IRS Publication 915: Combined Income, Tiers, and Lump-Sum Election

IRS Publication 915 is the guide that tells you how much of your Social Security or equivalent Tier 1 Railroad Retirement benefits count as taxable income on your federal return. The short version: somewhere between 0% and 85% of your benefits are taxable, depending on a figure the publication calls your combined income. At least 15% of your benefits are always tax-free, no matter how high your income goes.¹

The publication’s worksheets do the actual work of turning your income into a single number for Line 6b of Form 1040. What follows walks through the same steps the worksheets take, using the definitions and thresholds Publication 915 sets.

Combined Income: The Starting Number

Everything in Publication 915 flows from one calculation. Combined income (also called the base amount, or informally provisional income) is the sum of three pieces:

Your other income from Form 1040 — wages, pensions, IRA distributions, capital gains, business income, and other taxable amounts.
Your tax-exempt interest, most commonly from municipal bonds.
One-half of your total Social Security benefits for the year.

Add those three, and the result is what gets compared to the thresholds.²

One easy-to-miss step: certain exclusions you normally leave off your return have to be added back for this calculation. That includes excluded foreign earned income or housing, employer-provided adoption benefits, and income earned as a resident of American Samoa or Puerto Rico. If any of those applied on your return, put them back in when figuring combined income.²

The Three Tiers

Your combined income falls into one of three ranges, and the range determines the ceiling on how much of your benefits can be taxed. The thresholds have been fixed by statute since 1993 and are not indexed for inflation.¹

None taxable. Combined income below $25,000 (single, head of household, or qualifying surviving spouse) or below $32,000 (married filing jointly).
Up to 50% taxable. Combined income between $25,000 and $34,000 (single) or between $32,000 and $44,000 (joint).
Up to 85% taxable. Combined income above $34,000 (single) or above $44,000 (joint).

The 85% figure is a ceiling, not a rate. It caps how much of your benefits enter your taxable income; the actual tax on that amount depends on your ordinary bracket.

Married Filing Separately

Publication 915 sets a base amount of $0 for anyone who is married, files separately, and lived with their spouse at any point during the year. That eliminates the 0% and 50% tiers entirely — the 85% calculation applies from the first dollar of combined income.² If you filed separately but lived apart from your spouse for the entire year, you use the single thresholds instead.³

Working the 50% Tier

When combined income falls between the first and second thresholds, the taxable amount is the smaller of two numbers: half your total Social Security benefits, or half the amount by which combined income exceeds the first threshold.²

An example. A single filer has $18,000 in pension income, $2,000 in bank interest, and $20,000 in Social Security. Combined income is $18,000 + $2,000 + $10,000 = $30,000. That is $5,000 above the $25,000 threshold; half of the excess is $2,500. Half of total benefits is $10,000. The smaller wins, so $2,500 goes on Line 6b.

The most that can be taxed under this tier alone is $4,500 for a single filer and $6,000 for joint filers. Those caps are just the width of the tier ($9,000 or $12,000) multiplied by 50%.²

Working the 85% Tier

Once combined income crosses the second threshold, the calculation stacks. You take the 50% tier maximum (the lesser of $4,500 single or $6,000 joint, or 50% of total benefits) and add 85% of the amount by which combined income exceeds the second threshold. The taxable portion is the smaller of that sum or 85% of total benefits.²

Back to the same single filer, now with $35,000 in pension income and $5,000 in interest. Combined income is $50,000. The 50% tier piece is $4,500. The excess above $34,000 is $16,000, and 85% of that is $13,600. The sum is $18,100. But 85% of the $20,000 benefit is $17,000, which is smaller, so $17,000 goes on Line 6b.

No matter how far above the second threshold you land, the taxable amount cannot exceed 85% of what Social Security paid you.

Where the Numbers Go on Form 1040

Each January, the Social Security Administration issues Form SSA-1099. The boxes you use for the worksheet are Box 3 (total benefits paid), Box 4 (benefits repaid), Box 5 (net benefits, which is Box 3 minus Box 4), and Box 6 (any voluntary federal income tax withheld).² If your form did not arrive, you can pull a copy from your online my Social Security account.⁴

On Form 1040 or 1040-SR:

Line 6a: net benefits from Box 5, added across all SSA-1099s.
Line 6b: the taxable amount from the Publication 915 worksheet or the worksheet in the Form 1040 instructions.
Line 6c: check if you are using the lump-sum election.
Line 6d: check if you are married filing separately and lived apart from your spouse for the entire year.

Line 6b flows into your total income and is taxed at your ordinary rate.⁵

The Lump-Sum Election for Back Payments

Retroactive awards — common with disability claims and delayed applications — can arrive as one large payment covering benefits owed for prior years. Reporting the whole thing in the year of receipt can inflate your combined income and push more of your benefits into the 85% tier.

Publication 915 lets you elect to figure the taxable portion as if you had received each year’s benefits in the year they were owed. You refigure the taxable benefits for each earlier year using that year’s income, subtract any benefits already reported for those years, and add the differences. No amended returns are involved; the whole calculation happens on your current-year return, and you check Line 6c to signal the election.⁶

You are not forced into the election. Run both methods and use whichever gives you the lower tax. Publication 915 includes worksheets for each earlier year involved.²

When You Repaid Benefits

Repayments to the Social Security Administration reduce Box 5 automatically for the current year, so the worksheet already reflects them.²

If Box 5 is negative — meaning you repaid more than you received during the year — the treatment depends on the size of the negative amount. Repayments of $3,000 or less were historically deductible as a miscellaneous itemized deduction, but the Tax Cuts and Jobs Act suspended that category, so those repayments currently offer no federal tax benefit.⁷

If the negative amount is more than $3,000, you have two paths. You can claim the repayment as an itemized deduction on Schedule A, or you can calculate a credit equal to the tax you would have saved in the earlier year had those benefits not been included in your income. Compute both and take whichever produces the lower overall tax.²

SSDI Follows These Rules; SSI Does Not

Social Security Disability Insurance benefits run through the same combined-income calculation as retirement benefits and appear on the same SSA-1099. Supplemental Security Income is separate: SSI payments are not taxable at the federal level and do not appear on Form SSA-1099. If you receive both, only the SSDI portion enters the Publication 915 worksheet.³

Paying the Tax During the Year

Social Security does not automatically withhold federal income tax. If your worksheet shows a meaningful amount on Line 6b and nothing else in your income mix is covering the tax, you can end up with a balance due and possibly an underpayment penalty.

Two options handle this. First, voluntary withholding: file Form W-4V with the Social Security Administration and choose a flat rate of 7%, 10%, 12%, or 22% from each monthly payment. No custom percentages or dollar amounts are permitted.⁸ Second, quarterly estimated payments on Form 1040-ES. For 2026, the due dates are April 15, June 15, and September 15 of 2026, plus January 15, 2027; the January payment can be skipped if you file and pay in full by February 1, 2027.⁹

To avoid the underpayment penalty, you generally need to pay at least 90% of the current year’s tax or 100% of the prior year’s tax, whichever is less. The prior-year safe harbor rises to 110% if your prior-year AGI exceeded $150,000 ($75,000 if married filing separately). A separate waiver may apply if you or your spouse retired after reaching age 62 within the past two years.¹⁰

1
Internal Revenue Service. IRS Reminds Taxpayers Their Social Security Benefits May Be Taxable
2
Internal Revenue Service. Publication 915 (2025), Social Security and Equivalent Railroad Retirement Benefits
3
Internal Revenue Service. Social Security Income
4
Social Security Administration. Get Tax Form (1099/1042S)
5
Internal Revenue Service. 2025 Instructions for Form 1040
6
Internal Revenue Service. Back Payments
7
Internal Revenue Service. Publication 529 (12/2020), Miscellaneous Deductions
8
Internal Revenue Service. Form W-4V – Voluntary Withholding Request
9
Internal Revenue Service. 2026 Form 1040-ES
10
Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty