Roth IRA Conversion Calculator

How a Roth conversion is taxed

A Roth conversion moves money from a pre-tax account (a traditional IRA, a SEP or SIMPLE IRA, or an old 401(k)) into a Roth IRA. Nothing about the investment changes. What changes is when the tax gets paid. Pre-tax dollars have never been taxed, so moving them into a Roth account triggers the bill that was deferred, and everything that grows in the Roth afterwards comes out tax-free in retirement.

The converted amount is added to your ordinary income for that year. It is not taxed at a special rate, and it is not capital gains. It stacks on top of your wages, your business income, and everything else, which means a large conversion can push part of itself into a higher bracket than the one you started the year in. That stacking effect is the single most important thing to understand, and it is why the size of a conversion matters as much as the decision to do one.

There is no dollar limit on a conversion and no income limit on who may do one. You can convert $5,000 or $500,000. You can convert in a year when you earn nothing. The only real constraint is what you are willing to pay in tax to do it.

How to calculate the tax on a Roth conversion

The short version: multiply the amount converted by the marginal rate that applies to it, then check whether the conversion pushed you into a higher bracket partway through.

Worked through properly, it looks like this. Start with your expected taxable income for the year before any conversion. Find the top of your current bracket. The distance between the two is how much you can convert at your current rate. Anything above that is taxed at the next rate up, and so on. If you are sitting at $150,000 of taxable income and the top of your bracket is $210,000, then roughly $60,000 can be converted at the lower rate, and a $100,000 conversion means the last $40,000 costs more per dollar than the first $60,000.

This is why the calculator above asks for a marginal rate rather than computing brackets for you. Marginal rate is a decision, not a lookup: for a modest conversion it is simply your current bracket, but for a large one you should run the number at the blended rate you would actually pay. If the answer changes materially between the two, that is a signal to convert less this year and more next year.

What this calculator assumes

Underneath the interface the model does three things. It stacks the conversion on top of the taxable income you already have and runs it through the 2026 federal brackets, so the tax on the first dollar and the last dollar can differ. It adds your Medicare income (AGI plus the conversion) against the 2026 IRMAA thresholds. And it projects both paths to retirement: the traditional balance left alone, against the balance converted at your chosen pace, with the tax paid either from outside cash or from the converted amount. It is simple enough to rebuild in a spreadsheet, and worth understanding rather than trusting as a black box.

Every conversion calculator makes assumptions, and the ones it makes determine the answer it gives. Here are ours, stated plainly so you can judge whether they fit your situation.

• Taxable income is your AGI less the standard deduction ($16,100 single, $32,200 joint in 2026, plus $2,050 or $1,650 if you are 65). If you itemize, or you qualify for the temporary $6,000 senior deduction, your real taxable income is lower and you have more bracket room than shown.
• The same income and the same conversion every year. Real tax lives are lumpier than that. If your income varies year to year, or you have a gap between retiring and claiming Social Security, model those years separately rather than averaging them.
• State tax is a flat rate on the conversion. Enter your top state rate, or 0 if your state has no income tax. If you plan to retire in a different state, the retirement rate you enter should reflect the state you will be in then.
• Paying from outside cash means the whole conversion lands in the Roth, and the no-conversion path keeps that same cash invested in a taxable account with a 15% drag on its growth. Paying from the converted amount shrinks what reaches the Roth, and if you are under 59½ the withheld portion is also a penalized distribution.
• A flat return. Growth is applied evenly rather than modeled as a sequence. Conversions made after a market decline are worth more than this shows, because you are paying tax on a temporarily depressed balance.
• The conversion repeats annually until the traditional balance is exhausted or your horizon ends, whichever comes first. Enter 0 to model no conversion at all.

The 2026 federal brackets the calculator uses

These are the thresholds from Revenue Procedure 2025-32, applied to taxable income. The bracket you finish the year in is the rate the last dollar of your conversion pays.

Two things stand out. The 24% bracket is wide, running from $105,700 to $201,775 for a single filer and to $403,550 for a couple, and it is only two points above 22%. For most people with a meaningful traditional balance, the top of the 24% bracket is the natural ceiling for a conversion. Above it, the jump to 32% is eight points, and that is where the arithmetic usually turns against converting.

The 2026 IRMAA thresholds

Medicare Part B and Part D premiums are income-tested through a surcharge known as IRMAA, assessed on your modified adjusted gross income from two years earlier. A conversion at 63 raises your premiums at 65. IRMAA is a set of cliffs, not a slope: one dollar of income over a threshold moves you into the next tier for the entire year. These are the 2026 tiers and the approximate extra cost per person per year, Part B and Part D combined.

For a couple, the surcharge applies to each spouse on Medicare, so the first tier costs about $2,300 a year for two people. That is small against the tax on a large conversion, but it is a real cost that the brokerage calculators leave out, and sizing a conversion to stop just under a threshold is standard practice for anyone within a few years of Medicare.

How this differs from the brokerage calculators

The Roth conversion calculators at the large brokerages are fine for a first look, but most of them ask for a single marginal rate and apply it flat, which hides the stacking effect that makes the size of a conversion matter. Most leave state tax out. None that we have seen check the IRMAA thresholds, and few let you choose whether the tax comes from outside cash or from the IRA, which is the single assumption that most changes the answer. This one does all four. It is still a model, and the next section explains why the number it gives you is a starting point rather than a decision.

Filling a bracket instead of converting everything

The most common professional approach is not a single large conversion. It is a series of partial conversions sized to fill the remaining room in a particular bracket each year, and then stop. The logic follows directly from how the tax stacks: the first dollars of a conversion are cheap, the last dollars are expensive, and there is a point on that curve where converting stops being worth it.

The years this works best in are the ones where your income is unusually low. For most people that is the window between the last paycheck and the first required distribution: retired, not yet claiming Social Security, not yet subject to RMDs, and living off taxable savings. Taxable income in those years can be low enough to convert meaningful amounts at rates you will never see again. Business owners get a version of the same opportunity in a bad year, and anyone with a sabbatical, a career change, or a period of self-employment gets one too.

One note on timing that a lot of older material gets wrong: the argument for converting before tax rates went up in 2026 no longer applies. The individual rates set by the 2017 law were made permanent rather than allowed to expire, so there is no legislated deadline pushing conversions forward. The case has to rest on your own bracket trajectory now: your income today against your income at 75, with RMDs running.

Two costs to keep in view

Medicare premium surcharges

Covered above, and now checked by the calculator. The point to carry away is the two-year lag: the conversion you do this year sets the premiums you pay two years from now, and by then it is too late to do anything about it.

The five-year rule

Each conversion starts its own five-year clock. If you are under 59½ and withdraw converted amounts before that clock runs out, you may owe a 10% penalty on money that was already taxed at conversion. There is a separate five-year rule governing tax-free treatment of Roth earnings, which runs from your first Roth contribution of any kind. The practical implication is the same either way: conversions are for money you do not need soon.

One related trap is worth naming, because it is expensive and easy to walk into. If you are under 59½ and have the conversion tax withheld from the IRA rather than paying it from outside funds, the withheld portion is a distribution rather than a conversion, and it is subject to the 10% early withdrawal penalty. That is a direct argument for paying the tax from cash you already hold.

When a conversion usually is not worth it

The honest answer is that conversions lose more often than the enthusiasm around them suggests. The clearest cases against:

• You expect a lower bracket in retirement than you are in now. This is the ordinary case for high earners heading toward a modest retirement income, and it is exactly what deferral was designed for.
• You would have to pay the tax from the IRA. Paying from outside funds is most of what makes conversions work. Without that, the maths is much thinner.
• You are charitably inclined. Qualified charitable distributions let you move IRA money to charity without ever paying income tax on it. Converting first and giving later throws that away.
• Your heirs are in lower brackets than you are. The ten-year distribution rule for inherited IRAs made conversions look better for legacy planning, but it depends entirely on the beneficiary's own tax situation, not yours.
• You hold other pre-tax IRA money. The pro-rata rule aggregates all your traditional, SEP, and SIMPLE IRAs and taxes any conversion proportionally across them. You cannot cherry-pick after-tax dollars to convert.

What none of this arithmetic settles is the part that actually decides the answer: sizing each year's conversion against bracket thresholds, IRMAA tiers, Social Security taxation, and ACA subsidy cliffs at the same time. Those all move together when you convert, and getting one right by getting another wrong is the usual result of working through it alone.