To calculate the social security tax percentage as of 2015, add the 6.2 percent rate for the employer and the 6.2 percent rate for the employee, which totals 12.4 percent of wages, states the Internal Revenue Service. Social Security tax has a wage base limit of $118,500 for 2015 earnings.
The wage base limit is the maximum taxable wage for the year. The wage limit may change from year to year, and it may be different for agricultural employers. Social Security tax is included in the Federal Insurance Contributions Act. The tax is the combination of survivors, old-age and disability insurance taxes, explains the IRS.