Roth Conversion at the 85% Social Security Tax Torpedo

The decision: Margaret’s $30,000 conversion

Margaret and her husband are 68, married filing jointly, and living in Texas (no state income tax). Their income for the year is a $48,000 Social Security benefit between them and roughly $16,000 of pension and interest income. Their adviser suggested converting $30,000 from his traditional IRA to a Roth this year to “use up the 12% bracket.” On the surface it looks cheap: $30,000 × 12% = $3,600 of tax.

It is not cheap. Before the conversion, their provisional income is about $40,000 — AGI of $16,000 plus 50% of their $48,000 benefit ($24,000). That already puts them in the 50% taxation phase but below the $44,000 line where 85% kicks in. The $30,000 conversion lands fully in AGI, pushing provisional income to roughly $70,000. That clears the $44,000 threshold and drags the top slice of their benefits into income. The real cost of the conversion is not $3,600 — the effective rate on those conversion dollars climbs to roughly 22.2%, because each dollar converted also makes up to $0.85 of their benefits taxable. The decision lever: convert less this year, or convert in a year before benefits started.

How provisional income works (IRC §86)

The taxation of Social Security benefits is governed by IRC §86, and it turns on a figure the statute calls provisional income (often labeled “combined income” by the SSA). The formula:

• Your adjusted gross income, excluding Social Security benefits, plus
• Any tax-exempt interest (yes, municipal bond interest counts here), plus
• One-half of your annual Social Security benefits.

A Roth conversion is fully taxable ordinary income, so it enters AGI and lifts provisional income one-for-one. Tax-free Roth distributions in later years do not appear in this formula — which is precisely why accepting a controlled tax hit now can keep future provisional income low and protect benefits from the 85% formula in the RMD years.

The two thresholds (1983 levels, never indexed)

The thresholds that decide how much of your benefit is taxable were written into law in 1983 and 1993 and have never been adjusted for inflation. That is the structural reason the torpedo catches more middle-income retirees every year — the dollar lines stand still while benefits and prices rise.

Note the word “up to.” Crossing $44,000 MFJ does not instantly make 85% of your benefit taxable — it starts the 85% formula, which phases the taxable share upward as provisional income climbs. The maximum taxable share is capped at 85% of your benefit no matter how high your income goes (source: SSA; IRC §86).

Why the effective rate is 22.2%, not 12%

Inside the 85% phase-in zone, every extra dollar of ordinary income (including a conversion dollar) does two things at once: it gets taxed at your nominal bracket, and it makes $0.85 of your Social Security benefit newly taxable. So one converted dollar produces $1.85 of taxable income.

1. You are in the 12% federal bracket (MFJ taxable income up to $96,950 in 2026).
2. Each $1 converted is taxed at 12% → $0.12.
3. That same $1 makes $0.85 of benefits taxable → $0.85 × 12% = $0.102.
4. Combined tax per converted dollar: $0.12 + $0.102 = $0.222, an effective rate of 22.2%.

The same arithmetic at the 22% bracket produces 22% × 1.85 = 40.7% — higher than the 32% bracket rate on paper. This is the torpedo: a middle-income retiree can face a marginal rate that exceeds a high earner’s. It is a phantom rate that never appears on the bracket table.

The math, side by side

Here is Margaret’s $30,000 conversion against a smaller $4,000 conversion that stays under the $44,000 line. Both assume the 12% federal bracket and no state income tax (Texas).

The $30,000 conversion does not produce a flat $3,600 bill — the benefit drag pushes the true cost meaningfully higher because tens of thousands of benefit dollars get pulled into income alongside the conversion. The $4,000 conversion captures cheap bracket space without firing the torpedo. The lever is sizing, not avoidance.

What most people miss: the torpedo has an exit

The most common misconception is that once you are “in the torpedo,” every additional dollar is punished forever. It is not. The torpedo is a bubble, not a permanent surcharge. Once 85% of your benefits are already counted as taxable, there are no more benefit dollars left to drag into income — so the next dollar of conversion is taxed at the plain bracket rate again.

This changes the strategy in a non-obvious way. If you are going to be forced through the torpedo zone anyway (large pre-tax balances, looming RMDs at 73 or 75 under SECURE 2.0 §107), it can be cheaper to convert aggressively in a single year — blow all the way through the 85% phase-in to the far side — rather than dribble small conversions that keep you parked inside the bubble year after year. Three myths worth correcting:

• “The 85% threshold means I lose 85% of my benefit.” No. It means up to 85% of your benefit becomes taxable income — you then pay your ordinary rate on that portion, not lose it.
• “Municipal bond interest keeps me safe.” No. Tax-exempt interest is explicitly added back into provisional income under §86. Munis do not dodge the torpedo.
• “The thresholds rise with inflation like the brackets do.” No. The $25K/$34K and $32K/$44K lines are frozen at 1983/1993 levels and have never been indexed.

The gap-year window before you claim

The cleanest place to convert is the window between retiring and claiming Social Security — often the years from 62 to full retirement age (67 for those born 1960 or later) or even to 70 if you delay for the 8%-per-year delayed credits. In those years your benefit is $0, so the provisional-income formula has no benefit half to add. You can fill the 10% and 12% brackets with conversions and never touch the torpedo, because there are no benefits in the math yet.

Once benefits begin, the same conversion can cost the nominal rate plus the 85% drag. So the sequencing decision is concrete: convert early and heavily before claiming, then let RMDs land on a smaller pre-tax balance. The Roth conversion deadline is December 31 of the tax year — not April 15 like an IRA contribution — and recharacterization was eliminated by the TCJA, so a conversion cannot be undone (source: IRC §408A; stats §12).

The second torpedo: IRMAA on the same conversion

A conversion that triggers the Social Security torpedo can also trip the IRMAA surcharge on Medicare Part B and Part D — an income-related cliff, not a phase-in. For 2026 premiums (based on 2024 MAGI), a married couple staying at or under $206,000 pays the base $185.00/month Part B premium each; one dollar over moves both spouses to $259/month plus a Part D surcharge. Margaret’s $70,000 of provisional income is nowhere near that cliff, so IRMAA is not her problem — but for a higher-income retiree, the same conversion can detonate two cliffs at once.

The planning point: when you model a conversion, check it against three lines, not one — your federal bracket, the $34K/$44K Social Security thresholds, and the nearest IRMAA tier (source: CMS 2026; stats §9). A conversion that is cheap on the bracket table can be expensive once all three are layered.

How to size the conversion to the room you have

The actionable rule is a subtraction. Take the 85% threshold for your filing status, subtract your current provisional income, and the difference is your conversion room before the torpedo engages:

1. Compute current provisional income: AGI excluding Social Security + tax-exempt interest + 50% of benefits.
2. Subtract from your 85% threshold: $34,000 (single) or $44,000 (MFJ).
3. The gap is your low-cost conversion room. Margaret’s gap is $44,000 − $40,000 = only $4,000.
4. Decide: convert within the gap for the cheap rate, or deliberately blow through to the far side of the 85% phase-in if you have a large balance and RMDs coming.

Key takeaways

• A Roth conversion raises provisional income (IRC §86) dollar-for-dollar; cross $34,000 single or $44,000 MFJ and up to 85% of your benefits become taxable.
• Inside the 85% phase-in, each conversion dollar carries $1.85 of taxable income — an effective 22.2% at the 12% bracket, ~40.7% at 22%. That phantom rate never shows on the bracket table.
• The thresholds are 1983/1993 levels and are not inflation-indexed — the torpedo catches more middle-income retirees every year.
• The torpedo is a bubble with an exit: once 85% of benefits are counted, the next dollar is taxed at the plain bracket rate. Large balances may be cheaper to convert through in one big year than to dribble.
• Convert in the gap years before you claim, when benefits are $0 and absent from the formula. Deadline is December 31; recharacterization is gone (TCJA).
• Check the conversion against three lines at once: federal bracket, the $34K/$44K SS thresholds, and the nearest IRMAA cliff ($206K MFJ MAGI for 2026 base premiums).