Self-employment tax gets described as a flat 15.3%, and that's close enough for rough planning. But the actual calculation is more layered than that — and understanding the layers matters when you're trying to figure out what to reserve, what you can deduct, and why the number the IRS expects is slightly different from what you'd get if you simply multiplied 15.3% times your gross income.
What self-employment tax actually is
When you're employed by someone else, your FICA taxes — Social Security and Medicare — get split between you and your employer. You pay 7.65% of your wages (6.2% Social Security, 1.45% Medicare), and your employer pays a matching 7.65%. Neither of you sees the employer half in your paycheck; it's a cost the employer absorbs directly.
When you're self-employed, there's no employer to absorb the other half. You pay both halves yourself. That's where the 15.3% comes from: 12.4% for Social Security and 2.9% for Medicare, both halves combined. It's called self-employment tax (often abbreviated SE tax), and it's calculated on Schedule SE and filed with your return.
SE tax is separate from income tax. You owe both, calculated separately, paid in combination through quarterly estimated payments or at filing. The Gig Tax Optimizer handles both layers together so you can see the full picture.
The two parts: Social Security and Medicare
Social Security is the larger portion at 12.4%. The Social Security portion applies up to an annual wage base set by the IRS each year — above that threshold, only the 2.9% Medicare portion applies. For most freelancers below six figures, this cap doesn't come into play.
Medicare at 2.9% applies to all net self-employment income with no cap. Above certain income levels set by the IRS, an Additional Medicare Tax applies on top of the standard rate — relevant for high-income freelancers who should model their actual liability rather than using a flat heuristic.
Why the base is 92.35%, not 100%
Here's the part that often surprises people: SE tax isn't applied to your full net self-employment income. It's applied to 92.35% of that amount. The calculation is specifically: net self-employment income × 0.9235 × 0.153.
The reason is structural. When an employer pays the employer half of FICA, that payment is a deductible business expense — the employer gets to deduct what they spend on payroll taxes. The 92.35% base is the IRS's mechanism for giving self-employed people the equivalent treatment: you effectively deduct the employer-equivalent half before calculating the tax, even though the deduction is built into the formula rather than taken separately. The result is that your effective SE tax rate on full net income is closer to 14.13% than 15.3%.
The deduction for half of SE tax
SE tax produces one more benefit beyond the 92.35% base calculation. Half of the SE tax you owe — the employer-equivalent half — is deductible as an adjustment to income on Form 1040. This reduces your adjusted gross income, which in turn lowers the base for your income tax calculation.
It's a small but real offset. If you owe $7,065 in SE tax (on $50,000 net income, roughly), you can deduct $3,532 from income before applying federal income tax brackets. That deduction doesn't eliminate SE tax — it reduces the income tax calculated on top of it. The two taxes operate independently; the deduction is just a mechanical link between them that keeps the effective combined rate from being as high as it might appear on first glance.
How SE tax compares to W-2 employment
The net tax cost difference between self-employment and W-2 employment is smaller than the 15.3% headline suggests, for two reasons. First, the 92.35% base reduces the effective rate to about 14.13%. Second, the deduction for half of SE tax cuts the income tax bill slightly. The actual incremental cost of SE tax over W-2 employment — everything else held equal — is usually closer to 7–8% of net income.
The comparison also depends on what the employer was paying in total compensation. A W-2 employer offering $100,000 is absorbing roughly $7,650 in payroll taxes on top of that. A client paying you $100,000 as a contractor expects you to absorb that cost yourself. True compensation comparisons between W-2 and 1099 arrangements should account for this difference. The Take-Home Pay Calculator handles the W-2 side; use the Gig Tax Optimizer for the 1099 side.
When the Social Security wage base matters
If your net self-employment income exceeds the Social Security wage base for the year, the 12.4% Social Security portion of SE tax stops applying to the excess. Only the 2.9% Medicare portion continues. The IRS adjusts this threshold annually, so check the current year's limit if your income is in or approaching the high-five-figure range.
High-income freelancers also need to consider the Additional Medicare Tax that applies above certain income thresholds set by the IRS. The effective SE tax rate above those thresholds is higher than the standard 15.3% calculation would suggest. For freelancers well above six figures, these changes affect both the reserve percentage and the after-tax comparison with W-2 employment, so it's worth modeling the actual number rather than using the flat heuristic.
Running your own numbers
For planning purposes, the steps are: take your net self-employment income (gross income after business expenses), multiply by 0.9235, multiply by 0.153, and that's your approximate SE tax. Add federal income tax on taxable income (net income minus the SE tax deduction, minus any other deductions), and add state income tax if applicable. That total is your reserve target.
Most freelancers land in the 25–30% reserve range when they account for all three layers — SE tax, federal income tax, and state income tax. If you want a more granular breakdown of how that range decomposes in your situation, the article on how much freelancers should reserve for taxes covers the heuristic and when to adjust it. For the actual numbers, run your income through the Gig Tax Optimizer.
This is an educational guide, not personalized financial, tax, or legal advice.