In the US your employer pays around 15% of your salary as tax. If you're not working for someone else that's an extra 15.3% you have to pay - it's called "self employment tax".
NB: it drops to 2.9% (just the employer/employee portion of Medicare) after the cap, which is currently $118,500 for both 2015 and 2016. And then the employee portion of Medicare rises by 0.9% above $200K, but that's not specific to self-employment as it's the employee portion only.