SE Tax Rate (15.3%, 2013 ~)

https://www.irs.gov/businesses/small-businesses-self-employed/self-employment-tax-social-security-and-medicare-taxes#2

Self-employment tax is a tax consisting of Social Security and Medicare taxes primarily for individuals who work for themselves. It is similar to the Social Security and Medicare taxes withheld from the pay of most wage earners.

You figure self-employment tax (SE tax) yourself using Schedule SE (Form 1040). Social Security and Medicare taxes of most wage earners are figured by their employers. Also you can deduct the employer-equivalent portion of your SE tax in figuring your adjusted gross income. Wage earners cannot deduct Social Security and Medicare taxes.

The 2010 Tax Relief Act reduced the self-employment tax by 2% for self-employment income earned in calendar year 2011. The self-employment tax rate for self-employment income earned in calendar year 2011 was 13.3% (10.4% for Social Security and 2.9% for Medicare). The Temporary Payroll Tax Cut Continuation Act of 2011 extended the self-employment tax reduction of 2% for calendar year 2012 so the rates for 2011 remained in effect for 2012. For self-employment income earned in 2013 and 2014, the self-employment tax rate is 15.3%. The rate consists of two parts: 12.4% for social security (old-age, survivors, and disability insurance) and 2.9% for Medicare (hospital insurance).

https://www.ssa.gov/pubs/EN-05-10022.pdf
Published 2017

If you work for an employer, you and your employer each pay a 6.2 percent Social Security tax on up to $127,200 of your earnings and a 1.45 percent Medicare tax on all earnings. If you’re self-employed, you pay the combined employee and employer amount, which is a 12.4 percent Social Security tax on up to $127,200 of your net earnings and a 2.9 percent Medicare tax on your entire net earnings. If your earned income is more than $200,000 ($250,000 for married couples filing jointly), you must pay 0.9 percent more in Medicare taxes.

Example of SE tax
https://www.cbsnews.com/news/when-to-pay-the-self-employment-tax/
https://quickbooks.intuit.com/r/professional/what-is-the-self-employment-tax-and-why-am-i-paying-it/
http://www.investopedia.com/articles/personal-finance/030216/social-security-selfemployed-how-it-works.asp
https://www.fool.com/investing/general/2016/03/06/self-employment-tax-2016-what-you-need-to-know.aspx
http://www.aarp.org/work/social-security/info-2015/selfemployment-and-social-security.html