For 2017, the first $127,200 of your combined wages, tips, and
net earnings is subject to any combination of the Social Security
part of self-employment tax, Social Security tax, or railroad
retirement (tier 1) tax. The amount increased to $128,400 for 2018.
(For SE tax rates for a prior year, refer to the Schedule SE for
that year).
All your combined wages, tips, and net earnings in the current
year are subject to any combination of the 2.9% Medicare part of
Self-Employment tax, Social Security tax, or railroad retirement
(tier 1) tax.
If your wages and tips are subject to either social security tax
or the Tier 1 part of railroad retirement tax, or both, and total
at least $127,200, do not pay the 12.4% social security part of the
SE tax on any of your net earnings. However, you must pay the 2.9%
Medicare part of the SE tax on all of your net earnings.