You are self-employed if you operate a trade, business or profession, either individually or as a partner, and have net earnings of $400 or more in a taxable year. The Social Security tax rate for 2009 is 15.3 percent on self-employment income up to $106,800. If your net earnings exceed $106,800, you continue to pay only the Medicare portion of the Social Security tax, which is 2.9 percent, on the rest of your earnings.