In order to enter a formula, you must first enter = ( + and - may also be used).

Let's try to add 2 + 2 and have Excel give us the answer of 4.

In cell B3 type:
2 + 2 Notice the results
Now go to cell C3 and type:
= 2 + 2 You should see the number 4.

  Formulas can also reference other cells:
Go to cell C4 and type:
= C3 + 5
You should see the number 9 in cell C4 (value of 4+5)

  Go to cell B4 and write a formula with reference to cell B3
You should see the error: #VALUE!

 

This means that cell B3 contains labels. Therefore, referenced cell must contain a value.