of entering the formula manually (Home tab, Editing group).

You can also use functions (such as the SUM function) to calculate values in your worksheet. For more information see Overview of formulas.

To take it one step further, you can use cell references and names instead of the actual values in a simple formula. For more information, see Use cell references in formulas and Define and use names in formulas.
Examples
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Data 

2 

5 

Formula 
Description 
Result 
'=A2+A3 
Adds the values in cells A1 and A2 
=A2+A3 
'=A2A3 
Subtracts the value in cell A2 from the value in A1 
=A2A3 
'=A2/A3 
Divides the value in cell A1 by the value in A2 
=A2/A3 
'=A2*A3 
Multiplies the value in cell A1 times the value in A2 
=A2*A3 
'=A2^A3 
Raises the value in cell A1 to the exponential value specified in A2 
=A2^A3 
Formula 
Description 
Result 
'=5+2 
Adds 5 and 2 
=5+2 
'=52 
Subtracts 2 from 5 
=52 
'=5/2 
Divides 5 by 2 
=5/2 
'=5*2 
Multiplies 5 times 2 
=5*2 
'=5^2 
Raises 5 to the 2nd power 
=5^2 