TSP · Roth Conversion

TSP Roth Conversion Tax Estimator

Estimate the federal tax on your TSP in-plan Roth conversion, see which brackets you fill, flag your IRMAA Medicare surcharge risk, and compare converting all at once against a multi-year ladder.

New in 2026

Starting January 28, 2026, federal employees can convert traditional (pre-tax) TSP balances to Roth (after-tax) directly inside My Account — no IRA rollover required. The converted amount is fully taxable in the year of conversion; TSP does not withhold a cent.

Conversion inputs

Current year gross income ($)

Wages + pension + SS + other income before the conversion. Standard deduction applied automatically.

Conversion amount ($)

Min $500 per conversion. The full amount becomes taxable income — TSP does not withhold taxes.

Filing status

Show multi-year ladder comparison (spread conversion over N years vs. all at once)

How the conversion tax is calculated

A Roth in-plan conversion is treated as ordinary income in the year you execute it. The converted amount is added on top of your other taxable income — wages, pension, Social Security, dividends — and taxed progressively through the federal brackets. Your standard deduction reduces gross income first; the conversion amount then stacks on top of the result.

Gross income − standard deduction = baseline taxable income
Baseline taxable income + conversion = post-conversion taxable income
Tax on post-conversion − tax on baseline = conversion tax owed

Because federal taxation is progressive, only the portion of the conversion that falls into a higher bracket is taxed at that higher rate. A $50,000 conversion on $80,000 of income (single filer) would straddle the 22% and 24% brackets — the calculator shows this bracket-by-bracket fill precisely.

2026 federal income tax brackets (single filers)

TCJA brackets were permanently extended by OBBBA, signed July 4, 2025. These rates apply to conversions executed in 2026.

Taxable income range	Rate	Notes
$0 – $12,300	10%	Below standard deduction
$12,300 – $47,150	12%
$47,150 – $105,700	22%	Most federal employees land here
$105,700 – $201,775	24%	GS-14/15 + large conversions
$201,775 – $251,625	32%
$251,625 – $626,350	35%
$626,350+	37%

Source: IRS / Tax Foundation 2026 — TCJA brackets permanently extended by OBBBA (signed July 4, 2025). MFJ thresholds are roughly 2× the single amounts.

The IRMAA trap: how a conversion raises Medicare premiums

IRMAA is a Medicare surcharge applied when your Modified Adjusted Gross Income (MAGI) exceeds annual thresholds. For 2026, the first tier kicks in at $109,000 for single filers — a $1,148/year surcharge per person on top of the standard Part B premium of $202.90/month.

The insidious twist: IRMAA uses income from 2 years prior. A large conversion in 2026 affects your 2028 Medicare premiums — which are often the first or second year you are actually on Medicare if you retire around age 65. Unlike a one-time tax bill, IRMAA is a permanent surcharge for as long as your income stays elevated.

MAGI (single, 2026)	IRMAA tier	Annual surcharge
$0 – $108,999	No surcharge	$0
$109,000 – $136,999	Tier 1	+$1,148/yr
$137,000 – $170,999	Tier 2	+$2,884/yr
$171,000 – $204,999	Tier 3	+$4,615/yr
$205,000 – $499,999	Tier 4	+$6,355/yr
$500,000+	Tier 5	+$6,936/yr

Source: CMS 2026 Medicare Fact Sheet. Married filing jointly thresholds are exactly 2× the single amounts. IRMAA is per person — both spouses on Medicare pay the surcharge independently.

Strategy: the safest conversion years are those where the IRMAA spike falls in FEHB years (before Medicare enrollment). Federal employees who retire at their Minimum Retirement Age (MRA) and enroll in Medicare at 65 have roughly 8 years of conversion runway — ages 57 to 63 — before IRMAA timing starts to bite.

The conversion ladder: spread it out, pay less tax

Because federal income tax is progressive, converting a large traditional TSP balance in a single year can push a substantial portion into higher brackets. Spreading the same conversion over multiple years keeps each year's conversion amount within lower brackets, reducing total tax paid.

Example: Converting $100,000 as a single filer with $80,000 of other income means $59,300 hits the 24% bracket (a heavy cost). Spreading $20,000/year over 5 years keeps every dollar in the 22% bracket — saving roughly $1,700 in total federal tax. The ladder comparison in this calculator shows your specific savings.

Caveat: a ladder works best when your income stays relatively stable across conversion years. If you expect your income to rise (e.g., RMDs starting at 73 stacking on a pension and Social Security), front-loading conversions in low-income years may be better than spreading them evenly.