How calculate bruto to netto?

I have been here in Holland from 20 of March. I have a job. I have problem with signing the contract. Now I feel they misleading me. They promise they will pay 15 euro netto per hour, but in the contract they wrote 16,11 euro bruto. What can I do now? How much is my netto salary per hour when my bruto is 16,11 or how much must the bruto be to have netto 15 euro per hour?