Roth Conversion Ladder: How to Model the Tax Math

Imagine retiring at 55 with most of your savings locked inside a traditional 401(k). You have no paycheck, Social Security does not start for years, and your taxable income is suddenly very low. For many early retirees, this window of low income can last a decade or more. That window is exactly where a Roth conversion ladder becomes worth modeling carefully.

The core idea is straightforward: during years when your income is low, convert a portion of your pre-tax retirement savings into a Roth IRA, pay tax now at a lower rate, and let that money grow tax-free for the rest of your life. After five years, converted funds (not just earnings) can be withdrawn without penalty, even before age 59½. For early retirees, that five-year rule creates a bridge to penalty-free income before traditional retirement accounts open up.

But the strategy only works well when the math is done carefully. This guide walks through how the ladder works, how to identify the optimal conversion amount, and the specific thresholds you need to know before you convert a single dollar.

A Roth conversion means moving money from a traditional IRA, 401(k), or other pre-tax account into a Roth IRA. The amount converted is added to your ordinary income for that tax year and taxed accordingly. In exchange, the money grows tax-free inside the Roth and qualified withdrawals in retirement are never taxed again.

The ladder part refers to converting in annual installments rather than all at once. Each year's conversion is a separate rung of the ladder, and each has its own five-year clock. Here is a simplified picture of how that plays out:

• Year 1: Convert $40,000 from your traditional IRA. The five-year clock starts.
• Year 2: Convert another $40,000. A new five-year clock starts for this rung.
• Year 3 through Year 5: Continue converting annually. Each rung matures five years after it was converted.
• Year 6: The first rung matures. You can now withdraw that $40,000 (the converted principal) penalty-free, regardless of age.

This structure allows an early retiree to fund living expenses from maturing Roth conversions while continuing to convert new tranches in low-income years. It is worth noting that the five-year rule for conversions is separate from the five-year rule for Roth IRA earnings. Each conversion has its own five-year period for penalty-free access to principal, per IRS rules (see IRS Publication 590-B for the full detail on ordering rules).

For a deeper look at how Roth accounts compare to traditional accounts over time, this comparison of Roth vs Traditional IRA performance over 20 years walks through the long-term numbers in detail.

This is where the tax math becomes both powerful and precise. The goal for many people is to convert as much as possible while staying within a specific tax bracket. Converting one dollar too many can push income into a higher bracket, and in some cases, trigger IRMAA surcharges that cost thousands of dollars per year in added Medicare premiums.

Step 1: Estimate your total ordinary income for the year. This includes part-time work, rental income, interest, taxable Social Security (if applicable), capital gains that count as ordinary income, and any other sources. For early retirees with no earned income, this number may be quite low.

Step 2: Identify your standard deduction or itemized deductions. For 2025, the standard deduction is $15,000 for single filers and $30,000 for married filing jointly. Your taxable income is your gross income minus these deductions. If your deductions exceed your income, you may have room to convert up to the top of the 0% or 10% bracket at very little tax cost.

Step 3: Calculate the gap to the next bracket ceiling. For 2025, the federal income tax brackets for ordinary income are:
(Note: these thresholds are indexed annually. Always verify current figures at irs.gov.)

• 10%: Up to $11,925 (single) / $23,850 (married filing jointly)
• 12%: Up to $48,475 (single) / $96,950 (married filing jointly)
• 22%: Up to $103,350 (single) / $206,700 (married filing jointly)
• 24%: Up to $197,300 (single) / $394,600 (married filing jointly)

Many financial planners discuss the 12% and 22% bracket boundary as a common target. Converting up to the top of the 12% bracket is a frequently discussed approach because the jump from 12% to 22% is significant. But whether that boundary is the right target for any individual depends on many variables, including state income taxes, Social Security benefit taxation, and IRMAA thresholds.

Step 4: Run the IRMAA check. This is the step many people skip, and it can be costly.

IRMAA stands for Income-Related Monthly Adjustment Amount. It is a surcharge added to Medicare Part B and Part D premiums for people whose income exceeds certain thresholds. As of 2025, IRMAA kicks in when Modified Adjusted Gross Income (MAGI) exceeds $106,000 for individuals or $212,000 for married couples filing jointly (per Medicare.gov). These thresholds are adjusted annually.

The critical detail: Medicare uses your income from two years prior to determine IRMAA. A Roth conversion completed in 2025 is counted in your 2025 MAGI, which Medicare uses to set your 2027 premiums. A conversion that pushes your income over an IRMAA threshold could add hundreds of dollars per month to your Medicare costs two years later.

The IRMAA tiers in 2025 for Part B premiums range from the standard $185.00 per month up to $628.90 per month for the highest income tier (per Medicare.gov 2025 data). For a married couple, accidentally crossing the first IRMAA threshold could add more than $800 per year in combined premiums, which can offset a meaningful portion of the tax savings from the conversion itself.

Modeling a Roth conversion therefore means running two calculations simultaneously: the income tax impact in the conversion year, and the potential IRMAA impact two years forward. A retirement tax calculator that accounts for both layers of cost can help make this analysis more concrete.

Important considerations when mapping IRMAA exposure:

• Capital gains realizations, even if taxed at 0%, still count toward MAGI for IRMAA purposes.
• Roth conversions count as ordinary income and are included in MAGI.
• Qualified Roth IRA distributions do not count toward MAGI, which is part of their long-term appeal.
• You can appeal an IRMAA determination if your income has dropped due to a life-changing event, such as retirement itself. The Social Security Administration (SSA) handles these appeals using Form SSA-44.

The Tax Cuts and Jobs Act of 2017 (TCJA) significantly lowered individual income tax rates and nearly doubled the standard deduction. Many of those provisions are currently scheduled to expire after December 31, 2025. If Congress does not act, tax rates in 2026 would revert to pre-TCJA levels, meaning higher marginal rates for many income levels and a lower standard deduction.

For Roth conversion planning, this potential shift matters. If rates rise in 2026, conversions done in 2024 and 2025 at current rates may prove to have been well-timed. However, legislation is always uncertain. The most thoughtful approach is to model conversions under both the current rate structure and a higher-rate scenario, then evaluate how the outcomes compare. It is also worth noting that tax legislation is actively evolving. Recent tax legislation signed in 2025 has introduced additional changes that may affect retirement planning decisions, including deductions relevant to retirees.

A key misconception to address: some people assume they should simply convert as much as possible before 2026 in case rates rise. But this approach ignores bracket management, IRMAA exposure, state income taxes, and the real cost of paying tax early on funds that may not be needed for decades. The decision is genuinely multi-variable, and generic urgency is rarely a sound basis for an irreversible financial move.

And that word, irreversible, is worth emphasizing. Prior to 2018, it was possible to undo a Roth conversion through a process called recharacterization. The TCJA eliminated that option for conversions. A conversion done today cannot be undone if the market drops or your circumstances change. That is precisely why modeling the tax impact thoroughly before converting is not optional, it is essential.

Consider a hypothetical scenario for illustrative purposes only. This is not a plan or recommendation for any individual.

Hypothetical profile: A married couple, both age 57, who retired early. They have $1.2 million in a traditional IRA, $200,000 in a taxable brokerage account, and no Social Security income yet. Their annual living expenses are $70,000, funded from the taxable account. Their taxable income for the year, before any conversion, is $12,000 in dividends and interest.

Step 1 - Gross income before conversion: $12,000
Step 2 - Standard deduction (married filing jointly, 2025): $30,000
Taxable income before conversion: $12,000 minus $30,000 = $0 (the deduction exceeds their income)

This means they have room to convert up to $30,000 before reaching even the bottom of the 10% bracket (because the deduction absorbs income first), and up to roughly $96,950 in taxable income before reaching the 22% bracket ceiling.

Step 3 - IRMAA check: The first IRMAA threshold for married filers is $212,000 in MAGI. Adding a $80,000 conversion to their $12,000 in other income gives a MAGI of $92,000, well below the threshold. A $100,000 conversion would bring MAGI to $112,000, still below the first IRMAA tier.

In this illustrative case, converting somewhere in the range that fills the 12% bracket while staying comfortably below IRMAA thresholds and considering state tax implications might be a reasonable framework to explore with a qualified adviser. The numbers shift considerably if the couple also realizes capital gains, receives rental income, or lives in a state with significant income tax.

This example underscores why a Roth conversion calculator that incorporates federal brackets, IRMAA tiers, and state taxes simultaneously is far more useful than rough mental math. Running multiple scenarios side by side helps clarify the actual after-tax cost of each conversion amount.

For those approaching RMDs, the calculus also changes. Pre-tax accounts that are not converted will eventually be subject to Required Minimum Distributions starting at age 73, which can force taxable income at potentially higher rates and with no ability to control the timing.

Even well-intentioned Roth conversion strategies can go sideways. A few patterns worth being aware of:

• Ignoring state income taxes. Many states tax Roth conversions as ordinary income. A conversion that looks efficient at the federal level may be less attractive when state taxes are added. A handful of states have no income tax, which changes the calculation significantly.
• Forgetting that Social Security income may become partially taxable. Once combined income (adjusted gross income plus nontaxable interest plus half of Social Security benefits) exceeds $32,000 for married filers or $25,000 for single filers, up to 85% of Social Security benefits can become taxable. A Roth conversion increases AGI and can make more of your Social Security taxable in that same year.
• Failing to account for the source of funds to pay the tax bill. Paying the tax on a conversion from the converted funds themselves reduces the amount that ends up in the Roth and diminishes the long-term benefit. Many financial planners point to using funds from a taxable account to cover the tax bill as a way to preserve the full conversion amount inside the Roth.
• Converting during a high-income year by accident. A large capital gain event, a part-time consulting project, or an inheritance in the same year as a planned conversion can push total income well beyond the target bracket. Projecting full-year income before converting is important.
• Starting too late in the year. Conversions must be completed by December 31 of the tax year. Waiting until late December leaves little time to assess the full income picture or correct errors.

A note on next steps: The concepts in this article are intended as general financial education. Roth conversion ladders involve federal and state tax law, Medicare rules, Social Security benefit calculations, and individual financial circumstances that vary widely from person to person. The examples used here are hypothetical and illustrative only. Before converting any amount, it is worth building a multi-year projection that accounts for all income sources, tax brackets, IRMAA thresholds, and your state's tax rules. A qualified financial adviser or CPA with retirement planning experience can help model those scenarios in the context of your full financial picture. For IRS guidance on Roth conversions, see IRS Publication 590-A (contributions) and 590-B (distributions), available at irs.gov.

This article is for general educational purposes only and does not constitute financial, tax, or investment advice. fidser. is not a registered investment adviser or financial planner. Please consult a qualified financial adviser or tax professional before making any decisions about Roth conversions or retirement planning.