Social Security tax is the old-age, survivors, and disability insurance charge that the Internal Revenue Code imposes on wages paid for employment and on self-employment income. Unlike its Medicare counterpart it stops at an annual ceiling, and unlike income tax it is measured against wages rather than against taxable income. The same two words are also used loosely for something else entirely, the income tax that can reach Social Security benefits once they are received, and that second sense has its own section, its own thresholds and its own taxpayer. Both are below, in that order.
The Three Sections That Impose It
Each is a separate tax on a separate taxpayer, which is why one name can look like several charges at once.
The employee’s tax. In addition to other taxes, there is imposed on the income of every individual a tax equal to 6.2 percent of the wages, as defined in section 3121(a), received by the individual with respect to employment, as defined in section 3121(b) (26 U.S.C. 3101(a)).
The employer’s own tax. In addition to other taxes, there is imposed on every employer an excise tax, with respect to having individuals in his employ, equal to 6.2 percent of the wages paid by the employer with respect to employment (26 U.S.C. 3111(a)). It is the employer’s liability, not a share of the employee’s, and nothing about it is deducted from anyone’s pay.
The self-employed person’s tax. In addition to other taxes, there is imposed for each taxable year, on the self-employment income of every individual, a tax equal to 12.4 percent of the amount of that self-employment income (26 U.S.C. 1401(a)). What that base is built from, and the two steps that produce it, are set out in self-employment tax.
The employee owes the first of those but does not remit it. The tax imposed by section 3101 is collected by the employer of the taxpayer, by deducting the amount of the tax from the wages as and when paid (26 U.S.C. 3102(a)).
The Cap Is Written as an Exclusion, and It Reaches Only Two of the Three
There is no sentence in the Code that says the tax stops. What exists instead is a hole cut in the definition of wages, and the hole is addressed to named sections.
For purposes of the chapter, wages means all remuneration for employment, including the cash value of all remuneration, including benefits, paid in any medium other than cash, except that the term does not include, in the case of the taxes imposed by sections 3101(a) and 3111(a), that part of the remuneration which, after remuneration other than remuneration referred to in the succeeding paragraphs of that subsection equal to the contribution and benefit base, as determined under section 230 of the Social Security Act, with respect to employment has been paid to an individual by an employer during the calendar year with respect to which that base is effective, is paid to that individual by that employer during that calendar year (26 U.S.C. 3121(a)(1)).
Three things in that sentence do real work and are easy to lose.
- It names only 3101(a) and 3111(a). The hospital insurance taxes under 3101(b) and 3111(b) are not in the list, which is why the Medicare side has no ceiling at all. That contrast is worked through in Medicare tax.
- The base is per employer, per calendar year. The exclusion is measured against remuneration “paid to an individual by an employer“, so an individual with two employers meets the ceiling twice over, once at each.
- There is a successor-employer rule sitting immediately after it, which lets remuneration paid by a predecessor count toward the successor’s measurement where substantially all the property used in the trade or business has been acquired and the individual is employed immediately after the acquisition (26 U.S.C. 3121(a)(1)). Without it an acquisition would restart the base mid-year.
Which figures land in which wage-statement box because of this exclusion is set out in gross wages.
The Figure for the Current Year
The Social Security Administration publishes the base rather than the Code. The OASDI program limits the amount of earnings subject to taxation for a given year, the same annual limit applies when those earnings are used in a benefit computation, the limit changes each year with changes in the national average wage index, and it is called the contribution and benefit base, also commonly referred to as the taxable maximum (SSA, Contribution and Benefit Base).
For earnings in 2026, this base is $184,500. The OASDI tax rate for wages paid in 2026 is set by statute at 6.2 percent for employees and employers, each, so an individual with wages equal to or larger than $184,500 would contribute $11,439.00 to the OASDI program in 2026, and his or her employer would contribute the same amount. The OASDI tax rate for self-employment income in 2026 is 12.4 percent (SSA, Contribution and Benefit Base).
Because the charge stops and the wage does not, the share of income it takes falls as the wage rises, and that structural point is worked through in progressive vs regressive taxes.
Where the Money Goes, and Why It Is Two Funds
The name OASDI carries two programs, and the statute creates two separate trust funds for them. There is created on the books of the Treasury of the United States a trust fund to be known as the Federal Old-Age and Survivors Insurance Trust Fund (42 U.S.C. 401(a)), and the Federal Disability Insurance Trust Fund is created and funded separately in the same section, with amounts transferred from the general fund on the basis of estimates by the Secretary of the Treasury of the taxes paid to or deposited into the Treasury, and proper adjustments made in amounts subsequently transferred to the extent prior estimates were in excess of or less than those taxes (42 U.S.C. 401(b)).
The Other Social Security Tax: Income Tax on the Benefits
This is a different charge on a different person at a different time, and it is the sense half the search results for the term are about. It is income tax under section 86, not a payroll tax, and the recipient of the benefits owes it.
Who it reaches. A taxpayer is described in the operative subsection if the sum of the modified adjusted gross income of the taxpayer for the taxable year plus one-half of the social security benefits received during the taxable year exceeds the base amount (26 U.S.C. 86(b)(1)). Modified adjusted gross income means adjusted gross income determined without regard to that section and sections 85(c), 135, 137, 221, 911, 931, and 933, and increased by the amount of interest received or accrued during the taxable year which is exempt from tax (26 U.S.C. 86(b)(2)). Both halves of that definition matter, and the second is why tax-exempt interest can make taxable a benefit that would otherwise escape. The wider family of these computed figures is set out in modified adjusted gross income, and the interest that gets added back in tax-exempt interest income.
The two thresholds. The base amount means, except as otherwise provided in that paragraph, $25,000; $32,000 in the case of a joint return; and zero in the case of a taxpayer who is married as of the close of the taxable year within the meaning of section 7703 but does not file a joint return for that year and does not live apart from his spouse at all times during the taxable year (26 U.S.C. 86(c)(1)). The adjusted base amount means, except as otherwise provided in that paragraph, $34,000; $44,000 in the case of a joint return; and zero in the same married-not-filing-jointly case (26 U.S.C. 86(c)(2)).
The zero limb has two conditions and both have to hold. Married at the close of the year, and not living apart at all times during the year. A married taxpayer filing separately who did live apart for the whole year is not in that limb.
How much is included. Except as provided in the additional-amount paragraph, gross income includes social security benefits in an amount equal to the lesser of one-half of the benefits received during the taxable year, or one-half of the excess described in the first threshold test (26 U.S.C. 86(a)(1)). The opening words there are not decoration: the rule is a default that a second paragraph overrides.
Where the amount determined under the first threshold test exceeds the adjusted base amount, the amount included in gross income is the lesser of two computed figures: the sum of 85 percent of that excess plus the lesser of the amount determined under the first paragraph or an amount equal to one-half of the difference between the adjusted base amount and the base amount, or 85 percent of the social security benefits received during the taxable year (26 U.S.C. 86(a)(2)).
So the ceiling on inclusion is 85 percent of the benefits received during the taxable year, never all of them, and the lower tier is capped at one-half of them (26 U.S.C. 86(a)(1) and 86(a)(2)). Those two fractions are what the familiar shorthand about benefits being partly taxable is describing.
What Social Security Tax Is Confused With
Medicare Tax
The same statutes impose both, one subsection apart, and only one of them is capped. The hospital insurance charge on the employee is 1.45 percent of the same wages, and a further 0.9 percent is imposed above a threshold on the employee alone (26 U.S.C. 3101(b)). Both are set out in Medicare tax and additional Medicare tax.
Payroll Tax
Social Security tax is one component of that set rather than a synonym for it. What else sits inside the term, including federal unemployment tax and the income tax withholding that is not a payroll tax at all, is set out in payroll tax.
Self-Employment Tax
Same programme, different chapter and different base. The self-employed pay the whole 12.4 percent themselves rather than half of it, and the base is computed rather than paid, which is worked through in self-employment tax.
Common Questions
The rate on wages is, on either side of the pay run. The load is not, because an employee’s 6.2 percent is matched by a separate employer tax at the same rate, while a self-employed individual owes 12.4 percent alone (SSA, Contribution and Benefit Base).
The exclusion is measured against remuneration paid to an individual by an employer during the calendar year (26 U.S.C. 3121(a)(1)), so each employer measures its own payments. The successor-employer rule in the same paragraph is the named case where a predecessor’s payments carry across, and it turns on an acquisition of substantially all the property used in the trade or business rather than on a job change generally.
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