How do you calculate the tax on a Roth conversion in 2026?

TL;DR. Roth conversions are taxed as ordinary income in the year of conversion — add the converted amount to your existing taxable income, apply 2026 federal brackets, and check for bracket crossings and IRMAA surcharges two years out. A Roth conversion tax calculator automates the traditional-to-Roth tax calculation so you can size the conversion to stay within a target bracket and avoid surprise Roth conversion costs.

How the tax calculation works

A Roth conversion is mechanically simple: move dollars from a pre-tax IRA (or 401(k) rolled to an IRA) into a Roth IRA. The IRS treats the converted amount as ordinary income in the year you convert. No special rate, no capital-gains treatment — it stacks on top of everything else on your 1040.

Here is a worked example for 2026. Assume a married-filing-jointly couple with $150,000 of taxable income before any conversion. Their last dollar falls in the 22% bracket, which for MFJ in 2026 runs from $96,950 to $206,700. They have $56,700 of room before crossing into the 24% bracket ($206,700 minus $150,000).

They convert $50,000 from a traditional IRA. New taxable income: $200,000. The entire $50,000 is taxed at 22%, producing $11,000 of federal conversion tax. They stay below the 24% threshold and below the $212,000 IRMAA Tier 1 threshold for MFJ — so no Medicare premium surcharge in 2028.

If they instead converted $70,000, the first $56,700 would be taxed at 22% and the remaining $13,300 at 24%. Total tax: $12,474 + $3,192 = $15,666. And their 2026 MAGI of $220,000 would trigger Tier 1 IRMAA in 2028 — roughly $1,050 in extra annual Medicare premiums. That Roth conversion cost is easy to miss without a calculator that models the IRMAA lookback.

Try it with your numbers

What a good Roth conversion tax calculator should show

• Year-by-year tax bracket impact with marginal and effective rates
• IRMAA two-year lookback projection showing Medicare premium surcharges
• Side-by-side comparison of convert vs. no-convert scenarios over a 10-30 year horizon
• RMD reduction estimate under SECURE 2.0 rules (age 73 or 75 start)
• Estate value comparison showing Roth inheritance vs. traditional IRA 10-year rule impact

AdvisorCal's Roth Conversion Calculator handles all of the above. If you also want to model required minimum distributions, IRMAA bracket thresholds, or backdoor Roth mechanics, those tools are included in the same subscription.

Key facts

• 2026 federal brackets (MFJ): 10% up to $23,850 | 12% to $96,950 | 22% to $206,700 | 24% to $394,600 | 32% to $501,050 | 35% to $751,600 | 37% above.
• 2026 federal brackets (Single): 10% up to $11,925 | 12% to $48,475 | 22% to $103,350 | 24% to $197,300 | 32% to $250,525 | 35% to $626,350 | 37% above.
• IRMAA Tier 1 threshold (2026): MAGI above $106,000 single / $212,000 MFJ triggers higher Medicare Part B and Part D premiums two years later.
• Conversion deadline: December 31 of the conversion year. No extensions, no recharacterization (eliminated by TCJA).
• Mega backdoor Roth tax treatment: After-tax 401(k) contributions converted to Roth are not taxed again — only the earnings portion is taxable. A mega backdoor Roth tax calculation differs from a standard conversion.
• 10% early withdrawal penalty: Does not apply to Roth conversions, but does apply if you withdraw converted dollars within five years and are under 59 1/2.

Common follow-ups

How do I figure out my Roth conversion tax bracket for 2026? Start with your projected adjusted gross income minus the standard deduction (or itemized deductions). That is your taxable income before conversion. Look up where that number falls in the 2026 bracket table. The gap between your current taxable income and the top of your target bracket is your "room" for a tax-efficient conversion. A Roth conversion tax calculator does this arithmetic automatically and layers in IRMAA, NIIT, and Social Security provisional income effects.

Does a Roth conversion increase my Medicare premiums? It can. Medicare uses MAGI from two years prior. A 2026 conversion increases 2026 MAGI, which CMS uses to set 2028 Part B and Part D premiums. The first IRMAA tier for MFJ starts at $212,000. If your conversion pushes MAGI above that line, you will pay an income-related monthly adjustment amount on top of the standard premium. See our IRMAA threshold breakdown for the full tier schedule.

Should I convert all at once or spread it over several years? Most advisors recommend staged conversions — converting enough each year to fill a target bracket without crossing into the next one. This spreads the tax hit, keeps IRMAA under control, and avoids a single year of very high marginal tax. The traditional-to-Roth tax calculation changes each year as income, brackets, and deductions shift, so rerun the numbers annually.

Is there a way to undo a Roth conversion? No. Since 2018 (TCJA), recharacterization of Roth conversions is permanently eliminated. Once the conversion is reported, the tax is owed. This makes accurate upfront calculation essential — you cannot reverse a conversion that pushes you into a higher bracket or triggers IRMAA.

When this doesn't apply

Roth conversions are not beneficial in every scenario. If a client expects significantly lower income in future years (for example, early retirement before Social Security begins), it may be better to wait and convert during those low-income years. Clients with very short life expectancy and no legacy objective gain little from prepaying tax. Non-deductible traditional IRA contributions require Form 8606 pro-rata calculations, which change the Roth conversion cost substantially. And clients in states with no income tax may face different tradeoffs than those in high-tax states. Always model the specific situation rather than applying a blanket rule.

Sources

• IRS — Roth IRAs
• IRS — IRA FAQs: Rollovers and Roth Conversions
• CMS — Medicare Part B Premiums and IRMAA

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