The maximum amount of an employee's 2018 earnings (and a self-employed person's net income) that is subject to the Social Security tax is $128,400. This amount is also known as the Social Security annual limit, wage base, contribution and benefit base, ceiling, tax cap, and maximum taxable earnings. The maximum amount is adjusted annually based on the national average wage index.
[For the calendar year 2017, the maximum earnings subject to the Social Security tax was $127,200.]
This earnings limit is used in computing the employee's Social Security tax and therefore the employer's Social Security tax. (In effect, the employer must match each employee's Social Security tax.)
The combination of the Social Security tax and the Medicare tax is often referred to as the FICA tax. However, there is no annual limit or cap on the earnings when computing the Medicare tax for the employee, the self-employed person, or the employer.