Related Function:
The LOG function returns the logarithm of a number to the base you specify.
Syntax
=LOG(number,[base])
Arguments
Argument  Description  

number  The positive real number for which you want the logarithm  
[base]  Optional. The base of the logarithm

Examples
A  B  C  D  

1  Formula  Result  Notes  Description 
2  =LOG(10)  1  Logarithm of 10. Because the second argument (base) is omitted, it is assumed to be 10  10^{1} = 10; therefore, log_{10} = 1 
3  =LOG(8,2)  3  Logarithm of 8 with base 2  2^{3} = 8; therefore, log_{2}(8) = 3 
4  =LOG(64,2)  6  Logarithm of 64 with base 2  2^{6} = 64; therefore, log_{2}(64) = 6 
5  =LOG(4,0.5)  2  Logarithm of 4 with base 0.5  0.5^{2} = 4; therefore, log_{0.5}(4) = 2 
Common Function Error(s)
Problem  What went wrong 

#NUM!  Occurs if either the supplied number argument or the supplied [base] argument is negative or zero 
#DIV/0!  Occurs if the supplied [base] argument is equal to 1 
#NAME?  Occurs if the supplied number argument or the supplied [base] argument is not a numeric value 
The logarithm of a number, to a given base, is the power to which the base must be raised to give that number; it is the inverse operation to exponentiation. Therefore, if
then
See Wikipedia for more information on Logarithm.