“Is Social Security taxable?” has a frustrating answer: it depends — on a formula most people have never seen. The thresholds are statutory and famously not inflation-indexed, which means more retirees cross them every year. Every number below comes from the same audited formula behind our Social Security estimate helper and Roth conversion planner.
1. Provisional income: the number the IRS actually uses
The IRS doesn't look at your gross benefit. It computes provisional income: all your other taxable income (RMDs, pensions, interest, dividends, taxable gains) plus halfof your Social Security. That's the number the thresholds apply to — which is why a big 401(k) withdrawal in one year can suddenly make most of your benefit taxable.
2. The three bands: 0%, 50%, 85%
For single filers: provisional income up to $25,000 → benefits untaxed. Between $25,000 and $34,000 → the lesser of 50% of benefits or 50% of the excess over $25,000 becomes taxable. Above $34,000 → a base amount plus 85% of the excess, capped at 85% of benefits. For joint filers the thresholds are $32,000 and $44,000. These numbers haven't moved since 1993 — no inflation adjustment, ever.
3. Worked examples, single and joint
Single, $48,000 benefit, $20,000 from an IRA: provisional income is $20,000 + $24,000 = $44,000 — above $34,000. Taxable portion: the lesser of 85% of the benefit ($40,800) or $4,500 + 85% × ($44,000 − $34,000) = $13,000. Result: 27% of the benefit is now taxable income.
Joint, $80,000 combined benefits, $40,000 of RMDs and pension: provisional income $40,000 + $40,000 = $80,000 — above $44,000. Taxable portion: lesser of $68,000 or $6,000 + 85% × $36,000 = $36,600 — about 46% of benefits taxable.
4. The torpedo: when a 22% bracket behaves like 40.7%
In the band between the first and second thresholds, each extra dollar of income drags up to 85 cents of benefit into taxable income. A 12%-bracket retiree faces an effective 22.2% marginal rate there; a 22%-bracket retiree faces up to 40.7%. This is the “tax torpedo” — and it's why the years before you claim benefits, when provisional income is low, are usually the cheapest Roth conversion window of your life.
5. The Roth fix
Qualified Roth withdrawals don't count in provisional income — ever. Converting traditional money to Roth in the gap years between retiring and claiming (or before RMDs begin at 73/75) permanently lowers the taxable share of your benefit and your Medicare IRMAA brackets two years later. The trade: conversions themselves raise provisional income in the year you convert, which is exactly the year-by-year optimization the planner exists for.
6. Compute your own share
Enter your benefit estimate and other income in the SS estimate helper for your taxable percentage, or run full multi-year scenarios — including conversions, IRMAA cliffs, and the torpedo curve — in the Roth conversion planner.