Social Security Taxes

The Social Security tax rate is 12.4%. A portion 6.2% is payed by the employer and 6.2% is payed by the employee.  Self-employed persons pay the full amount of 12.4% of their taxable income.  
The rate for Medicare is 1.45% for the employer and 1.45% for the employee, or 2.9% total.
Only the social security tax has a wage base limit. The wage base limit is the maximum wage that's subject to the tax for that year. For earnings in 2019, this base is $132,900.

Taxpayers who lives outside the USA are subject for Social Security and Medicare payments only on their income as self employed regardless on that income source location.