I recently got a new job as a contractor under W2. I'm currently under F1 OPT status and the company agreed to pay 30 per hour. According to this, how much will I get approximately after deductions if I work 20 days a month, 8 hrs a day.I work in California. What are the deductions applicable. Thanks.

On F1/OPT you are exempt from residency for 5-years. You must file non-resident tax return Form 1040NR or 1040NR-EZ and Form 8843. Your income is not subject to FICA taxes but you must pay federal and state income taxes. On your income you will get deduction based on tax treaty between the U.S. and your country.

If you are working as a contractor, you will not get W-2 instead you will get 1099-misc. With 1099-misc, you will also file Schedule C or C-EZ (Form 1040).
Your U.S. Tax Return: U.S. Tax Filing Requirements for Non-Residents (http://taxipay.blogspot.com/2008/03/us-tax-filing-requirements-for-non.html)

If you are under W-2, you are an employee. If you are under Form, 1099-MISC, you are a contractor.

In either case, you pay the same amount in income taxes, but as a contractor, no taxes are withheld from your salary.