TSP Roth Conversion Calculator

TSP launched in-plan Roth conversions on January 28, 2026 — you can now convert traditional (pre-tax) TSP to Roth TSP without leaving the plan. This calculator shows how much to convert each year using a fill-the-bracket strategy, what you'll pay in taxes, how conversions reduce your future Required Minimum Distributions, and whether you're at risk of crossing the IRMAA cliff.

How the fill-the-bracket strategy works

Your FERS pension, TSP withdrawals, and Social Security are all ordinary income taxed at progressive rates. Many federal retirees end up in the 22% or 24% bracket in early retirement — then hit 24% or 32% once Required Minimum Distributions kick in on a $700K–$1.5M traditional TSP balance. The fill-the-bracket strategy converts just enough traditional TSP to Roth each year to max out a lower bracket without spilling into the next one. You pay 12% or 22% now on those dollars rather than 24% or 32% on RMDs later.

Why the FERS supplement gap is the prime conversion window

Federal employees who retire before age 62 often have the lowest taxable income of their post-service lives: FERS pension covers base expenses, the FERS supplement replaces some SS income, but combined income is typically well below peak-earning years. This low-income gap — before Social Security starts at 62+ and before RMDs begin at 73 or 75 — is often the most tax-efficient window for converting. A FERS retiree at 57 with $55K in pension income might have $155K+ of bracket room in the 22% band.

TSP conversion mechanics: what the calculator assumes

• Minimum conversion: $500 per source (traditional pre-tax and tax-exempt combat zone are separate sources).
• Maximum per year: 26 conversions. The calculator assumes you can convert your full annual amount in one or several transactions.
• Tax payment: TSP does not withhold on in-plan conversions — you pay with outside funds. The calculator shows the tax cost separately so you can plan quarterly estimated payments.
• Irrevocable: TSP in-plan conversions cannot be undone. Roth IRA recharacterization doesn't apply to TSP conversions.

IRMAA: the Medicare income trap

Medicare Part B surcharges (IRMAA) kick in when your Modified Adjusted Gross Income exceeds $109,000 (single) or $218,000 (MFJ) — the 2026 first-tier thresholds.² TSP conversions are added to your MAGI in the conversion year. IRMAA applies two years later (the 2026 conversion shows up in your 2028 Medicare premium). If a large conversion pushes you past the threshold, you'll owe ~$974–$4,644 in extra Part B premiums per person per year. The calculator flags this risk so you can size your conversion to stay below the cliff.

The 5-year clock

TSP has a single Roth 5-year clock — different from Roth IRAs, which track each conversion separately. The clock starts on January 1 of the first year you made any Roth TSP contribution or conversion. Earnings on converted balances are not tax-free until the clock completes AND you are 59½. The converted amount itself (your basis) is always withdrawn tax-free. If you haven't previously contributed to Roth TSP, the clock starts in the year of your first conversion. See the full guide for the 5-year mechanics and how they interact with separation.