Line 6b of Form 1040 is where you report the taxable portion of the Social Security benefits shown on Line 6a. That portion ranges from zero to 85% of your total benefits, depending on how your other income compares to fixed dollar thresholds set in Internal Revenue Code Section 86. To calculate the taxable Social Security amount for Line 6b, you run a figure called provisional income through the worksheet in IRS Publication 915. The result is what you enter.
What Line 6b Represents
Line 6a shows the total benefits you received during the year. Line 6b shows how much of that total the federal government actually taxes. The two lines can differ by a lot: a retiree with modest outside income may report the full benefit amount on 6a and zero on 6b, while a retiree with significant pension or investment income can end up with 85% of benefits on 6b. The 85% figure is a statutory ceiling. No matter how high your other income climbs, the IRS will never tax more than 85% of your Social Security benefits.
Get Your Starting Number From Form SSA-1099
The Social Security Administration mails Form SSA-1099 each January to everyone who received benefits during the prior year. Box 5 shows your net benefits — gross benefits paid minus any repayments to the SSA. That Box 5 figure goes on Line 6a of Form 1040 and feeds the rest of the calculation.1IRS.gov. Form SSA-1099 Social Security Benefit Statement
If your SSA-1099 never arrived or you can’t find it, log into your my Social Security account on ssa.gov to download a replacement.2Social Security Administration. Get Tax Form (1099/1042S) Report the full Box 5 amount on Line 6a even when the calculation shows nothing taxable on Line 6b.
Calculate Your Provisional Income
Provisional income is a test figure that exists only for this calculation. It captures your full economic picture, including income that would normally escape taxation. The formula:
Provisional income = Modified AGI + tax-exempt interest + 50% of Social Security benefits
Modified AGI here means your adjusted gross income calculated as if Social Security benefits weren’t taxable at all. For most retirees, that includes pensions, traditional IRA withdrawals, wages, interest, dividends, and capital gains. If you excluded foreign earned income under Section 911, you add that back.3Office of the Law Revision Counsel. 26 USC 86 Social Security and Tier 1 Railroad Retirement Benefits
Tax-exempt interest, most commonly from municipal bonds, gets added next. This catches income that doesn’t appear on your tax return but still reflects real spending power. Finally, add exactly half of your Line 6a benefits. The 50% inclusion is fixed regardless of income level.
Quick example: $30,000 in pension and investment income, $5,000 in tax-exempt municipal bond interest, and $20,000 in Social Security benefits. Provisional income equals $30,000 + $5,000 + $10,000 = $45,000. That $45,000 is what you compare to the thresholds.
Match Provisional Income to the Thresholds
Congress wrote the thresholds into the statute and never indexed them for inflation. The $25,000 single-filer figure has stood since 1984, and the upper thresholds have stood since 1993. That’s why more retirees end up with taxable benefits each year even when their real income hasn’t changed.
Single, Head of Household, and Qualifying Surviving Spouse
Provisional income at or below $25,000: none of your benefits are taxable. Between $25,001 and $34,000: up to 50% of benefits are taxable. Above $34,000: up to 85% are taxable.3Office of the Law Revision Counsel. 26 USC 86 Social Security and Tier 1 Railroad Retirement Benefits
“Up to” matters. The taxable amount is not simply 50% or 85% of your total benefits. In the first bracket, you’re taxed on the lesser of half your benefits or half the amount by which provisional income exceeds $25,000. In the upper bracket, the 85% rate applies only to the amount over $34,000, and you add the piece from the lower bracket on top. The total is capped at 85% of your benefits.
Married Filing Jointly
Joint filers get higher thresholds: $32,000 and $44,000. Below $32,000 in provisional income, nothing is taxable. Between $32,001 and $44,000, up to 50% is taxable. Above $44,000, up to 85% is taxable.3Office of the Law Revision Counsel. 26 USC 86 Social Security and Tier 1 Railroad Retirement Benefits
Married Filing Separately
The rules turn punitive here. If you’re married, filed separately, and lived with your spouse at any point during the year, your base amount is zero. That means 85% of your benefits are potentially taxable starting with the first dollar of provisional income.3Office of the Law Revision Counsel. 26 USC 86 Social Security and Tier 1 Railroad Retirement Benefits
If you filed separately but lived apart from your spouse for the entire year, you get the $25,000 and $34,000 thresholds. The word “entire” matters. A single day of shared residence during the tax year triggers the zero base amount.
Work the Publication 915 Worksheet
The IRS provides Worksheet 1 in Publication 915 to walk through every step. Tax software handles this automatically, but doing it once by hand makes it easy to spot errors and plan ahead.4Internal Revenue Service. Social Security and Equivalent Railroad Retirement Benefits
Consider Maria, a single filer with $28,000 in pension income, $2,000 in bank interest, $1,500 in tax-exempt municipal bond interest, and $22,000 in Social Security benefits.
Step 1. Provisional income. Modified AGI is $30,000 ($28,000 pension + $2,000 interest). Add $1,500 tax-exempt interest and $11,000 (half of $22,000 in benefits). Provisional income: $42,500.
Step 2. First threshold. Provisional income exceeds $25,000 by $17,500. The 50% tier amount is the lesser of half her benefits ($11,000) or half the excess ($8,750). That’s $8,750. But she also exceeds $34,000, so she continues.
Step 3. Second threshold. The gap between the thresholds is $9,000. Half of that is $4,500, the maximum from the 50% tier. Provisional income exceeds $34,000 by $8,500. Multiply by 85%: $7,225. Add the $4,500 from the first tier: $11,725.
Step 4. Apply the 85% cap. 85% of $22,000 is $18,700. Because $11,725 is less than $18,700, Maria’s taxable amount is $11,725. That’s what goes on Line 6b.
If you’re married filing separately and lived with your spouse, the worksheet skips the two-tier structure. You multiply provisional income by 85%, cap the result at 85% of total benefits, and enter that on Line 6b.4Internal Revenue Service. Social Security and Equivalent Railroad Retirement Benefits
Special Situations That Change the Line 6b Number
Lump-Sum Back Payments
When Social Security pays back benefits owed for prior years, the full amount lands on the SSA-1099 for the year you received it. Under the default rule, the taxable portion goes into current-year income even though the payment covers earlier years. For someone with otherwise modest income, that lump sum can spike provisional income and drag far more benefits into the taxable range than would have applied year by year.
The lump-sum election, made by checking the box on Line 6c of Form 1040, lets you recalculate as if each year’s portion had been received in the year it was owed. If splitting the payment across the prior years produces a lower taxable amount, you use that figure on Line 6b.5Internal Revenue Service. Back Payments
You don’t amend prior returns. The lower taxable amount replaces the default figure on your current return. The calculation uses Worksheets 2 through 4 in Publication 915, with a separate worksheet for each prior year involved. Once elected, the choice can’t be revoked without IRS consent.4Internal Revenue Service. Social Security and Equivalent Railroad Retirement Benefits
Benefit Repayments Exceeding Benefits Received
Box 4 of the SSA-1099 shows repayments to the SSA, and Box 5 reflects the net. Box 5 can be negative if repayments exceeded benefits received during the year. When the negative amount exceeds $3,000, you may either take an itemized deduction on Schedule A or refigure the earlier year’s tax as if the repaid benefits had never been included and claim a credit on Schedule 3 under IRC Section 1341. You use whichever produces less tax. If the negative amount is $3,000 or less, it falls into the miscellaneous itemized deduction category and is not currently deductible.4Internal Revenue Service. Social Security and Equivalent Railroad Retirement Benefits
Planning Moves That Reduce What Lands on Line 6b
Because Line 6b turns on provisional income, anything that reduces that figure can shrink the taxable amount. A few tools work particularly well for retirees.
Qualified charitable distributions. If you’re 70½ or older with a traditional IRA, you can transfer up to $111,000 per year directly to a qualified charity. The QCD satisfies your required minimum distribution without adding to AGI. A $25,000 QCD removes $25,000 from provisional income compared to taking the distribution and donating the cash separately.6Internal Revenue Service. 2026 Amounts Relating to Retirement Plans and IRAs
Roth conversions timed before benefits begin. Roth distributions don’t count toward provisional income, but the conversion itself does add to AGI in the year of conversion. The gap between retirement and claiming Social Security is often the best window. Once benefits start, Roth withdrawals won’t push you over the thresholds.
Income timing. Selling appreciated stock or taking large capital gains in the same year you’re collecting Social Security creates a double hit: the gain is taxed, and it also drags more benefits into taxable territory. When timing is flexible, concentrating large income events in years before benefits begin, or spreading distributions across several years, can keep provisional income under the thresholds.
None of these moves works in isolation. A Roth conversion done in the wrong year can spike provisional income and cause exactly the problem you were trying to avoid. Run the math before any major financial move in retirement.
Downstream Effects of a Higher Line 6b
The Line 6b figure adds to your AGI, and AGI feeds several other tests.
Medicare premiums. The Income-Related Monthly Adjustment Amount for Parts B and D uses your modified AGI from two years earlier. For 2026, a single filer with 2024 income above $109,000 pays a Part B premium of $284.10 per month instead of the standard $202.90, and the surcharges climb from there to $689.90 above $500,000. Part D adds its own surcharges on the same brackets.7Medicare.gov. 2026 Medicare Costs
Net Investment Income Tax. The 3.8% surtax on net investment income applies when modified AGI exceeds $200,000 (single) or $250,000 (married filing jointly). Social Security itself isn’t investment income, but the taxable portion counted in AGI still feeds the threshold test.8Internal Revenue Service. Net Investment Income Tax
State taxes. Eight states tax Social Security benefits to some degree: Colorado, Connecticut, Minnesota, Montana, New Mexico, Rhode Island, Utah, and Vermont. Each has its own exemptions and income thresholds. In states that use federal AGI as their starting point, a higher Line 6b can also affect state-level deductions and credits even when the state itself doesn’t tax benefits directly.