Have a look at this form - it will depend on what agency you work for but it will give you an idea of the amount that will be deducted from your salary.

http://www.accurity.ch/revenueestimate/

Its a lot more than just tax and the usual social security deductions. But on the other hand, as a contractor you usually get a certain amount tax free (for expenses claimed) which makes up for some of the extra deductions.