You can google it. It'll give you a breakdown of which weeks belong to which month.
There are anywhere between 4 and 5 weeks in a month. This chart will hopefully help.
1-4w=1m
5-8w=2m
9-13w=3m
14-17w=4m
18-21w=5m
22-26w=6m
27-30w=7m
31-35w=8m
36+w=9m
......how many weeks are in a month?