Excel allows us to calculate the logarithm of a number for a given base, by using the **LOG** function. This step by step tutorial will assist all levels of Excel users in getting the logarithm for a number

*Figure 1. The result of the LOG function*

**Syntax of the LOG Formula**

The generic formula for the LOG function is:

**=LOG(number, base)**

The parameters of the LOG function are:

**number**– a number for which we want to calculate the logarithm**base**– a base for the logarithm calculation. This parameter is optional. If we omit it, the function will take 10 as a base.

**Setting up Our Data for the LOG Function**

In column B (“Number”), we have the list of numbers and in column C (“Base”), a base for each number. In column D (“Logarithm”), we want to get the logarithm for number and base.

* Figure 2. Data that we will use in the LOG example*

**Calculate the Logarithm Using the LOG Function**

In our example, we want to get the logarithm in the cell D3, for 25 and base 8.

The formula in the cell D3 is:

**=LOG(B3, C3)**

The parameter **number **is the cell B3, while the parameter **base** is C3.

To apply the LOG function, we need to follow these steps:

- Select cell C3 and click on it
- Insert the formula:
`=LOG(B3)`

- Press enter
- Drag the formula down to the other cells in the column by clicking and dragging the little “+” icon at the bottom-right of the cell.

*Figure 3. Using the LOG function*

As you can see in the cell D3, the logarithm for 25 and base 8 is 1.547952063.

**Notes**

When using the LOG function, these are the possible errors:

- #VALUE! Error occurs if a number or a base are non-numeric
- #NUM! Error occurs if a number or a base are negative or zero
- #DIV/0! Error occurs if a base is 1.

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