Self-employment tax on a $40,000 side business, line by line

A side business that clears $40,000 of profit in tax year 2025 (the return you file in 2026) owes $5,652 of self-employment tax if its owner has no wages, and $4,307 if the owner also earned $150,000 on a W-2. Both figures come off the same thirteen lines of Schedule SE, and both are what TaxMath's Form 1040 calculator produces for those inputs. The $1,345 gap between them is one line, line 9, doing its job.

Every figure below was run through the calculator's engine with its 2025 rule set, which is marked official. The filer is single, has one Schedule C showing $40,000 of net profit on line 31, and has no farm income, church wages, unreported tips or optional-method election. The engine rounds each line to whole dollars as it is written, so the cents shown below are the arithmetic before rounding.

Case 1: $40,000 of profit, no W-2

Part I of the 2025 Schedule SE, worked top to bottom:

Line 4a is why the tax is not 15.3 percent of profit. §1402(a)(12) reduces net earnings by 7.65 percent of themselves, half the combined 12.4 and 2.9 percent rates, and 100% − 7.65% is the 92.35% the form prints. So $40,000 becomes $36,940, and the two rates apply to that.

Line 10 is 12.4 percent of $36,940, which is $4,580.56 and rounds to $4,581. That is the Social Security half, imposed by §1401(a). Line 11 is 2.9 percent of the same $36,940, $1,071.26, rounded to $1,071, the Medicare half under §1401(b)(1). Together they come to $5,652, which is 14.13 percent of the $40,000 profit.

The $400 test on line 4c does not bite here. It matters only to a filer whose 92.35 percent figure falls below $400, because §1402(b)(2) says net earnings under $400 are not self-employment income at all (2025 instructions).

Case 2: the same $40,000 beside a $150,000 W-2

Lines 2 through 7 do not change. Wages are not self-employment income, so line 6 is still $36,940 and line 7 is still $176,100. The difference starts on line 8a, where the filer enters $150,000 from box 3 of the W-2.

• Line 9: $176,100 − $150,000 = $26,100. That is all the Social Security ceiling has left for this person in 2025.
• Line 10: the smaller of $36,940 and $26,100 is $26,100. Times 12.4 percent, $3,236.40, rounded to $3,236.
• Line 11: unchanged at $1,071. Medicare has no ceiling; the SSA notes that there has been no limit on hospital-insurance earnings after 1993.
• Line 12: $3,236 + $1,071 = $4,307.
• Line 13: half of $4,307 is $2,153.50, rounded to $2,154.

The ceiling on line 7 is the Social Security Administration's contribution and benefit base, $176,100 for 2025. §1402(b)(1) is the reason a W-2 shrinks line 9: the 12.4 percent tax reaches only that base "minus … the amount of the wages paid to such individual." Social Security tax on the first $150,000 was already withheld through payroll, and Schedule SE charges the side business only for the room left above it.

Here $10,840 of net earnings ($36,940 − $26,100) sit above the ceiling and pay only the 2.9 percent. At 12.4 percent those dollars would have cost $1,344.16; after each line's rounding, line 12 is $1,345 lower than in Case 1. Put another way, the first $28,262 or so of profit fills the remaining $26,100 of ceiling (26,100 ÷ 0.9235), and every profit dollar after that costs about 2.7 cents of self-employment tax instead of 14.1.

The ceiling is per person. Line 8a counts the self-employed person's own W-2s, because §1402(b)(1) speaks of wages paid "to such individual," and on a joint return each spouse with self-employment income files a separate Schedule SE (2025 instructions). A spouse's $150,000 salary would leave this filer's line 9 at the full $176,100.

One more tax sits nearby and stays at zero here. The 0.9 percent Additional Medicare Tax of §1401(b)(2) applies to self-employment income over $200,000 for a single filer, a threshold reduced by wages subject to the tax (2025 instructions). Assuming the W-2's Medicare wages in box 5 are also $150,000, the threshold for the business falls to $50,000, and line 6 is $36,940. Form 8959 has nothing to collect, and the engine shows $0.

Where the two halves of line 12 go

Line 12 and line 13 leave Schedule SE for two different schedules, and they move the return in opposite directions.

The tax goes to Schedule 2, line 4, which totals on line 21 and lands on Form 1040 line 23, "Other taxes, including self-employment tax." It is added to income tax rather than figured inside it.

The deduction goes to Schedule 1, line 15, "Deductible part of self-employment tax." §164(f) allows half the §1401 tax as a deduction, excluding the 0.9 percent Additional Medicare Tax. Schedule 1 line 26 sums the adjustments and carries them to Form 1040 line 10, where they come off total income before adjusted gross income on line 11a:

Because it is an adjustment and not an itemized deduction, the half-tax deduction is taken whether the filer itemizes or not, and it lowers every figure that keys off AGI. The income tax that follows depends on more entries than this example makes, including the qualified business income deduction on Form 8995, so it is not worked here.

The same return in 2026

Of the figures this example uses, only line 7 changes for tax year 2026. The 12.4 and 2.9 percent rates are written into §1401, and the $400 floor into §1402(b)(2), with no adjustment clause. Line 7 follows the national average wage index, and the SSA set the 2026 contribution and benefit base at $184,500, $8,400 above 2025's. TaxMath's 2026 rule set carries that figure.

Case 1 does not notice. Its $36,940 of net earnings is nowhere near either ceiling, so 2026 self-employment tax on the same $40,000 is again $5,652.

Case 2 does. Line 9 grows to $184,500 − $150,000 = $34,500, line 10 becomes $34,500 × 12.4% = $4,278, and line 12 rises to $5,349, with $2,675 on line 13. The owner of a $40,000 side business with a $150,000 day job pays $1,042 more self-employment tax for 2026 than for 2025 on identical profit, all of it from the higher ceiling. To check either case against your own figures, enter the W-2 and the Schedule C in the Form 1040 calculator and read Schedule SE lines 9 and 12.

This article is for informational purposes only and does not constitute tax advice. Consult a qualified tax professional for guidance specific to your situation.