How to calculate the hourly wage if you work as a freelancer?

you can check the salary market range of the position then you can calculate hourly with higher rate 

You cannot calculate it. as a freelancer you can do1 of 2 things

Set a standard price for any work done.

or set a standard hourly rate after studying your competitors and how much they charge

If you do this you will be thee tough option to choose by employers. rather you should set your competitive rates per hour based on the market trend or quote your rates as per project. this quote may vary from project to project..