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A common logarithm is a logarithm of base 10. For example, log_(10) 1000 is called the common logarithm of 1000.
It is equal to 3 because 10^3 is 1000.
Since common logarithms are used so often, the base does not need to be written.
log m ≡ log_(10) m
In the identity above, m is a positive number. Recalling the definition of a logarithm, the common logarithm of m can be defined for positive values of m.
log m=n ⇔ 10^n=m
Common logarithms can be evaluated using a calculator. For example, to evaluate log 34, push LOG, enter 34, and then hit ENTER.
The common logarithm of 34 is about 1.53.