Model a Roth conversion window without hiding the assumptions
For many households there is a stretch of unusually low taxable income between the last paycheck and the first required minimum distribution: wages have stopped, Social Security may not have started, and withdrawals are discretionary. That stretch is often called the conversion window, because moving traditional-IRA dollars to Roth during it can mean paying tax at a lower rate than the money would otherwise face later.
Whether a conversion actually helps, and how large it should be, depends entirely on assumptions — several of which tools and articles routinely leave implicit. This guide names them, so you can check any conversion analysis (including one from our own planner) instead of taking its word.
The core comparison, stated plainly
A Roth conversion prepays tax. It helps exactly when the rate you pay now is lower than the rate those dollars — or your heirs — would pay later. Everything else is detail on top of that one comparison. So the honest question is never “how much tax does this save this year?” (a conversion raises this year’s tax) but “does prepaying at today’s rate beat paying at the future rate this money would actually face?”
What the future rate would have been
The later rate is driven by things a year-by-year model can make explicit: required minimum distributions pushing income up from the RMD age onward; the survivor filing single on nearly the same income after a spouse dies, in narrower brackets; and where current bracket law is scheduled to go. A model that just assumes “same bracket forever” has quietly answered the question in advance.
Who pays the conversion tax
Paying the tax from outside the IRA (taxable savings) converts the full amount and lets it all grow tax-free. Paying the tax from the converted dollars shrinks the amount that reaches the Roth and much of the arithmetic advantage with it. Any analysis should state which it assumed; the two can flip a marginal decision.
The cliffs between the brackets
The federal brackets are not the only rates in play during the window:
- Medicare income surcharges (IRMAA). Once you are within two years of Medicare enrollment, conversion income counts toward the surcharge thresholds — which are cliffs, not phase-ins. One dollar over a threshold raises premiums for a full year, two years later (Medicare.gov).
- ACA premium credits before 65. Marketplace subsidies fall as income rises, which acts as an extra marginal rate on every converted dollar while you are on a marketplace plan.
- The taxable share of Social Security. If benefits have started, conversion income can drag more of them into taxation, again raising the true marginal rate above the bracket table.
A conversion sized to “fill the 22% bracket” can carry a real marginal cost well above 22% once these interact. A year-by-year model that computes them together is the only way to see the true price of each converted dollar.
The heirs
Since the SECURE Act, most non-spouse heirs must empty an inherited IRA within ten years (IRS: inherited IRA rules). A traditional IRA lands in the heirs’ brackets during what are often their peak earning years; an inherited Roth generally empties tax-free — generally, because earnings are only tax-free to beneficiaries once the account has satisfied the five-year rule (IRS Publication 590-B), a detail that matters for a recently opened Roth. If leaving money to children in high brackets is likely, conversions are partly a gift of their tax rate — an assumption worth making consciously, since it can dominate the math.
Growth, longevity, and the state you’ll live in
Converted dollars must stay invested long enough for tax-free growth to repay the prepaid tax — early death with no surviving spouse or heirs weakens the case. And state tax matters on both ends: converting while in a high-tax state and retiring to a no-tax state reverses part of the benefit, while the opposite move strengthens it. A model that lets you change residence mid-plan can show this; a single-rate model cannot.
What honest output looks like
Given all that, be suspicious of any tool whose conversion answer is a single number with no ledger behind it. Useful output shows, for each candidate strategy, the year-by-year taxes with and without conversions, lifetime tax paid, and ending after-tax wealth — because minimizing lifetime tax is not the goal; ending with more after-tax wealth (for you or your heirs) is. Sometimes the strategy that pays more total tax wins.
The RetireGolden planner models conversions this way — bracket-fill strategies and an optimizer, computed against the full year-by-year tax ledger, with every threshold’s source cited — so the assumptions above are settings you can see and change, not defaults you have to guess at.
RetireGolden guides are educational — not financial, tax, legal, or investment advice. Thresholds and rules described here are as of the 2026 tax year and change over time; conversion decisions are consequential and worth reviewing with a qualified professional who knows your full situation.