[zrail]

It's not an extra 15%. The total amount of Social Security and Medicare taxes is 15.4%, with the employer paying half and the employee paying the other half. When you're self-employed, you pay both halves. And the employer half is always deductible on your federal income tax, which reduces the sting a little.

[sokoloff]

Good actual data, though the total figure is 15.3%

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.