A special logarithm is the common logarithm or the decadic logarithm. This is the logarithm to base 10:
Many calculators can calculate the logarithm only to base 10. Therefore, the following formula often helps in the calculation:
$\log_5(18)$ is not that easy to simplify and cannot be entered on some calculators.
Now use the formula, because almost every calculator should own the common logarithm.