Definition
Self-employment tax is the self-employed version of FICA. Employees split Social Security and Medicare tax with their employer, each paying 7.65%. When you work for yourself you pay both halves: 15.3% in total, made up of 12.4% for Social Security and 2.9% for Medicare.
It is figured on 92.35% of your net self-employment earnings, and you owe it once those earnings reach $400 for the year. The Social Security part stops at the annual wage base ($184,500 for 2026), while the Medicare part applies to all net earnings, with an additional 0.9% on earnings above $200,000 for single filers. You report it on Schedule SE, and you can deduct half of it when figuring your adjusted gross income.
Because it is separate from income tax, self-employment tax is easy to overlook when you estimate what you will owe on freelance or side income.
How this affects your W-4
Nothing withholds self-employment tax from freelance or 1099 income, and it is on top of the income tax you owe on that income. If you also have a W-2 job, the simplest way to cover both is to raise your withholding at that job using Step 4(c), which adds a flat dollar amount to every paycheck.
Use the calculator to see what your income tax withholding looks like, then add an amount in Step 4(c) to cover the self-employment tax as well. If your side income is large or irregular, you may also want to make quarterly estimated payments or meet an IRS safe harbor to avoid an underpayment penalty.
See how self-employment tax affects your withholding
Use the free W-4 calculator to calculate your exact withholding with your specific situation.
Open the W-4 Calculator →Read the guide
W-4 for Freelancers, 1099 and Side Gig Income (2026 Guide)
Side income has no withholding, so it shows up as a surprise tax bill. Cover income tax and self-employment tax through Step 4c or quarterly estimates, with worked examples and 2026 due dates.
IRS Safe Harbor: How to Avoid Underpayment Penalties on Your W-4 (2026)
The IRS charges penalties when you underpay by too much. The safe harbor rules let you optimize your W-4 aggressively without triggering them.
Related terms
Step 4c — Extra Withholding
A line on the W-4 where you specify a flat dollar amount to withhold from every paycheck in addition to the calculated amount.
Above-the-Line Deduction
A deduction taken from gross income to arrive at AGI — available whether or not you itemize.
Safe Harbor Rule
An IRS rule that protects you from underpayment penalties if your withholding meets either of two minimum thresholds.