An Excel formula is a way of defining a specific calculation in a worksheet. In Excel, all formulas must begin with an equals sign (=). For example, to add 100 to a value in A1, you can write a formula like this:

```
=A1+100
```

This formula will always add 100 to the value in A1, even when that value changes.

### Formulas with Functions

Excel contains hundreds of pre-built formulas called functions. You can use functions on their own in formulas, or even combine multiple functions in a single formula.

For example, the this formula uses only the MAX function to calculate the maximum value in the first 100 cells in column A:

```
=MAX(A1:A100)
```

This formula uses the WORKDAY function to return a date 5 working days from a date in A1:

```
=WORKDAY(A1,5)
```

In a more complex example, this formula uses two functions to count the letter "a" in A1:

```
LEN(A1)-LEN(SUBSTITUTE(A1,"a",""))
```

See this page for a full explanation of this formula.

### Formulas vs. Functions

All functions are formulas. However, you can create formulas in Excel without any functions at all. For example, this formula adds A1 to A2 without any functions:

```
=A1+A2
```