Social Security + Roth Conversion Tax Calculator (2026)

A Roth conversion raises your AGI — and under IRC § 86, a higher AGI makes more of your Social Security benefit taxable. In the worst zone, a $1 conversion costs as if you earned $1.85. This calculator quantifies your Social Security torpedo exposure and finds the conversion amount that avoids it, or shows you exactly what crossing each zone costs.

What the Social Security torpedo actually is

Most retirees understand that Roth conversions are taxed at ordinary income rates. Fewer realize there's a second layer: the conversion increases provisional income, which in turn makes a larger share of the Social Security benefit taxable. That additional SS income is also taxed — so you effectively pay tax on more than the conversion amount itself.

The torpedo gets its name because it's hidden and strikes in a narrow income band. Once your SS is already 85% taxable — which happens at provisional income above $44,000 MFJ / $34,000 single — additional conversions no longer amplify. The torpedo only fires during the phase-in zones.

The two IRC § 86 zones

Source: IRC § 86 — thresholds enacted 1983/1993, never inflation-adjusted.

Why the torpedo hits hardest in the 85% zone — with a worked example

Suppose a single retiree has $24,000 in annual SS and $28,000 in other income (provisional income = $28K + $12K = $40K, in the middle of the 85% phase-in zone). Converting $15,000 pushes provisional income to $55K — still in the 85% zone, but now SS goes from partially taxable to fully taxable.

• Without conversion: provisional income $40K → taxable SS ≈ $11,300
• With $15K conversion: provisional income $55K → taxable SS = $20,400 (at cap)
• Extra SS taxable from the conversion: $9,100
• Total extra taxable income: $15,000 (conversion) + $9,100 (SS) = $24,100
• At 12% bracket: $2,892 in extra tax
• Effective rate on the $15K conversion: 19.3% — even though you're nominally in the 12% bracket

The torpedo nearly doubled the apparent tax cost. This is exactly the interaction the calculator above quantifies for your specific numbers.

How to sidestep the torpedo

There are three practical strategies depending on your situation:

• Convert only up to the phase-in entry point. Keep provisional income below $32,000 MFJ / $25,000 single and zero SS is ever taxable. This requires a very low other-income baseline and limits conversion room.
• Convert past the 85% cap in one move. If SS is going to be 85% taxable anyway (provisional income already above $44K MFJ / $34K single with zero conversions), the torpedo has already fired on your baseline income. Additional conversions above the cap are taxed only at the marginal bracket rate — no further amplification. Many retirees with pensions, dividends, or RMDs are already past the cap before any conversion.
• Delay Social Security while converting heavily. For retirees not yet collecting SS, delaying SS to 70 keeps provisional income lower during the heaviest conversion years. See our SS timing guide for a worked example showing how this preserves an extra $110K of conversion room.

SS torpedo interaction with IRMAA

The torpedo and IRMAA are separate tax systems with separate thresholds — but they can stack. A conversion that crosses the IRMAA Tier 1 threshold ($218,000 MFJ MAGI in 2026) adds a Medicare surcharge on top of the SS torpedo. In practice, most retirees fully in the SS torpedo zone (provisional income $32K–$44K MFJ) have low enough MAGI that IRMAA isn't the binding constraint. But at higher incomes, both apply simultaneously. Use the IRMAA calculator alongside this one to see both constraints.