Each converted dollar can drag 85 cents of Social Security into taxable income with it.
Figures on this page are for the 2026 tax year. Rules that are fixed in law rather than indexed — the Social Security thresholds, for instance — are noted where they come up.
Social Security isn't taxed at a rate. There's no "Social Security bracket". Instead, a formula decides how much of your benefit counts as taxable income, and that formula keys off your other income — including anything you convert.
The consequence is a range of income where each extra dollar of conversion drags up to 85 cents of Social Security into taxable income alongside it. Your taxable income rises by $1.85 for every $1 converted. That multiplier is the torpedo.
The IRS calls the input provisional income, and it's a slightly odd construction:
All your other income, plus any tax-exempt interest, plus half your Social Security benefits.
Compare that against two thresholds, which for 2026 are:
| Filing status | First threshold | Second threshold |
|---|---|---|
| Married filing jointly | $32,000 | $44,000 |
| Single | $25,000 | $34,000 |
| Married filing separately | $0 | $0 |
Below the first, none of your benefit is taxable. Between the two, up to 50% becomes taxable. Above the second, up to 85%.
Three things about that table deserve attention.
Those thresholds have never been inflation-indexed. They were set in 1983 and 1993 and have not moved since. Every year, ordinary inflation pushes more retirees over them. This is not an oversight in the model — it's the law, and any calculator that quietly indexes them forward is flattering you.
Tax-exempt muni interest counts. It's not taxed itself, but it counts toward provisional income, so it can push your Social Security into taxability. Muni bonds are not invisible here.
Married filing separately is brutal. If you lived with your spouse at any point during the year, both thresholds drop to zero and your benefits are taxable almost immediately.
Above the second threshold, each additional dollar of provisional income makes 85 cents more of your Social Security taxable. Add that to the converted dollar itself and your taxable income rises by $1.85.
So the effective cost of a converted dollar is your bracket rate × 1.85:
| Statutory bracket | Actual cost inside the torpedo |
|---|---|
| 10% | 18.5% |
| 12% | 22.2% |
| 22% | 40.7% |
| 24% | 44.4% |
A couple who believe they're "safely in the 12% bracket" and convert into the middle of this range are paying 22.2%. They would have done better converting at 22% in a year when the torpedo wasn't in play.
Here's what makes the torpedo different from a bracket, and what makes it actionable.
Once 85% of your benefits are taxable, there is no more Social Security left to drag in. The effect is spent. Every dollar you convert after that point costs the plain bracket rate.
Concretely — a couple, both over 65, with $48,000 of Social Security and $20,000 of other income:
| Converting | Marginal cost | What's happening |
|---|---|---|
| $10,000 | 0% | still inside the deductions |
| $30,000 | 22.2% | mid-torpedo: 12% × 1.85 |
| $50,000 | 12.0% | torpedo exhausted |
| $70,000 | 12.0% | plain 12% |
| $110,000 | 24.6% | senior deduction phase-out, a different effect |
Which is not the same as saying "always convert more". The right move depends on where your own curve turns over — and on whether pushing further trips an IRMAA threshold or an ACA cliff, which are cliffs rather than rates and don't exhaust themselves.
The thresholds for a single filer are $25,000 and $34,000, against $32,000 and $44,000 for a couple. Not half, but close — and a surviving spouse keeps the larger of the two Social Security benefits, not both, while typically holding the same portfolio.
So the survivor faces a lower threshold on similar income. The torpedo hits sooner and harder. That interacts with the narrower single brackets in a way covered separately in the widow's penalty.
Convert before you claim, if the timing works. The single cleanest way to avoid the torpedo is to have no Social Security in the picture yet. The years between retiring and claiming — especially if you're delaying to 70 for the larger benefit — are the classic conversion window precisely because provisional income is low and there's nothing to drag in.
Look at the whole curve, not one point. The torpedo is a hump, not a wall. Its position and width depend on your benefit size and your other income, so there's no rule of thumb that survives contact with real numbers.
Watch your muni interest. If you hold tax-exempt bonds, they're feeding provisional income even though they never appear as taxable income. It's a common surprise.
Check what happens if you push through. If the torpedo is exhausted at $50,000 and your next threshold is a long way off, the dollars in between are some of the cheapest you'll ever convert.
The calculator models the Publication 915 worksheet exactly and shows the torpedo as a visible hump on the marginal cost curve, with the Social Security component named separately — so you can see how much of a given rate is the bracket and how much is the drag.