Self-Employment Tax Explained: 15.3%, Both Halves, and the Ceiling

Updated August 7, 2026. Quick answer: self-employment tax is 15.3%, and the IRS is explicit about what it is made of: “The rate consists of two parts: 12.4% for social security (old-age, survivors, and disability insurance) and 2.9% for Medicare (hospital insurance).” It is not instead of income tax — it is on top of it. You are paying both halves because you are both the employer and the employee, and you get to deduct the employer half in figuring your adjusted gross income. The 12.4% Social Security portion stops at the annual wage base; the 2.9% Medicare portion does not stop.

Why it feels like a surprise

In a job, 7.65% came out of your pay and your employer paid a matching 7.65% you never saw on a payslip. Self-employed, the two halves are visibly yours. Nothing has been added; what changed is that the invisible half became visible. That is genuinely most of the shock, and it is worth naming, because the common reaction — that self-employment is punitively taxed — is not quite right.

The offset the IRS provides: “You can deduct the employer-equivalent portion of your self-employment tax in figuring your adjusted gross income.” It is an above-the-line deduction, so you get it whether or not you itemise.

The ceiling, and the part with no ceiling

The 12.4% Social Security component applies only up to the annual wage base — $184,500 for 2026, set by the Social Security Administration in its annual cost-of-living determination. Above that, the Social Security piece stops entirely and only the 2.9% Medicare piece continues, which is why the effective rate on self-employment income falls sharply once you cross it.

Running the other way, an additional Medicare tax applies at higher incomes. The 2025 Schedule SE instructions state it as “A 0.9% Additional Medicare Tax” applying above “Married filing jointly—$250,000; Married filing separately—$125,000; Single, Head of household, or Qualifying surviving spouse—$200,000.” Those thresholds are quoted from the 2025 instructions because that is the most recent revision published, and they are not inflation-indexed.

The question an encore business actually asks

Not “what is the rate” but “what does this income do to my Social Security?” Two different things, constantly confused:

• Before full retirement age, self-employment earnings can cause benefits to be withheld under the earnings test — and the withheld months are not lost, which is the fact that changes the decision. The calculator sizes it.
• At any age, self-employment earnings are covered earnings, so they enter your benefit computation and can raise the benefit itself if they displace a low year.

Those two pages own that story. What belongs here is the connection people miss: the 12.4% you are paying is what makes the earnings count toward the benefit at all. The tax and the credit are the same transaction. The three systems an encore business runs into covers the rest, including the Medicare premium consequence two years later.

And it buys something else

Self-employment income is earned income, and earned income unlocks retirement accounts that pension and portfolio income cannot: what a small amount of side income actually lets you contribute to. For many encore businesses that is worth more than the deductions are.

Sources and limits

Rate, components and the employer-equivalent deduction quoted 2026-08-07 from the IRS’s Self-employment tax page; the Additional Medicare Tax thresholds from the Instructions for Schedule SE (tax year 2025). The $184,500 wage base is the SSA’s 2026 contribution and benefit base from its annual determination published 3 November 2025; irs.gov’s own current Schedule SE instructions still state the 2025 figure of $176,100, so check the year on any wage base you see quoted, including this one. General information, not tax advice.