Roth conversion calculator: what a conversion costs this year

A Roth conversion moves money from a pretax account, such as a traditional IRA, into a Roth account. The amount converted is added to taxable income for the year, so the decision comes down to one comparison: the tax rate paid on that slice today against the rate the same money would face when it is withdrawn later. The Roth Conversion Calculator puts dollar figures on both sides of that comparison and shows how much room is left in the current federal bracket before a conversion starts being taxed at the next rate up.

The decision it answers

The question this tool answers is narrow and practical: if I convert a given amount this year, how much federal income tax does it add, how much of it stays inside my current bracket, and how does that compare with paying tax on the same dollars later? It also projects the first required minimum distribution (RMD) the pretax balance would force when RMDs begin (age 73, or 75 for people born in 1960 or later), because a large pretax balance growing untouched is one of the main reasons people look at conversions in the first place.

How the tool computes it

Bracket room

The function calcA() finds the bracket your taxable income falls in using currentBracket(), then computes room as the top of that bracket minus your income, capped at your pretax balance. The tax on that room is room multiplied by the current marginal rate. The brackets are coded in the tool as 2026 federal estimates for Single and Married filing jointly; head of household and married filing separately are not offered.

Tax on a specific conversion

For any amount you enter, taxBetween() walks the brackets from your income up to income plus the conversion, taxing each segment at its own rate. Dividing that tax by the amount gives a blended rate. If the conversion spills past the top of your bracket, the part above is taxed at the higher rate, and the tool flags it.

Now vs later and the RMD angle

calcN() multiplies one amount by a rate now and a rate later that you choose and reports the difference. It deliberately compares rate against rate on one slice and does not model the tax-free growth inside the Roth after conversion. calcD() works out the RMD start age from your age today under SECURE 2.0 (73 if born 1951 to 1959, 75 if born 1960 or later), grows the pretax balance at your assumed rate to that age and divides by the IRS Uniform Lifetime Table factor for that age (26.5 at 73, 24.6 at 75), to estimate the first RMD.

Worked example

These inputs are hypothetical, not a real person or account. The results were produced by running the tool's own calculation code on them.

Filing status: Single
Taxable income this year: $70,000
Pretax balance: $400,000
Amount to convert: $40,000
Rate now / rate later (now vs later tab): 22% / 24%
Current age, growth assumption (RMD tab): 62, 5% a year

Taxable income of $70,000 sits in the 22% bracket, which the tool codes as running from $50,400 to $105,700 for a single filer.
Bracket room is $105,700 minus $70,000, which is $35,700. Converting exactly that much costs $7,854, all of it at 22%.
A $40,000 conversion walks from $70,000 up to $110,000. The first $35,700 is taxed at 22% and the remaining $4,300 at 24%, for a total of $8,886 and a blended rate of 22.2%.
On the now vs later tab, $40,000 at 22% is $8,800 of tax now, against $9,600 at 24% later, so the rate-only difference on that slice is $800.
Because income plus the conversion reaches about $110,000, the tool shows its Medicare IRMAA caution, which it triggers for a single filer above $109,000, the 2026 first-tier threshold published by CMS ($218,000 for married filing jointly).
Someone aged 62 in 2026 was born around 1964, so under SECURE 2.0 their RMDs start at 75. Left unconverted, $400,000 grown 13 years at 5% reaches $754,260 at 75, and the first RMD is $754,260 divided by 24.6, or $30,661 of forced taxable income.
If the $35,700 of room were converted every year, the ladder table shows $214,200 converted over six years for $47,124 of tax, leaving $185,800 pretax, assuming income and brackets hold.

In this hypothetical, the first $35,700 converts at a flat 22%; going to $40,000 adds $4,300 taxed at 24%.

Assumptions and limits

Federal income tax only. State income tax is not calculated, and many states tax conversions.
Brackets are the tool's 2026 federal estimates for Single and Married filing jointly only.
Taxable income is entered directly; the tool does not compute deductions or the standard deduction.
The IRMAA check is a yes or no caution using the 2026 first-tier threshold from CMS for the filing status you choose: above $109,000 for single filers and above $218,000 for married filing jointly. It shows no surcharge amount. ACA subsidy, Social Security taxability and capital-gains effects are text cautions, not calculations.
The RMD start age is estimated from your age today, assuming your birthday has already passed this year, so someone near the 1959/1960 cutoff should confirm their own birth year. The projection uses one constant growth rate. Medicare IRMAA is based on MAGI from two years earlier.
Now vs later ignores tax-free growth after conversion and assumes the tax is paid from outside money.

Sources for the rules on this page

Run it with your own numbers

The Roth Conversion Calculator is free, but it needs a free Obsidian Metrics account: signed-out visitors are sent to sign in first.

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Frequently asked

What does bracket room mean in a Roth conversion?

It is the distance between your taxable income and the top of the federal bracket you are currently in. Converting up to that amount keeps every converted dollar at your current marginal rate. In the tool, room is the bracket top minus your income, capped at your pretax balance.

Is a Roth conversion taxed as ordinary income?

Yes. The pretax amount converted is added to taxable income for the year and taxed at ordinary federal income tax rates, which is how the tool treats it. Any after-tax basis in the account changes the math through the pro-rata rule, which the tool lists as an item to check rather than calculating.

Does the calculator include state tax or IRMAA dollar amounts?

No. It calculates federal tax only. It shows an IRMAA caution when income plus the conversion passes the 2026 first tier for the filing status you choose ($109,000 single, $218,000 married filing jointly), but it does not price the Medicare surcharge.

Why does the tool project a first RMD?

A pretax balance that keeps growing produces larger required withdrawals later, and those withdrawals are taxable whether or not the money is needed. The tool grows your balance to your RMD start age (73, or 75 if born in 1960 or later) at your assumed rate and divides by the IRS Uniform Lifetime Table factor for that age to show the size of that first forced withdrawal.

Can I hand the result to a tax preparer?

The tool builds a plan summary with the estimated tax, the ladder option, the rate comparison and a list of items to check, including IRMAA, ACA, Social Security, state tax, the pro-rata rule and the 5-year rule. You can copy or print it. It is an estimate for discussion, not a tax return.

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