Go here. It takes out the taxes for you so you can see how much your paycheck will be if you get a new job, raise or work more or less hours.
I was trying to figure out how to calculate your federal withholdings and I could not figure it out. It differed so much depending on hours and I tried to figure out the % but it changed per hours. Then I found this and it worked!!! It was only like 40 cents off on the many and many paychecks I compared it to. Awesome!!