Roth Conversion Before December 31, 2026: How to Size It to the Bracket (With the Exact Math)

Published October 5, 2026. All brackets and limits below are the IRS's official 2026 figures. Educational information only — not tax advice. Confirm your situation with a qualified tax professional.

Of all the year-end retirement deadlines, the Roth conversion is the one people most often want to use and least often know how to size. Contribution limits are fixed. RMDs are mandatory. A Roth conversion is the opposite: you choose the exact dollar amount, and you can choose it right up to December 31.

That flexibility is the whole opportunity — and the whole risk. Convert too little and you leave cheap bracket space unused. Convert too much and the last dollar is taxed at a higher rate than it needed to be, with a possible Medicare surcharge two years later.

Here is how to size it on purpose.

Why 2026 Is an Unusually Good Window

Two facts make this a high-value year to look at conversions:

1. The TCJA-era individual brackets did not sunset. The 2025 reconciliation law, P.L. 119-21, made several individual tax provisions permanent under current law — so the previously anticipated 2025 jump in rates is not the basis for 2026 planning. (Future Congresses can still change the rules. See the Congressional Research Service summary.)
2. You know your income. By fall, you can see your actual taxable income for the year — which is what lets you make a precise conversion decision instead of a guess.

If you retired this year, took a sabbatical, had a low-income year, or simply have unused bracket space, you have a window that will not stay open forever: before RMDs and Social Security fully stack up, your taxable income is lower than it will be later. Converting now can move money into tax-free growth at a rate you may never see again.

The 2026 Brackets You're Filling

A conversion adds to your ordinary income, so the relevant question is: how much room is left before your next bracket starts? Here are the 2026 federal rates on taxable income (after the standard or itemized deduction).

The key word is taxable. Your conversion doesn't start at your last bracket — it starts at your taxable income, which is gross income minus your standard deduction ($15,000 single / $30,000 married for 2026) or your itemized deductions.

Room left in a bracket = top of bracket − your current taxable income.

That subtraction is the entire game. Everything else is execution.

Worked Example: Filling the 22% Bracket

A married couple, both 58, retired in June 2026, planning to delay Social Security.

Converting anywhere up to $119,400 stays inside the 22% bracket. The next dollar — the $119,401st — would be taxed at 24%, and every dollar after that, too.

That does not mean convert $119,400. It means you now know the boundary, and you can decide how close to sit to it based on three questions:

• Will you pay the tax from the IRA or from cash? Paying tax from outside the IRA is generally far more powerful — the full converted amount keeps growing tax-free. Paying from the IRA means less money lands in the Roth.

• What does the conversion do to your Medicare premium? (See the IRMAA section below — for a 58-year-old not yet on Medicare, this is a 2028-decade problem, not a 2027 one.)
• What rate will you pay later on that same dollar? Compare the conversion rate to your expected future rate, not to zero.

The Three Rules That Resolve Most "How Much?" Debates

1. Fill the bracket, do not blow through it — unless you planned to. Going $1 past a threshold taxes that dollar at the higher rate. That is fine if you decided the long-term tax-free growth justifies it. It is a mistake if it was an accident. Sitting exactly at the line maximizes the cheap dollars.

2. Watch the hidden cliffs, not just the marginal rate. The marginal bracket is not the only threshold. Near the boundary, the net investment income tax and, for seniors, IRMAA and the Social Security taxability formula can create effective rates well above the headline number. Run the whole return; do not stop at the bracket table.

3. Respect the pro-rata rule if you can't fund the conversion cleanly. If you hold a pre-tax IRA balance (including the original deductible IRA, SEP, or SIMPLE) your conversion is taxed pro rata across all your IRAs — you cannot convert only your after-tax basis. This is a common trap for high earners doing a "backdoor Roth." If you have a large pre-tax IRA, model the taxable portion before you convert. See Roth Conversion Strategies for the mechanics.

The Deadline: December 31, and Why It's Final

A Roth conversion for tax year 2026 must be completed by December 31, 2026 — not the April filing deadline the way an IRA contribution can be. The conversion is reported on Form 1099-R and Form 8606 (for any after-tax basis), and the taxable amount lands on your 2026 return.

Two operational details that cost people money:

• Do not send a paper conversion form on December 29. Custodians process slowly at year-end. Initiate the conversion in the custodian's portal with days to spare.
• Conversion recharacterization is not available. Once you convert, it is permanent — you cannot "undo" it if the market drops. That is exactly why sizing it before you execute matters.

The related tax traps, for context: December 31 is also the deadline for 401(k) deferrals, HSA contributions, and tax-loss harvesting — and the deadline for most 2026 RMDs. Miss an RMD and the penalty is 25% of the amount you should have withdrawn (reduced to 10% if corrected in time). See the Year-End Retirement Tax Checklist 2026 for all nine moves.

Don't Forget IRMAA (the Two-Year Delayed Surcharge)

Here is the part that surprises people who convert carefully but plan only one year at a time.

IRMAA — the Medicare income surcharge — uses a two-year lookback. A conversion you complete in 2026 affects your 2028 Medicare Part B and Part D premiums, because 2026 is the income year Medicare examines for 2028. It is a cliff, not a phase-in: crossing a threshold by a dollar can raise both premiums for the year.

This does not mean "never convert." It means model the surcharge as part of the conversion cost:

Total cost of the conversion = federal tax + state tax + any IRMAA surcharge two years later.

A 22% bracket fill that triggers an IRMAA cliff can still be the right answer — but only if you decided it with the surcharge on the page. Filling the 22% bracket and then tripping an IRMAA cliff by accident is the failure mode to avoid.

If a life-changing event (retirement, for example) reduced your income, SSA Form SSA-44 can request a reduction in the surcharge. And if you are coordinating this with your Medicare plan choice, see Medicare Open Enrollment 2027: The Decision Tree — both decisions land on the same December calendar.

A 5-Step Decision Process

1. Find your taxable income for 2026 (last pay stub, year-to-date, or a projection).
2. Subtract it from the top of your target bracket to get your conversion room.
3. Decide where in the room to land — full bracket fill, partial, or past it on purpose.
4. Add the second-order costs — state tax, NIIT, IRMAA two years out, and the taxbill source (cash vs. IRA).
5. Execute in the portal before mid-December, and confirm the trade settled before December 31.

Write down the reasoning while you do it. Next year's conversion decision starts with this year's answer.

Put Your Own Numbers Behind It

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Your Next Step

1. Today: Add up your 2026 taxable income.
2. This week: Subtract it from your target bracket's ceiling — that's your conversion room.
3. Before deciding: Model the state tax, NIIT, and the 2028 IRMAA cliff.
4. Before mid-December: Execute in the custodian's portal, not by paper.
5. December 31: Conversion window closes. There is no extension.

⚠️ Educational information only. This article is not tax, legal, or financial advice. Figures reflect 2026 IRS data and can change; verify against IRS.gov. Conversion outcomes depend on your full return — consult a qualified tax professional before executing.

Related Reading

• Year-End Retirement Tax Checklist 2026: 9 Moves Before December 31
• Roth Conversion Calculator: The Optimal Strategy
• Roth Conversion Strategies
• Medicare Open Enrollment 2027: The Decision Tree
• The 7 Most-Searched Retirement Questions of 2026