If you’re looking for Line 5b to report taxable Social Security benefits on your Form 1040, that line no longer holds them. On the current Form 1040, Social Security goes on Line 6: total benefits on Line 6a, and the taxable portion on Line 6b. Lines 5a and 5b now handle pensions and annuities. The 2018 and 2019 versions of the form did use Lines 5a and 5b for Social Security, which is why the older reference still circulates. The calculation behind the number hasn’t changed with the renumbering.
What Lines 5a, 5b, 6a, and 6b Report Today
On the redesigned Form 1040 you’ll use for a 2025 return filed in 2026, Line 5a is total pensions and annuities, and Line 5b is the taxable portion of those pensions and annuities. Line 6a is the total Social Security benefits you received during the year, and Line 6b is the taxable portion of those benefits. If you’re working from a worksheet or older guidance that says “Line 5b,” translate that to Line 6b for Social Security purposes on any current return.
How the Taxable Amount Is Calculated
The number that lands on Line 6b comes from a figure the IRS calls combined income, sometimes called provisional income. It doesn’t appear anywhere on the return itself, but it drives the whole worksheet. The formula:
Combined Income = Adjusted Gross Income + Tax-Exempt Interest + Half of Total Social Security Benefits
AGI pulls in wages, pension distributions, investment income, traditional IRA and 401(k) withdrawals, and most other taxable income. Tax-exempt interest from municipal bonds counts here even though it stays out of AGI elsewhere on the return. And you add only half of your Social Security, not the full amount. That half-benefit figure is what the IRS uses to test whether your overall financial picture warrants taxing any of the benefits.
A few sources behave in ways people don’t expect. Qualified Roth IRA distributions don’t count toward combined income because they don’t flow into AGI. Traditional IRA and 401(k) withdrawals do, because they do. A Roth conversion adds the converted amount to AGI for the year of the conversion, which can temporarily push combined income well past the top threshold. Supplemental Security Income is not a Social Security benefit for tax purposes: SSI payments don’t appear on Form SSA-1099, aren’t reported on your return, and aren’t part of the combined income calculation at all.
The Thresholds That Determine How Much Is Taxable
Once you have combined income, you compare it against dollar amounts Congress set in 1993. These thresholds are fixed in the tax code and don’t adjust for inflation, so more retirees cross them every year.
Single, Head of Household, or Qualifying Surviving Spouse
- Combined income below $25,000: none of your benefits are taxable.
- Combined income between $25,000 and $34,000: up to 50% of your benefits are taxable.
- Combined income above $34,000: up to 85% of your benefits are taxable.
The statute uses the word “exceeds,” so combined income of exactly $25,000 does not trigger any tax on your benefits.
Married Filing Jointly
- Combined income below $32,000: none of your benefits are taxable.
- Combined income between $32,000 and $44,000: up to 50% taxable.
- Combined income above $44,000: up to 85% taxable.
Both spouses’ incomes and benefits go into the calculation on a joint return, even if only one spouse receives Social Security.
Married Filing Separately
If you file separately and lived with your spouse at any point during the year, the base amount drops to zero. Up to 85% of your benefits become taxable on the first dollar of combined income, with no 50% tier available. If you filed separately but lived apart for the entire year, you use the $25,000 base amount, the same as a single filer, and check the box on your return indicating you lived apart all year.
Working Through the Math
“Up to 50%” and “up to 85%” are ceilings, not flat rates. The actual taxable amount depends on how far your combined income exceeds each threshold.
Suppose you’re single and received $18,000 in Social Security, $20,000 in pension income, and $2,000 in tax-exempt municipal bond interest.
- Half of Social Security: $18,000 × 50% = $9,000
- Combined income: $20,000 + $2,000 + $9,000 = $31,000
That $31,000 clears the $25,000 base but stays under $34,000, which puts you in the 50% tier. The taxable amount is the lesser of:
- 50% of your benefits: $18,000 × 50% = $9,000
- 50% of the excess over the base: ($31,000 − $25,000) × 50% = $3,000
The smaller number wins. So $3,000 of your $18,000 in benefits is taxable, roughly 17% of the total, not the full 50%. The 50% figure is a cap you’d only hit if combined income climbed higher within that tier.
If combined income crosses the adjusted base ($34,000 single, $44,000 joint), the calculation adds a second layer: 85% of the amount above the adjusted threshold, plus the lesser of half your benefits or $4,500 ($6,000 for joint filers). The result is capped at 85% of your total benefits, and that cap is absolute. No matter how high your income climbs, you’ll never pay tax on more than 85% of your Social Security.
The IRS lays this out step by step in Worksheet 1 of Publication 915, which is about 18 lines long. Tax software runs it automatically, but the worksheet is useful if you’re filing by hand or want to verify the number your software produced.
Getting the Number for Line 6a From Form SSA-1099
The Social Security Administration mails Form SSA-1099 to every recipient each January. The figure you need is Box 5, labeled “Net Benefits.” That number goes on Line 6a of your Form 1040.
Box 5 already accounts for Medicare Part B premiums withheld from your checks. Don’t subtract them again in the taxable-amount calculation. Use Box 5 as-is.
Lost the form? You can download a replacement through your online my Social Security account at ssa.gov. Statements are available for the current year (after January 31) and the prior five years. You can also call the SSA at 1-800-772-1213 to request one by mail.
Paying the Tax During the Year
Social Security doesn’t automatically withhold federal income tax from your benefits. If you expect to owe, you have two ways to stay ahead of the bill.
Voluntary Withholding
File Form W-4V with the Social Security Administration to have federal tax withheld from each payment. You choose a flat percentage: 7%, 10%, 12%, or 22%. There’s no option for a dollar amount or a percentage between those choices. Retirees in the 50% taxability range often find 7% or 10% enough; a higher bracket may call for 12% or 22%.
Quarterly Estimated Payments
The alternative is quarterly estimated payments using Form 1040-ES. For the 2026 tax year, the four deadlines are April 15, June 15, September 15, and January 15, 2027.
The IRS charges an underpayment penalty if you don’t pay enough during the year. You avoid it if your total tax due at filing is under $1,000, or if you paid at least 90% of the current year’s tax or 100% of last year’s tax (whichever is smaller). If your AGI last year exceeded $150,000, the safe harbor rises to 110% of the prior year’s tax.
Lump-Sum Payments for Prior Years
If the SSA paid you a lump sum covering benefits for an earlier year, that payment still appears on your current-year SSA-1099. Reporting the entire amount in the current year can inflate your combined income and push more benefits into the taxable range than if you’d received the money on schedule.
The IRS offers a lump-sum election that lets you recalculate as if you’d received the benefits in the year they were intended for. Work through Worksheet 4 in Publication 915 using your income from that earlier year, then compare the result to the standard calculation. If the lump-sum method produces a lower taxable amount, you can elect it by checking the box on Line 6c of Form 1040.