What is bracket filling?

Bracket filling is the practice of converting enough traditional IRA balance to Roth each year to use up a chosen marginal tax bracket before Required Minimum Distributions begin at age 73 or 75 and push you into a higher one. For a married couple with $1.4M in traditional IRA assets who fill to the top of the 22% bracket during their pre-RBD window, the lifetime federal tax and IRMAA difference versus doing nothing is $119,811. That number is reproducible with the default inputs in the calculator; the full scenario is worked through below.

The mechanic

A Roth conversion is a taxable distribution from a traditional IRA that is immediately recontributed to a Roth IRA. The converted amount is added to ordinary income in the year of conversion, taxed at current marginal rates, and then grows tax-free thereafter. RMDs are never required from a Roth IRA during the owner's lifetime. The strategy is deliberately forward-looking: you pay tax now at a known rate under the 2026 brackets (Rev. Proc. 2025-32) to avoid paying tax later at a higher rate driven by the compounding of the untaxed balance and the forced distribution schedule in Pub. 590-B Appendix B Table III.

The name is literal. Think of the brackets as a bucket: income fills it from the bottom, and each slice of income is taxed only at the rate of the band it lands in, never retroactively. The diagram shows a married couple with $150,000 of taxable income and the conversion room left above it.

Why the 60-to-72 window is the inflection point

The strategy works because retirement creates a narrow window of unusually low ordinary income that typically closes again when Social Security and RMDs stack on top of each other. Once you leave a W-2 job, wages fall to zero or near-zero. Social Security has not yet started (or is only partially in play if claimed early). No RMDs are required yet. The result is that large amounts of bracket space at 12% and 22% sit unused, available to absorb a conversion at rates that will likely be unavailable again. For SECURE 2.0 cohorts born 1951-1959, the RBD is age 73; for those born 1960 or later, it is age 75. Once RMDs begin, the IRA itself forces distributions that consume that headroom, and the conversion decision becomes progressively less valuable with each year of delay.

Worked example: the default MFJ scenario

The calculator's default scenario: spouses born 1966 and 1968, $1.4M traditional IRA, $200k Roth, MFJ, $250k wages (still working), $3,400/month Social Security at FRA, retirement at 65, life expectancy 92, 6% nominal growth, 2.5% inflation. No state tax in this baseline (add California and the numbers change further).

The 2026 MFJ brackets after the standard deduction ($32,200 plus $1,650 per spouse age 65+) work out as follows:

The three-way lifetime comparison for this couple, combining federal income tax and IRMAA premiums through age 92:

The fill-22 strategy wins here. The inflation-indexed brackets grow each year, so the couple's RMD projections land primarily within the 22% band rather than spilling into 24% at scale. Converting at 24% now accelerates taxes on dollars that the projected growth and inflation-adjusted brackets would have kept in the 22% range anyway, making the extra tax upfront a net negative. Fill-22 beats fill-12 by only $193 in this scenario, so the strategies are nearly tied; either outperforms doing nothing by roughly $120,000. Fill-24 still saves $60,455 versus the do-nothing baseline but falls well short of the 22% strategy. RMD divisors come from the Uniform Lifetime Table in Pub. 590-B Appendix B Table III; at age 73 the divisor is 26.5, meaning roughly 3.8% of the account must be distributed whether you want the income or not.

Interaction effects that shift the optimal answer

Several thresholds complicate the straightforward bracket math, and each can flip the apparent optimum if ignored.

Social Security taxation under Pub. 915 phases in at provisional income above $32,000 (MFJ base) and reaches the 85% inclusion ceiling at $44,000 MFJ, creating a band where each dollar of conversion income costs effectively 1.85x its face rate.

IRMAA is assessed on MAGI from two years prior, so 2026 Medicare premiums reflect 2024 income. The first MFJ tier kicks in above $218,000 of MAGI and lifts Part B from the $202.90 base to $284.10 per enrollee per month, an $81.20 surcharge. With both spouses on Medicare that is $162.40 a month, or about $1,950 a year for the couple. Add the Part D IRMAA surcharge of $14.50 per enrollee at tier 1 and the cliff cost lands closer to $2,300. A conversion that lands $1 above the tier boundary therefore costs more than the marginal tax rate alone suggests, which is why the calculator's scenario comparison treats IRMAA premiums as a first-class part of lifetime cost.

For retirees under 65 on ACA marketplace coverage, the 400% FPL cliff (restored for 2026 after the ARPA enhanced subsidies expired) can make a conversion that appears to cost 22% actually cost 22% plus the entire lost subsidy, often an effective marginal rate above 50%.

The 3.8% Net Investment Income Tax under IRC §1411 applies to investment income once MAGI clears $250,000 MFJ; a conversion raises MAGI and can drag previously safe dividend or capital-gain income above that threshold.

The SALT deduction under OBBBA phases out above $500,000 AGI: the $40,000 cap is reduced by 30 cents for each dollar of AGI over $500,000, reaching the $10,000 floor at $600,000. A conversion that pushes AGI from $490,000 to $550,000 costs not just the marginal bracket rate but also the lost SALT benefit on the last $50,000 of income above $500,000.

When bracket filling does not work

Households with very large traditional balances, roughly $5M or above for a married couple with typical other income, often project into the 24% bracket on RMDs alone. In that situation, converting at 22% to avoid 22% later is a breakeven at best, and breakevens do not justify the tax acceleration. The correct question is whether conversions can stay inside the 22% bracket today to avoid 24% later, which depends on the specific trajectory. The calculator surfaces this by comparing the marginal rate at conversion against the marginal rate on the projected RMD each year.

The second case where conversion is actively negative: an IRA designated to pass entirely to a qualified charity. A charity inherits traditional IRA assets tax-free regardless of whether the account was ever converted. Converting and paying income tax now only reduces the amount available for the charitable bequest; the conversion provides no benefit because the income tax that would otherwise apply to withdrawals will never be owed. The same logic applies, partially, to a mixed bequest that is partly charitable, scaled to the charitable fraction of the estate plan.

Related reading

How to size the annual conversion amount, and why the answer changes every year as Social Security and RMDs approach, is covered in How much should I convert to Roth this year? The two-year Medicare lookback and how a conversion that crosses the IRMAA tier boundary shows up in premiums two years later is covered in Roth conversion and IRMAA. The 1.85x marginal-rate band that Social Security creates and how it affects conversion sizing is covered in Roth conversion and Social Security. How the length of the conversion window varies by birth cohort under SECURE 2.0 is covered in The Roth conversion window. The four scenarios where conversion produces little or no benefit are covered in When not to do bracket filling.

Go deeper

• Which bracket to fill: the strategy compared across four scenarios
• A worked example, end to end
• The seven rules that bound every plan
• FAQ with computed answers
• Heard it called bracket topping? Same strategy

Run your own numbers.