Combined Income: The Social Security Tax Formula and Its Fixed Thresholds

Two numbers decide whether a Social Security check is partly taxable income: a sum the filer computes, and a fixed dollar figure set by statute. The sum is called combined income on the Social Security Administration's pages and is built from adjusted gross income, nontaxable interest, and one-half of the benefits. The fixed figure is called the base amount in IRS Publication 915. Everything else in this corner of the tax code follows from comparing the first to the second.

The structure catches people out in two directions. Some assume benefits are never taxed, which is true only below the base amount. Others assume that crossing a line means 85 percent of the check disappears into tax, which misreads what the 85 percent is. Neither figure in this system is a tax rate. Both are caps on how much of the benefit may be pulled into gross income before ordinary rates are applied to it.

What follows is the mechanics as the primary documents state them: what goes into the sum, where the lines sit, when Congress put them there, and which surrounding numbers move each year while these do not.

The Test Is a Sum, Not a Rate

The Social Security Administration's benefits planner on income taxes lays out the addition in four lines: adjusted gross income, plus nontaxable interest, plus one-half of the Social Security benefits, equals what it labels combined income. The third line is the one that surprises. Only half the gross benefit enters the test, no matter how large the benefit is.

Figure 1. The combined-income addition, as structured in IRS Publication 915 (2025) and the SSA benefits planner on income taxes and Social Security benefits.

The IRS states the same test in a single sentence in Topic no. 423: "Your benefits won't be taxable unless the sum of your modified adjusted gross income (MAGI) plus one half of your benefits received in the tax year is more than the base amount for your filing status." The wording differs from the SSA page — the IRS says modified adjusted gross income where SSA writes adjusted gross income plus nontaxable interest — but both describe a sum compared against a status-specific floor.

One detail of sequencing matters and is easy to get backwards. The adjusted gross income that goes into the test does not already contain the taxable portion of the benefits, because that portion is what the test is being run to determine. Publication 915 resolves the circularity with a worksheet rather than a formula in prose: "To figure the total of one-half of your benefits plus your other income, use Worksheet A, discussed later. If the total is more than your base amount, part of your benefits may be taxable." The worksheet, not any summary of it, is the operative computation.

It is also worth separating this from the benefit calculation itself. The dollar figure that gets halved here is the output of an entirely different formula — the one that pays 15 cents per additional dollar of earnings above the second bend point. That formula determines the size of the check. This one determines how much of the check the tax code looks at.

The Base Amount Is the First Line

Publication 915 (2025) sets the base amount by filing status, and by one additional fact about living arrangements. Its list reads:

• "$25,000 if you are single, head of household, or qualifying surviving spouse;"
• "$25,000 if you are married filing separately and lived apart from your spouse for all of 2025;"
• "$32,000 if you are married filing jointly; or"
• "$0 if you are married filing separately and lived with your spouse at any time during 2025."

The last entry is not a typographical oddity. A base amount of $0 removes the exempt zone entirely for that filer, so the comparison starts above the floor from the first dollar. The phrase governing it is "at any time during" the year, not a majority of the year.

Note also what the base amount does not depend on. It does not vary with age, with the number of years benefits have been received, or with the size of the benefit. A filer receiving the maximum possible retirement benefit and a filer receiving a small one are measured against the same $25,000 if both file as single.

The Second Line Raises a Cap, Not a Rate

Above the base amount, Publication 915 describes the first tier plainly: "Generally, up to 50% of your benefits will be taxable." A second, higher line then raises that ceiling to 85 percent. For a single filer the two lines sit at $25,000 and $34,000; for a joint return, at $32,000 and $44,000, the higher figure in each pair taken from the SSA Office of the Chief Actuary rather than from the base-amount list.

Figure 2. The two lines per filing status on a single combined-income scale, drawn to one common dollar axis. Base amounts from IRS Publication 915 (2025); the $34,000 and $44,000 lines from the SSA Office of the Chief Actuary.

The words "up to" carry the whole meaning. Fifty percent and 85 percent are ceilings on the includable share of the benefit, not percentages of tax owed and not marginal rates. Each is a maximum rather than the share that applies at every level of combined income above a line, and the includable dollar amount for any actual return comes from Worksheet A.

A capped share computed on a worksheet behaves quite differently from the IRMAA surcharge, where one dollar over a line moves an entire premium tier. It should also not be confused with the retirement earnings test, which withholds benefits rather than taxing them — that test reduces the payment itself and is measured against earnings, not against combined income.

Where the Two Pairs of Numbers Came From

The two tiers were enacted at different times, and the SSA Office of the Chief Actuary records both. The first pair appears there as the basis on which the trust funds receive tax revenue: "The funds receive taxes on up to 50 percent of benefits from single taxpayers with incomes over $25,000 and from taxpayers filing jointly with incomes over $32,000."

The second pair came with legislation the same page dates to 1993: "Legislation enacted in 1993 extended taxation of benefits. The legislation increased the limitation on the amount of benefits subject to taxation from 50 percent to 85 percent for single taxpayers with incomes over $34,000 and for taxpayers filing jointly with incomes over $44,000."

The 1993 change also split the destination of the money. Per the same SSA page: "All additional tax income resulting from the 1993 legislation is deposited in Medicare's Hospital Insurance Trust Fund." So the tax on benefits funds two different programs depending on which tier produced it — a detail that explains why the rules survive as a financing mechanism rather than purely as a tax provision.

Set those figures beside the current publication and the point is immediate: the $25,000 and $32,000 base amounts printed in Publication 915 for 2025 are the same two amounts the actuary's page attaches to the 50 percent tier.

What Moves Each Year, and What Does Not

Almost every other number in this neighborhood is indexed. SSA states that "Social Security benefits will increase by 2.8 percent beginning with the December 2025 benefits, which are payable in January 2026," and that "COLAs are based on increases in the Consumer Price Index for Urban Wage Earners and Clerical Workers (CPI-W)." The 2026 COLA fact sheet puts the average monthly benefit for all retired workers at $2,015 before the adjustment and $2,071 after it. The same sheet sets 2026 maximum taxable earnings at $184,500 and the retirement earnings test exempt amounts at $24,480 a year under full retirement age and $65,160 in the year full retirement age is reached.

Each of those figures is restated annually. The four combined-income thresholds are not among them.

Figure 3. The four thresholds to scale, with the average retired-worker benefit annualized for comparison. Base amounts from IRS Publication 915 (2025) and the higher tier lines from the SSA Office of the Chief Actuary; benefit figure from the SSA 2026 COLA fact sheet. The annualization is arithmetic applied here, not a figure SSA publishes.

Running the Average Benefit Through the Test

Working the numbers through shows where the lines actually bite. Taking the post-COLA average monthly retired-worker benefit of $2,071 and multiplying by 12 gives $24,852 for a full year. Half of that — the amount that enters the test — is $12,426.

For a single filer whose only income is that average benefit, combined income is $12,426. Against a base amount of $25,000, that leaves $12,574 of other income before the first line is reached. Below it, no part of the benefit enters gross income.

For a joint return where both spouses receive that same average benefit, gross benefits total $49,704 and the half that enters the test is $24,852. Against the joint base amount of $32,000, that leaves $7,148 of other income before the first line is reached — a materially smaller cushion than the single filer has, relative to the benefits involved.

Figure	Single filer	Joint, two average benefits
Gross annual benefits	$24,852	$49,704
One-half, entering the test	$12,426	$24,852
Base amount	$25,000	$32,000
Other income before the first line	$12,574	$7,148
Where the 85% cap begins	$34,000	$44,000

The annual benefit figures in this table are the SSA monthly averages multiplied by 12; the base amounts and tier thresholds are the statutory figures. Because the thresholds hold still while the benefit is adjusted upward each year, the size of that remaining cushion narrows with every COLA for a household whose other income is flat.

Withholding Is a Separate Question

Whether tax is owed and whether it is prepaid are different matters. No tax is withheld from Social Security benefits by default. SSA's page on requesting withholding states the available settings: "You may choose to withhold 7%, 10%, 12%, or 22% of your monthly payment." Those are fixed percentage options rather than a free-entry figure, and they apply to the payment, not to the includable portion.

That same SSA page compresses the test into one sentence: "You will pay federal income taxes on your benefits if your combined income (50% of your benefit amount plus any other earned income) exceeds $25,000/year filing individually or $32,000/year filing jointly." It is a plain-language summary and reads slightly narrower than the controlling text — Topic no. 423 puts modified adjusted gross income on the other side of the sum and Publication 915 puts "your other income" there, either of which is broader than earned income alone. Where a short page and the publication differ in scope, the publication and its worksheet govern.

Numbers to Re-check

Several figures above are pinned to a specific year and should be re-read against the current documents before being relied on.

• The base amounts and tier thresholds — $25,000, $32,000, $34,000, $44,000 — appear with these values in the SSA actuary's account and, for the two base amounts, in Publication 915, but the current-year publication is the place to confirm they still read that way.
• The married-filing-separately wording is year-stamped in the publication ("for all of 2025," "during 2025"). A later edition carries a later year.
• The 2.8 percent COLA and the $2,015 to $2,071 average benefit figures are from the 2026 fact sheet and are restated annually.
• Maximum taxable earnings of $184,500 and the earnings-test exempt amounts of $24,480 and $65,160 are 2026 figures.
• The withholding percentages of 7, 10, 12, and 22 are the options SSA lists on its current page.
• The $24,852 annual and $12,426 half figures are arithmetic on the SSA monthly average, not published annual statistics, and they describe an average rather than any particular record.

Where This Doesn't Apply

The mechanics above are federal and narrow in scope, and several adjacent situations are governed elsewhere.

• Scope of the publication. Publication 915 is titled Social Security and Equivalent Railroad Retirement Benefits. Payments outside that scope are not governed by the base-amount test described in it.
• State income tax. Every figure here is federal. Whether a state taxes benefits, and on what measure, is set by that state and is independent of these thresholds.
• Filers with a $0 base amount. For someone married filing separately who lived with a spouse at any time during the year, the first zone in Figure 2 does not exist, and the walkthrough above does not describe that situation.
• The earnings test and IRMAA. Both involve income thresholds, and neither uses combined income. They are separate tests with separate measures and separate consequences.
• The worksheet itself. This describes which lines exist and what they cap. It does not reproduce Worksheet A, and the includable dollar amount for any actual return comes from that worksheet.

Notes

This article describes federal rules as stated in IRS and SSA source documents as of the 2025 and 2026 editions cited. It is general information about how the rules are written, not tax, legal, or insurance advice, and it is not a determination about any individual return. Thresholds, publication wording, and annual figures change. Anyone applying these rules to a specific situation should consult a qualified tax professional or contact the IRS or SSA directly, and should verify every figure against the current primary source.