I started plasma donations in early 2023, and expect annual income of about $3,000. I plan to continue the activity indefinitely. I planned to file 2023 taxes using Schedule C. But I am concerned about how the self-employment tax would apply in my case. I exceed the Social Security wage cap from my full-time employment, so I did not expect to pay the Social Security 6.4% (or 12.8% for both parts) on the Schedule C activity - even though I expected to pay the Medicare 1.25% (or double) plus Medicare surcharge, based on my AGI. Am I wrong?
You are correct. The calculation of self-employment tax on Schedule SE takes into consideration the Social Security tax that was withheld on your W-2. If you have already paid the maximum amount of Social Security tax on your wages, you will not pay the Social Security portion of the self-employment tax.
The Social Security tax rate is 6.2%. The basic Medicare tax rate (before additional Medicare tax) is 1.45%.