The Logarithm of a number, to a given base, is the power to which the base must be raised to give that number.
The Base 10 Logarithm of a number, (also known as the Common Logarithm and written as Log_{10}(x) ), is the logarithm to the base 10.
Therefore,
The Excel LOG10 function calculates the base 10 logarithm of a given number.
The format of the function is:
where the number argument is a positive real number that you want to calculate the base 10 logarithm of.
The spreadsheet below provides four examples of the Excel Log10 function:
Formulas:
 Results:

Further examples of the Excel Log10 function can be found on the Microsoft Office website.
If you get an error from your Excel Log10 function, use the following table to diagnose the likely cause:
#NUM!    Produced if the supplied number argument is negative or zero. 
#VALUE!    Produced if the supplied number argument is not a numeric value. 