The Only Question That Matters
Strip away the complexity and a Roth conversion is one bet: the rate you pay today is lower than the rate you — or whoever inherits the account — would pay later.
Everything else in this article is about answering that question honestly, because almost every mistake in Roth planning comes from getting one of three things wrong: misjudging today's rate, misjudging tomorrow's, or forgetting that the tax bracket is not the only cost.
A conversion moves money from a traditional pre-tax account into a Roth IRA. The converted amount is added to your ordinary income for the year. In exchange, the money grows tax-free, qualified withdrawals are tax-free, and — importantly — the balance no longer generates required minimum distributions during your lifetime.
This Is Not About Beating the Market
A conversion doesn't change what your money is invested in or what it earns. The same dollars in the same funds simply move to a differently-taxed wrapper. All of the value comes from rate arbitrage and from removing future RMDs. If the rate you pay today equals the rate you'd pay later, a conversion is exactly break-even — and the tax you prepay was pure cost.
The Tax Math, Done Properly
Here is where most explanations go wrong. They tell you a conversion is taxed "at your marginal rate," which makes people think a conversion into the 22% bracket costs 22%. It doesn't, and the difference is large enough to change decisions.
A conversion stacks on top of your other income and fills brackets from the bottom up. Only the last dollar converted is taxed at your top rate.
Take a married couple, both retired, with $40,000 of other taxable income in 2026. The standard deduction is $32,200, so their taxable income before converting is $7,800 and their federal tax is $780.
| Conversion | Taxable income after | Total federal tax | Tax caused by the conversion | Blended rate on the conversion |
|---|---|---|---|---|
| $0 | $7,800 | $780 | — | — |
| $93,000 | $100,800 (tops the 12% bracket) | $11,600 | $10,820 | 11.6% |
| $203,600 | $211,400 (tops the 22% bracket) | $35,932 | $35,152 | 17.3% |
Filling the 22% bracket costs 17.3%, not 22%. That is the number to compare against your expected future rate — and it is nearly five points better than the figure most people are working from.
The same logic applies to a single filer: with a $16,100 standard deduction, the 12% bracket tops out at $50,400 of taxable income and the 22% bracket at $105,700.
The 2026 brackets you're filling
Bracket tops, stated as taxable income — that is, after the standard deduction:
| Rate | Single | Married filing jointly |
|---|---|---|
| 10% | $12,400 | $24,800 |
| 12% | $50,400 | $100,800 |
| 22% | $105,700 | $211,400 |
| 24% | $201,775 | $403,550 |
| 32% | $256,225 | $512,450 |
| 35% | $640,600 | $768,700 |
| 37% | above | above |
Standard deductions for 2026: $16,100 single, $32,200 married filing jointly, $24,150 head of household.
The Conversion Window
For most retirees there is a stretch of years when taxable income falls to its lifetime low: wages have stopped, Social Security may not have started yet, and RMDs haven't begun. That gap is the single most valuable Roth planning opportunity most people will ever get, and it is finite.
When it closes depends on your birth year. SECURE 2.0 staggered the required beginning age, and the common shorthand of "72" is now wrong for nearly everyone still planning:
| Birth year | RMDs begin at age |
|---|---|
| 1950 or earlier | 72 |
| 1951 – 1959 | 73 |
| 1960 or later | 75 |
Someone born in 1962 who retires at 62 has thirteen years of conversion window. Someone born in 1955 who retires at 68 has five. That difference should drive completely different conversion paces.
Once RMDs begin the math degrades in a specific way: RMDs cannot be converted. They must come out, they land in your lowest brackets first, and any conversion then stacks on top of them at a higher marginal rate. The cheap bracket space you were filling voluntarily gets filled involuntarily instead.
Charitably inclined retirees can partly reopen that space with qualified charitable distributions, which satisfy the RMD without adding to AGI. For the mechanics of the distributions themselves, see required minimum distributions explained.
Delaying Social Security Widens the Window
Every year you delay claiming is a year with less taxable income competing for cheap bracket space — and it permanently raises the guaranteed, inflation-adjusted income that reduces future portfolio withdrawals. The two decisions are usually analyzed separately and shouldn't be: delaying Social Security to 70 can add several years of high-value conversion room while also improving the underlying plan.
The Five Costs That Aren't in Your Tax Bracket
This is where real conversion planning separates from bracket arithmetic. Your marginal bracket is only one of the things a dollar of conversion income can trigger.
1. IRMAA — the Medicare cliff
Cross an income threshold and your Medicare Part B and Part D premiums rise. It is not a phase-in. One dollar over the line triggers the entire surcharge, and it applies with a two-year lookback — a conversion in 2026 changes your 2028 premiums.
2026 thresholds and the annual cost per person on Medicare:
| MAGI (single) | MAGI (married filing jointly) | Annual surcharge, per person |
|---|---|---|
| Up to $109,000 | Up to $218,000 | $0 |
| Over $109,000 | Over $218,000 | $1,148 |
| Over $137,000 | Over $274,000 | $2,885 |
| Over $171,000 | Over $342,000 | $4,620 |
| Over $205,000 | Over $410,000 | $6,355 |
| Over $500,000 | Over $750,000 | $6,936 |
For a couple where both are on Medicare, double it.
Now notice the collision. Our couple filling the 22% bracket converted $203,600, reaching a MAGI of $243,600 — which is past the $218,000 IRMAA threshold. The tax-bracket answer and the Medicare answer disagree. Filling that bracket costs them an extra $1,148 per person, or $2,296 for the couple, two years later.
Stopping the conversion at $178,000 instead lands MAGI exactly at $218,000 — inside the 22% bracket but under the cliff. Whether the last $25,600 of conversion room is worth $2,296 is a real decision, not a rounding error. See IRMAA surcharges and the Medicare cliff for the full treatment.
2. Social Security taxation
Conversion income raises provisional income, which can push more of your Social Security benefit into taxable status — up to 85% of it. In the phase-in range each additional dollar of conversion makes an additional 50 or 85 cents of benefit taxable, so a retiree nominally in the 12% bracket can face an effective marginal rate of 22.2% or more on the conversion. That's the tax torpedo, and it is the most commonly missed cost in the entire analysis.
3. ACA premium subsidies
For anyone retiring before 65 and buying marketplace coverage, conversion income reduces or eliminates the premium tax credit. This is frequently the largest hidden cost of all — a conversion that looks like it costs 12% can cost far more once a subsidy is lost. See bridging to Medicare at 65.
4. Net investment income tax
A conversion is not itself net investment income, but it raises MAGI — which can push your other investment income above the NIIT threshold and expose it to the additional 3.8%.
5. State income tax
Some states tax conversions fully, some exempt retirement income up to a cap, and a handful have no income tax at all. If a move across state lines is plausible within the conversion window, the sequencing matters enormously: converting after relocating from a high-tax state to a no-tax state can save the entire state-level cost.
Add the Costs Before You Compare Rates
The honest comparison is not "22% today versus 24% later." It is the blended federal rate, plus the IRMAA surcharge, plus any additional Social Security taxation, plus any lost ACA subsidy, plus state tax — against the same full set of costs in the future year. A conversion that wins on brackets alone can easily lose once the rest is counted.
Pay the Tax From Outside the Account
If there is one rule with almost no exceptions, this is it. Pay the conversion tax from a taxable account, not from the IRA.
Converting $100,000 and paying $17,000 of tax from savings puts the full $100,000 into the Roth. Withholding that $17,000 from the IRA puts only $83,000 into the Roth — the other $17,000 never gets the tax-free treatment you just paid for. Same tax bill, materially less benefit.
The Under-59½ Trap
If you are under 59½, the portion withheld for taxes is not treated as converted — it is treated as a distribution. That means a 10% early-distribution penalty on top of the income tax. Someone converting at 57 and withholding $17,000 owes an extra $1,700 for the privilege. Under 59½, paying from outside funds isn't merely better; paying from the IRA is actively penalized.
The Pro-Rata Rule
If you hold any after-tax basis in a traditional IRA — from non-deductible contributions, typically — you cannot choose to convert just the after-tax dollars.
The IRS aggregates all of your traditional, SEP, and SIMPLE IRAs and treats every conversion as a proportional slice of the whole. Someone with $500,000 across their IRAs and $50,000 of basis has a 10% after-tax ratio, so a $100,000 conversion is $90,000 taxable and $10,000 tax-free, regardless of which account the money physically leaves. Basis is tracked on Form 8606, and failing to track it means paying tax twice on the same dollars.
One consequential asymmetry: the aggregation rule covers IRAs, not employer plans. A 401(k) balance is excluded from the calculation, which is why rolling an IRA into an active 401(k) before converting is a recognized technique for isolating basis.
The Five-Year Rules
Roth accounts carry two separate five-year clocks — one governing tax-free earnings, another governing penalty-free access to converted principal for those under 59½. They are frequently confused, and each converted amount starts its own clock. This is enough of a subject on its own that it has a dedicated article; read it before converting if you are under 59½ or may need the money within five years.
Who Should Not Convert
Conversions are oversold. The cases where they're wrong are real and common:
- You're in peak earning years and will retire into a lower bracket. Converting at 32% to avoid 22% later destroys value. Wait for the window.
- You'd have to pay the tax from the IRA. This strips out much of the benefit, and adds a penalty under 59½.
- The IRA is going to charity. A charity pays no income tax on an inherited IRA. Prepaying that tax is a pure loss — as is converting money you'll later give through QCDs.
- You're on an ACA plan with a meaningful subsidy. The subsidy loss often swamps the rate arbitrage.
- Your heirs are in a lower bracket than you. A child in a low-income year may pay less on an inherited traditional IRA than you'd pay converting it.
- You need the money within five years and are under 59½. The clocks can bite.
The Case That's Usually Underweighted: Your Survivor and Your Heirs
The strongest argument for converting frequently has nothing to do with the retiree's own tax rate.
The widow's penalty. When one spouse dies, the survivor's brackets and IRMAA thresholds roughly halve while much of the household income continues. Our couple filling the 22% bracket to $211,400 of taxable income would, as a single filer, see that same income taxed well into the 24% bracket — and their IRMAA threshold drop from $218,000 to $109,000. Converting while both spouses are alive is often the single most valuable thing a couple can do for the survivor. See the widow's penalty.
The 10-year rule. Most non-spouse heirs must now empty an inherited IRA within ten years. A traditional IRA left to a child in their peak earning years gets distributed on top of that income, often at 32% or 35%. An inherited Roth is also subject to the ten-year window, but the distributions are tax-free. See the inherited IRA 10-year rule.
This is why conversions can still make sense at a rate equal to or slightly above your own expected future rate: you may not be arbitraging your rate at all, but your survivor's or your children's.
Common Mistakes
- Converting a lump sum in one year. It wastes the low brackets of every other year in the window and usually spikes IRMAA. Spread it.
- Using the marginal rate instead of the blended rate. It overstates the cost of every conversion and kills good ones.
- Using stale bracket figures. Thresholds move annually; a plan built on last year's numbers mis-sizes every conversion.
- Assuming RMDs start at 72. For anyone born in 1960 or later it's 75 — three extra years of window most plans never claim.
- Forgetting the two-year IRMAA lookback. The bill arrives long after the decision, and surprises clients who thought they were done.
- Ignoring the pro-rata rule. Converting "just the after-tax money" isn't a thing you can do.
- Converting right up to a bracket top without checking the cliffs. As shown above, the bracket top and the IRMAA threshold rarely line up.
- Stopping the analysis at your own lifetime. The survivor and heir cases often dominate the answer.
Why This Matters for Advisors
Roth conversion planning is the clearest example of a recommendation that cannot be made responsibly on a single year's arithmetic. The answer depends on a multi-year projection of income, brackets, RMDs, Medicare thresholds, and the surviving-spouse scenario — and it changes every time tax law, the market, or the client's situation moves.
It is also unusually easy to get approximately right and expensively wrong. The gap between 17.3% and 22% changes which bracket a client should fill. The gap between $218,000 and $243,600 of MAGI is worth $2,296 a year. Neither shows up in a conversation conducted from memory.
The defensible version of this advice shows the client the year-by-year schedule, names the thresholds it is deliberately stopping short of, and documents the assumptions — because a conversion is irreversible. Recharacterization of conversions was eliminated by the Tax Cuts and Jobs Act; there is no undo.
How RetirementForge Helps
The Roth Conversion Analyzer projects a conversion schedule year by year against 2026 federal brackets, state tax, per-account RMDs under the SECURE 2.0 age schedule, and pro-rata basis recovery on Form 8606 — with caps that stop a conversion at a chosen marginal rate or IRMAA tier rather than blowing through them. It models the heir's ten-year distribution window at their own tax rate, so the legacy case is quantified rather than asserted.
Pair it with the IRMAA Calculator to see exactly where the next threshold sits, the RMD Calculator to size the problem you're converting away from, and the Social Security Optimizer for the claiming decision that widens the window. Every assumption is documented and every client session is captured in an immutable audit trail. Get started free.
This article is for educational purposes only and does not constitute tax, legal, or investment advice. Tax figures are 2026 amounts and change annually. Roth conversions are irreversible and interact with your complete financial picture. Consult a qualified tax professional before converting.