Instead of reaching for your calculator, use Excel to do the math!
On a sheet, you can enter simple formulas to add, subtract, multiply, and divide two or more numeric values. Once you have created a formula, you can fill it into adjacent cells — no need to create the same formula over and over again. You can also enter a formula that uses the SUM function to quickly total a series of values without having to enter any of them manually in a formula.
Do any of the following:
Use a simple formula to add, subtract, multiply, or divide numeric values
To create a simple formula, you enter values and math operators into a cell, or the formula bar, to receive a result. Instead of entering values directly into the formula, you can also refer to the cells that contain the values that you want to calculate. Using a cell reference in a formula ensures that the result is updated if the values change.

Type a couple of values into cells.
For example, in cell A1, type 5, and in cell B1, type 20.

Click any blank cell, and then type an equal sign (=) to start a formula.

After the equal sign (=), you can type two numbers and a math operator to create a simple formula.
For example, you could simply type =5+20, or =5*20. But to create a formula that you would not have to change, even if you change one of the values, type the cell reference and a math operator. For example, A1 + B1.

After you have tried the formula with a plus sign (+), type a minus sign () to subtract values, an asterisk (*) to multiply values, and a forward slash (/) to divide values.
If you use the example numbers, the results are 25, 15, 100, and 0.25
Note: You can use variations on this formula for all basic math calculations in Excel.
Add a formula to adjacent cells
After you create a formula, you can easily drag it into adjacent cells, either in a row or column. Once you do, the formula automatically adjusts to calculate the values in the corresponding row or column.

Click a cell that contains a formula.
The cell outline shows a square in the lowerright corner, called the fill handle.

Drag the fill handle to an adjacent cell.
The formula is copied there, and automatically adjusts the cell references.
Use the SUM function to total numeric values in a column or row
To calculate the total of a series of numeric values in a row or column, you do not have to enter all those values manually into a formula. Instead you can use a predefined formula that uses the SUM function.

On a sheet, type three numbers in a row.
For example, in cell A1, type 5, in cell B1, type 20 and in cell C1, type 8.

Click the empty cell to the right of the values you typed in.
For example, click cell D1.

On the Formulas tab, under Function, click AutoSum .
Excel outlines the cells that will be included in the result.

Press RETURN .
If you use the numbers that are given in the first step, the result is 33.