You have a job and a side business. How much self-employment tax?

Working a job and running something on the side is the most common self-employed situation there is, and it is the one most tax calculators handle worst. The reason is a single interaction between two systems that otherwise ignore each other.

Get it right and the same side profit can cost a quarter of what you were told.

The one place the two systems meet

Your salary is subject to FICA: 6.2% Social Security and 1.45% Medicare, with your employer paying a matching half. Your side business is subject to self-employment tax: 12.4% Social Security and 2.9% Medicare, both halves yours.

Those look like separate worlds, and for the Medicare part they are. But Social Security tax only applies to the first $184,500 of earnings in 2026 — the wage base — and that ceiling is counted per person, not per source.

Your W-2 wages fill it first. Whatever is left over is all your business income can be charged on.

What that is worth

The same $40,000 of net business profit, filing single, at different salary levels:

Your W-2 salary	Social Security portion	Medicare portion	Total SE tax
$0	$4,581	$1,071	$5,652
$100,000	$4,581	$1,071	$5,652
$150,000	$4,278	$1,071	$5,349
$184,500 or more	$0	$1,071	$1,264

Identical business, identical profit, and a $4,388 difference.

Below the wage base nothing changes — at $100,000 of salary there is still $84,500 of headroom, more than the business needs. The effect only begins once your salary plus your business earnings together cross $184,500, and it is complete once your salary alone does.

At that point the 12.4% simply stops. Your side business is left paying 2.9% Medicare and the surtax below, which is why a high earner’s side income is taxed far more lightly than people expect.

Why the arithmetic is not quite the salary figure

Two details that make hand-calculations disagree with a good calculator.

Only 92.35% of profit counts. Self-employment tax applies to net earnings from self-employment, which is net profit × 0.9235. The $40,000 above becomes $36,940 before any rate is applied. That factor exists to mirror the deduction an employer gets for its half of FICA.

The remaining wage base is compared against that number. At $150,000 of salary, $34,500 of base is left. Net earnings are $36,940 — larger — so Social Security is charged on $34,500 only: $34,500 × 12.4% = $4,278. The last $2,440 escapes it.

The 0.9% surtax works differently

The Additional Medicare Tax is the exception: it looks at wages and self-employment earnings combined against a fixed threshold — $200,000 single, $250,000 married filing jointly, $125,000 married filing separately.

So it can apply even when neither source would trigger it alone. In the $184,500 row above, salary plus net earnings is $221,440, which is $21,440 over the single threshold, producing $193 of surtax. That is the difference between the $1,071 of Medicare and the $1,264 total.

One more wrinkle worth knowing: your employer starts withholding the surtax once your wages alone pass $200,000, without any knowledge of your business or your spouse. Under- and over-withholding are both routine here, and both are settled on your return.

Half of it is still deductible — but only half of the 15.3%

You deduct half your self-employment tax as an above-the-line adjustment. Note what the table implies: at $184,500 of salary the deductible half is only $536, because there is barely any tax to halve.

The surtax is never deductible. That is why the deductible half in that row is half of $1,071 rather than half of $1,264.

The withholding trick, which is genuinely useful

Having a job gives you an option a full-time freelancer does not have, and it is the most practical thing on this page.

Instead of making quarterly estimated payments on your side income, you can increase the withholding at your job — a new W-4 with an extra amount on the “additional withholding” line.

Why it is better than it sounds: estimated payments are credited on the date you make them, so an underpayment early in the year draws a penalty even if you overpay later. Withholding is treated as paid evenly across the whole year regardless of when it actually happened.

That means a large extra withholding in November can cure an underpayment from March. A November estimated payment cannot. If you reach the autumn and realise your side business has outrun your planning, this is the repair, and it is the reason to check your W-4 before writing a catch-up cheque.

It is also simply less work: no four dates to remember, no vouchers, nothing to forget.

What does not change

• Schedule C. You still file one, and you still report all business income and expenses on it, whatever your job pays.
• The $400 threshold. Net earnings of $400 or more mean self-employment tax is due, regardless of your salary. There is no allowance that a job uses up. See the $600 myth.
• The QBI deduction. Your business income can still qualify; your salary cannot.
• Deductions. Business expenses come off business income exactly as they would otherwise.
• Safe harbour. The rule still protects you, and your withholding counts toward it — which is what makes the trick above work. See the safe harbour guide.

Check your calculator

This is the single most useful test of whether a self-employment tax calculator is any good: does it ask for your W-2 wages?

If it does not, it cannot know how much wage base you have left, so it must assume you have all of it — and it will overstate your tax by up to 4.5× at high salaries, exactly as the table above shows. Plenty of well-known calculators do not ask.

Ours does. It is the first field inside the collapsible “Other income and withholding” section, and the calculator prompts you for it whenever you enter profit without a salary — because that is exactly the case where leaving it blank produces a badly wrong number.