As with all basic math operations in Excel, if you wish to add two or more numbers, create a formula. Formulas offer a quick way to find the sum of a few digits.
Instructions in this article apply to Excel 2019, 2016, 2013, 2010, 2007; Excel for Office 365, and Excel for Mac.
Excel Formula Syntax
Some important points to remember about Excel formulas:
 Formulas in Excel begin with an equal sign.
 The equal sign is typed into the cell where the answer will appear.
 The addition sign in Excel is the plus symbol.
 Formulas are completed by pressing the Enter key.
Use Cell References in Addition Formulas
In the example data shown below, rows 2 through 4 use a formula that is located in column C to add the data in columns A and B. In row 2, the result is computed using an addition formula, =5+5.
Rows 3 and 4 show how it is better to first enter the data into worksheet cells and then use the addresses, or references, of those cells in the formula. For example, =A3+B3.
One advantage of using cell references rather than the actual data in a formula is that if you want to change the data, you'll replace the data in the cell rather than rewrite the entire formula. The results of the formula update automatically when the data changes.
To add several numbers that are located in a single column or row in a worksheet, use the SUM function, which offers a shortcut to creating a long addition formula.
Enter Cell References With Point and Click
Although it is possible to type the above formula into cell C3 and have the correct answer appear, it's easier to use point and click to add the cell references to formulas. This approach reduces the possibility of errors created by typing the wrong cell reference.
Point and click involves selecting the cell containing the data to add the cell reference to the formula instead of manually typing it into the cell.
Use the Addition Formula in Excel
Creating the example shown below in cell C3 is simple when you use a formula to add the values of cells A3 and B3.
Here's how to create an addition formula:

Select cell C3 and type an equal sign to begin the formula.

Select cell A3 to add that cell reference to the formula after the equal sign.

Type the plus sign into the formula after A3.

Select cell B3 to add that cell reference to the formula after the addition sign.

Press Enter to complete the formula.

The answer 20 appears in cell C3.
Select the cell to display the formula in the formula bar above the worksheet.
Change the Formula
To change or correct a formula, select from one of two options:
 Doubleclick on the formula in the worksheet to place Excel in Edit mode and then make changes to the formula.
 Select the cell containing the formula and recreate the entire formula.
Create More Complex Formulas
To write more complex formulas that include other mathematical operators, use the steps listed above to get started and then continue to add the correct mathematical operators followed by the cell references containing the new data.
Before mixing different mathematical operations together in a formula, make sure you understand the order of operations that Excel follows when evaluating a formula.
Create a Fibonacci Sequence
A Fibonacci sequence, created by the 12thcentury Italian mathematician Leonardo Pisano, forms a continuous series of increasing numbers. These series are used to explain, mathematically, different patterns found in nature such as:
 The spiral shape of sea shells.
 The arrangement of leaves on a tree branch.
 The reproduction pattern of bees.
After two starting numbers, each additional number in the series is the sum of the two preceding numbers. The simplest Fibonacci sequence, shown in the image above, begins with the numbers zero and one.
Since a Fibonacci series involves addition, it can be created with an addition formula in Excel, as shown in the image above.
The steps below detail how to create a simple Fibonacci sequence using a formula. The steps involve creating the first formula in cell A3 and then copying that formula to the remaining cells using the fill handle. Each iteration, or copy, of the formula adds together the previous two numbers in the sequence.
To create the Fibonacci series shown in the example:

In cell A1, type a zero and press Enter.

In cell A2, type 1 and press Enter.

In cell A3, type the formula =A1+A2 and press Enter.

Select cell A3 to make it the active cell.

Place the mouse pointer over the fill handle (it's the dot in the bottom right corner of cell A3). The pointer changes to a black plus sign when it is over the fill handle.

Drag the fill handle down to cell A19.

Cell A19 contains the number 2584.