Tax Torpedo — Social Security

The Tax Torpedo is not a single threshold — it is a range of income where each additional dollar of income (from a Roth conversion, for example) triggers both the direct tax on that dollar AND causes additional Social Security income to become taxable. The compounding effect creates an effective marginal rate well above the stated bracket.

Social Security benefits are taxed on a sliding scale based on "provisional income" — a calculation that adds one-half of your SS benefits to your other income. Up to 50% of benefits become taxable when provisional income exceeds $25,000 (single) / $32,000 (MFJ). Up to 85% become taxable above $34,000 (single) / $44,000 (MFJ).

The torpedo effect occurs in the 50%–85% phase-in zone. In this range, every $1 of additional income (from a Roth conversion, IRA withdrawal, etc.) causes 85 cents of Social Security to become newly taxable. If you are in the 22% bracket, the effective rate on each new dollar is approximately 22% × (1 + 0.85) = 40.7%.

• Provisional income = AGI + tax-exempt interest + 50% of gross SS benefits
• Below $25,000 (single): 0% of SS is taxable
• $25,000–$34,000 (single): up to 50% of SS becomes taxable
• Above $34,000 (single): up to 85% of SS is taxable
• The phase-in rate in the 50%–85% zone: each $1 of income makes $0.85 of SS taxable
• Effective marginal rate in this zone = statutory rate × 1.85 (for a 22% bracket retiree: ~40.7%)

The torpedo zone typically costs $2,000–$10,000 in additional effective tax compared to what a simple marginal rate analysis would suggest. The exact amount depends on the size of Social Security benefits and how much of the torpedo band a Roth conversion traverses.

Once 85% of SS is fully taxable (above the upper threshold), the torpedo effect ends. After that point, additional income is taxed at only the statutory marginal rate. This is why some conversions are better done in large chunks rather than small annual slices — crossing through the full torpedo zone in one year may be more efficient than crossing partially multiple times.