You are generally required to pay self-employment taxes if you had net profit from self-employment of $400 or more. There is no upper limit beyond which Medicare tax is not owed. For the 2023 tax year, the Social Security tax portion of self-employment tax only applies to the first $160,200 of taxable income. This means that the maximum amount of Social Security tax a self-employed person can pay on that income in 2023 is $19,864.80.
For example, if your self-employment net income for 2023 is $200,000, you would pay taxes on $184,700 (92.35% of your net business profit is taxable for self-employment tax purposes).
The full amount of $184,700 would be subject to the 2.9% Medicare portion of the self-employment tax, for a total of $5,356.30, because there is no upper limit for taxable income for Medicare tax. However, instead of paying the full 12.4% portion for Social Security tax on the full amount, you would only be responsible for 12.4% of the maximum income subject to Social Security tax. Since the income limit in 2023 is $160,200, there is a maximum Social Security tax of $19,864.80.
It is also worth noting that if you have more than one source of earned income, all of your earned income is added together when determining whether you are over the maximum income threshold for Social Security tax.