Interactive tool
A conversion’s sticker price is the tax bracket it fills. Its true cost includes the cliffs it crosses. This tool shows both.
Set your income, then drag the conversion slider (or the chart itself) and watch where the dollars land. Uses 2026 federal figures and assumes the standard deduction. Deciding where new savings should go, rather than converting dollars you already have? That’s the Roth vs. Traditional calculator.
Built by Jesse Plunkett, CEPA, CRPC · Last reviewed July 30, 2026 · Verify on 
The cost of each additional $1,000 converted
Measured from your income including the conversion set above.
This is an educational illustration based on the assumptions you enter, not tax advice and not a recommendation to convert. Tax law changes. Talk with your CPA and your advisor before acting. Every calculation happens right here in your browser; nothing you enter is sent anywhere or stored.
One year’s cost is not the whole decision. These are the levers an advisor pressure-tests before any conversion; every one depends on facts this tool can’t see.
The stack. The standard deduction comes off ordinary income first. The conversion is taxed as ordinary income on top of your other ordinary income, through the 2026 federal brackets. Qualified dividends and long-term gains then stack on top of everything else, which is how a conversion can push gains that were taxed at 0% into the 15% rate without selling a single share.
The deduction, if you're 65 or older. The tool starts with the 2026 standard deduction ($32,200 joint, $16,100 single), then uses your ages to add what the bracket tables leave out: the extra standard deduction at 65 ($1,650 per spouse filing jointly, $2,050 single), and the temporary senior deduction of up to $6,000 per person, current law for 2025 through 2028. That senior deduction shrinks by 6 cents for every dollar of income above $150,000 joint or $75,000 single, and a conversion is exactly the kind of income that shrinks it. The tool counts that shrinkage as part of the conversion's cost, because it is.
Social Security. You enter your full benefit and the tool applies the IRS provisional-income formula to decide how much is taxable, from 0% up to the 85% maximum. A conversion can pull more of your benefit into taxable income on its way up; once you're at the 85% cap, further conversion dollars stop adding Social Security tax, and the tool shows that too.
The cliffs. Medicare's income surcharges (IRMAA) are true cliffs: cross a tier by one dollar and the full surcharge applies for the year, per person, for Parts B and D, two years later. Within a tier, every dollar costs the same; the first joint tier runs from $218,000 to $274,000, so a conversion that lands anywhere inside it carries one flat surcharge until the next line at $274,000. The ACA cutoff works the same way in 2026: above 400% of the poverty level, the entire premium credit is gone. Below that line, the credit also shrinks gradually as income rises, because the share of income you're expected to pay toward the benchmark plan climbs from roughly 2% to 10% on the way up; this tool shows only the final cliff, so for marketplace households the true cost of a below-the-line conversion runs somewhat higher than shown. The net investment income tax is gentler but similar in spirit: a conversion is not investment income, yet if it lifts your income past $250,000 joint or $200,000 single, it exposes dividends and gains to an extra 3.8%. The tool counts only the dividends and gains you entered as investment income; interest, rent, and short-term gains also count in real life, so treat the NIIT line as a floor.
Standard deduction vs. itemizing. This tool assumes the standard deduction plus the age-based additions above. If you itemize, your bracket math starts from a different deduction and the dollar figures shift. The cliffs do not move, though: IRMAA, the ACA line, the NIIT threshold, and the senior deduction phase-out are all measured before deductions, so they sit exactly where the tool shows them either way.
What it deliberately leaves out. State income tax. Tax-exempt interest, which counts toward the Medicare and ACA income lines even though it is not taxed. Itemized deductions, the blindness addition, and married-filing-separately, which has its own harsher IRMAA rules. The IRMAA figures assume both Part B and Part D and use 2026 tiers to illustrate a surcharge that would actually be set two years later, under tiers that will have adjusted for inflation by then. Underpayment penalties and withholding timing. And the biggest question of all: whether converting is a good idea, which depends on future tax rates, your heirs, and the rest of your balance sheet. This tool shows what a conversion costs this year, not what it saves over a lifetime.
Yes, and it's new. Since January 28, 2026, the Thrift Savings Plan allows in-plan Roth conversions: you can move traditional TSP dollars to your Roth TSP balance directly inside the plan, no rollover to an IRA required. Active employees, separated participants, and spousal beneficiaries are eligible, the minimum is $500 per conversion, and there's no cap on how often you convert. The converted amount is taxed as ordinary income in the year of the conversion, which is exactly the math this calculator illustrates.
Yes. Whether the dollars come from a traditional TSP, a traditional IRA, or a 401(k), a Roth conversion is ordinary income on your federal return. It fills the same brackets, counts toward the same Medicare (IRMAA) income lines two years later, can pull more of your Social Security into taxable income, and can shrink the senior deduction. One TSP-specific wrinkle worth raising with your advisor: how the tax bill itself will be paid, since paying it well usually means money from outside the account.
No dollar limit and no income limit. Contribution limits apply to contributions, not conversions. The practical limit is the tax math above: each additional dollar converted is taxed at your top bracket or worse once cliffs get involved, which is why many people convert in measured annual amounts rather than all at once.
Yes. A conversion raises your modified adjusted gross income, and Medicare premium surcharges look back two years, so a conversion at 63 can show up in your very first premium at 65. IRMAA is a cliff: one dollar over a tier line triggers the full surcharge for that year, per person, for Parts B and D. The first joint tier starts at $218,000 of income in 2026.
The year-one cost climbs when a conversion crosses lines rather than filling brackets: an IRMAA tier, the ACA subsidy cutoff, the point where gains lose the 0% rate, the NIIT threshold, or the senior deduction phase-out. Whether the conversion saves money over a lifetime is a different question that depends on future tax rates and who eventually spends the dollars, which is a planning conversation, not a calculator output.
Not on the conversion itself, at any age. But if taxes are withheld from the retirement account to pay the bill and you're under 59½, the withheld portion is treated as a distribution and can be penalized. Each conversion also starts its own five-year clock for penalty-free access to those converted dollars before 59½.
The Roth vs. Traditional calculator answers a contribution question: where should new savings go this year. This tool answers a conversion question: what does it cost to move dollars you already saved from traditional to Roth. Conversions are taxed all at once in the year you convert, which is why cliffs dominate this decision and barely touch the other one.
No. Every calculation happens in your browser as you move the sliders. Nothing you type is transmitted, saved, or seen by anyone, including me. Close the tab and it's gone. That's a deliberate design choice, not a technical accident.
Almost always. Everything is handled virtually, so where you live is rarely a barrier. The only exception is a handful of states where I'm not registered yet, and if that's you, I'll tell you on our first call.
The sliders can’t see your other accounts, your state, your conversion window, or what you want the money to do. A short call can cover all of that.
Writers, teachers, and other advisors are welcome to reference this tool or place it on their own page. Both options below keep the calculator with its assumptions, its disclosures, and a link back here, which is what makes it useful to a reader who lands on it somewhere else.
Plunkett, Jesse. “Roth Conversion Calculator: Brackets, Cliffs, and the True Cost.” True Stewards Advisory, https://truestewards.com/roth-conversion-true-cost. Accessed [date].
<iframe src="https://truestewards.com/roth-conversion-true-cost?embed=1"
title="Roth Conversion Calculator by True Stewards Advisory"
width="100%" height="1200" loading="lazy"
style="border:1px solid #ece3d2;border-radius:10px"></iframe>
The embedded version runs the same math in your reader’s browser, collects nothing, and carries the required disclosures with it. Please leave the attribution and disclosure lines in place. Questions, or want a different size? Email me.