This article describes the formula syntax and usage of the LOG function in Microsoft Excel.
Description
Returns the logarithm of a number to the base you specify.
Syntax
LOG(number, [base])
The LOG function syntax has the following arguments:

Number Required. The positive real number for which you want the logarithm.

Base Optional. The base of the logarithm. If base is omitted, it is assumed to be 10.
Example
Copy the example data in the following table, and paste it in cell A1 of a new Excel worksheet. For formulas to show results, select them, press F2, and then press Enter. If you need to, you can adjust the column widths to see all the data.
Formula 
Description 
Result 
=LOG(10) 
Logarithm of 10. Because the second argument (base) is omitted, it is assumed to be 10. The result, 1, is the power to which the base must be raised to equal 10. 
1 
=LOG(8, 2) 
Logarithm of 8 with base 2. The result, 3, is the power to which the base must be raised to equal 8. 
3 
=LOG(86, 2.7182818) 
Logarithm of 86 with base e (approximately 2.718). The result, 4.454, is the power to which the base must be raised to equal 86. 
4.4543473 