The United States tax Calculator uses Income tax information from the tax year 2015 to calculate the deductions made on a salary.
Federal Income tax is paid only on TAXABLE income, which is however much of your salary is left after the deductions have been made.
The tax is calculated according to US Tax deductions and rules.
This app is the easiest way to get your take home salary.
https://itunes.apple.com/in/app/us-tax-calculator/id1038773189?mt=8