Multi-Year Roth Conversions: Better or Worse Than Lump Sum?

Should I convert my 401(k) to a Roth IRA? For most retirees and pre-retirees holding $1 million or more in pre-tax accounts, the answer is often yes, but the decision that actually matters is how fast. A conversion moves money into a Roth IRA, taxes it once as ordinary income, and removes that balance from the required minimum distributions that begin at age 73. Paced wrong, the tax cost climbs.

Last reviewed: August 2026 | Written and reviewed by Craig Wear, CFP®, founder of Q3 Advisors

Should you convert your 401(k) to a Roth IRA?

For most households with a large pre-tax balance, converting a 401(k) to a Roth IRA is often worthwhile because that balance would otherwise trigger growing required minimum distributions starting at age 73 (age 75 for anyone born in 1960 or later, with the first age-75 RMD year arriving in 2035). The real decision is not whether to convert, but how quickly.

The question is rarely binary. Converting everything in one year and stretching thin over two decades are the two extremes, and the outcome that fits a household usually sits between them. Building a deliberate Roth conversion strategy means matching the pace to your income, Medicare timeline, and estate goals rather than a single rule of thumb.

What does converting a 401(k) to a Roth IRA actually do?

A Roth conversion moves money from a pre-tax account (a traditional 401(k) or IRA) into a Roth IRA. The IRS treats the converted amount as ordinary income in the year you convert. After that, the balance grows tax-free, comes out tax-free in retirement, and is not subject to required minimum distributions during the original owner’s lifetime.

The conversion is uncapped, so there is no annual limit on how much you can move. It is also irreversible, and it must be completed by December 31 to count for that tax year, unlike an IRA contribution. If you are already 73 or older, you must take your required minimum distribution first, because you cannot convert an RMD. Q3’s overview of the 2026 conversion deadline covers the timing rules.

How much tax will you pay to convert?

A Roth conversion is taxed as ordinary income at your marginal rate, which ranges from 10% to 37% in 2026. There is no separate conversion tax and no flat rate. The converted amount stacks on top of your other taxable income, so a large conversion can fill several brackets in a single year. The 2026 upper tiers are especially relevant for conversion planning.

A worked example: a married couple filing jointly with $100,000 of other taxable income who converts $200,000 in 2026 does not pay one flat rate. The added income stacks on their existing income, so a lower slice is taxed at 22% and the higher slice at 24%, the band that runs to $403,550 for joint filers. Sizing conversions to the top edge of a target band drives how much to convert each year.

Why the usual reasons to skip a conversion don’t hold up

Many IRA owners are told a conversion does not make sense for them, usually citing one of four reasons. For a household with a seven-figure pre-tax balance and future required minimum distributions on the horizon, these objections often weaken once the full retirement timeline is modeled rather than a single year.

• “You’ll be in a lower bracket in retirement.” Required distributions can push retirement income higher than expected, especially for a surviving spouse filing single.
• “You make too much this year.” A high-income year can still leave room inside a target bracket for a partial conversion.
• “Your IRA is too large.” A larger balance often means a larger future RMD problem, not a smaller reason to convert.
• “You have no outside cash for the tax.” Outside dollars help, but they are not strictly required to convert.

The case for converting all at once (lump sum)

Converting an entire balance in one year is an aggressive approach, and it fits a narrow set of situations. A single-year conversion locks in today’s known federal rates, simplifies planning to one tax filing and one Form 8606, and starts every converted dollar compounding tax-free immediately with no future tax claim on the gains.

This pattern can suit a household with a one-time drop in other income, a strong expectation of high growth inside the Roth, or a specific estate goal that rewards moving the full balance out of the taxable pool quickly. In those cases, the choice tends to follow a lifetime projection rather than a gut reaction.

The hidden costs of converting everything at once

For most IRA owners, a lump-sum conversion carries costs that a single-year snapshot hides. A million-dollar-plus conversion can reach the top of the bracket table and stack several separate penalties in the same year, from surtaxes to Medicare surcharges, which is why many households choose to pace the move instead.

• Top-bracket exposure. A large conversion can push income into the 37% federal bracket (starting at $768,700 taxable income for married couples in 2026), plus state income tax.
• The 3.8% NIIT. A conversion is not itself net investment income, but the added ordinary income can push your other investment income past the $200,000 single or $250,000 joint threshold where the Net Investment Income Tax applies.
• A two-year IRMAA spike. Medicare’s income-related adjustment uses a two-year lookback, so one big conversion year raises Part B and Part D premiums later.
• Liquidity drain. Paying a seven-figure tax bill often means selling brokerage assets and realizing capital gains in the same year.

The case for converting over several years

Spreading conversions across several years addresses most lump-sum drawbacks while keeping the strategic value of converting. A multi-year plan smooths the tax liability so no single year reaches the top brackets, and it compresses Medicare IRMAA exposure into years that can be managed against the thresholds. It also preserves flexibility to react to tax-law changes and market conditions.

Why “just convert up to the top of your bracket” usually backfires

Converting only up to the top of your current bracket each year sounds safe, but it often stretches a conversion across 10 to 20 years. That long window keeps reported income high for a decade or more, produces higher cumulative IRMAA charges, and lets a still-growing balance generate required minimum distributions the household may not need.

The right pace is frequently faster than feels comfortable. A slower stretch can leave a large pre-tax balance intact right when a surviving spouse moves to single-filer brackets, where the same income is taxed at higher rates and the survivor loses roughly half of the joint standard deduction and bracket width.

Why the conversion tax works like a prepayment

One mental shift helps many IRA owners accept a faster pace. The conversion tax is not money lost, it is a prepayment that settles the bill on a slice of the account the IRS would otherwise tax later through required distributions. Every dollar converted also moves permanently out of the future-tax pool, so that portion of the balance is never taxed again.

Consider a $1,000,000 IRA where the owner converts $200,000 at roughly a 24% effective rate, which produces about $48,000 of federal income tax that year. That payment is not a loss on the account; it retires the future tax on that $200,000 for good, while a fifth of the balance leaves the pool that would otherwise drive required distributions. Framing each year this way often makes a faster pace easier to accept.

Will converting push you into a higher bracket or raise your Medicare premiums?

A conversion can do both, which is why sizing matters. Because a conversion is ordinary income, a large one can move income into a higher bracket and lift Medicare premiums. In 2026 the standard Part B premium is $202.90 per month, and income-related surcharges (IRMAA) begin above $109,000 of modified adjusted gross income for single filers and $218,000 for joint filers.

IRMAA uses a two-year lookback, so a conversion at age 63 affects premiums at 65. The last conversion year that does not touch a future Medicare premium is age 62. Households often size conversions to stay under a specific IRMAA tier, or to clear it cleanly, rather than straddling it year after year.

Do you need cash outside your 401(k) to pay the tax?

Outside cash makes a conversion cleaner, but it is not strictly required. You can have federal tax withheld from the conversion itself, though that reduces the amount landing inside the Roth and, before age 59½, the withheld portion can count as an early distribution. Paying the tax from a taxable brokerage account keeps the full converted balance intact.

Many households compare paying from outside dollars against withholding from the conversion, weighing the capital gains friction of selling brokerage assets against the value of preserving the full Roth balance. Paying from outside cash usually lets more of the converted amount keep compounding tax-free, which is why many investors favor it when the cash is available.

What the 59½ and 5-year rules mean for getting to your money

Converted Roth principal is accessible without penalty once you are past age 59½. The five-year rule applies to earnings and to each conversion’s growth, not to the original converted principal after 59½, so a retiree in their 60s is not locking the money away by converting.

In practical terms, that makes Roth assets nearly as liquid as a brokerage account, without the ongoing tax drag from dividends and capital gains. The main constraint is the five-year clock on withdrawing gains tax-free, which starts separately for each conversion.

What actually drives the right pace for you

Tax brackets are what most online calculators lead with, but they are not the main input. The problem a conversion solves is required-distribution risk, not a guess about future rates. The variables that move the pacing decision are more specific to the household.

• Other income and its timing. Pensions, Social Security, deferred compensation, and rental income stack on top of conversion income.
• IRMAA thresholds. Conversions can be sized to sit below or above the $109,000 single and $218,000 joint Medicare tiers.
• Charitable giving. Qualified Charitable Distributions (available at age 70½ from an IRA, not directly from a 401(k)) can offset income in giving years.
• ACA subsidies. For early retirees before Medicare, conversion income can affect marketplace premium credits.
• Estate goals. The SECURE Act 10-year rule forces heirs to empty an inherited traditional IRA within a decade of taxable withdrawals.

Common mistakes IRA Millionaires make

Households with large pre-tax balances tend to repeat a handful of avoidable errors, and most trace back to judging a conversion by the current year instead of the full retirement horizon. The same patterns show up whether the balance is one million or five, and each one tends to raise the lifetime tax bill rather than the single-year one.

• Treating it as all-or-nothing. The optimal plan usually sits between a lump sum and a 20-year stretch, often three to seven years.
• Letting one year’s tax bill override lifetime math. Flinching at a conversion bill can lock in far larger RMD taxes later.
• Ignoring the surviving-spouse shift. When one spouse passes, the survivor moves to single-filer brackets the next year.
• Relying on a single-year CPA spreadsheet. A conversion plan needs a multi-decade projection, not one filing.
• Waiting for the perfect year. The useful year is often the first one the analysis actually runs.

Frequently asked questions

Should I convert my 401(k) to a Roth IRA all at once or over several years?

For most IRA owners with a large pre-tax balance, converting over several years produces a smoother tax outcome than a single lump sum, because it keeps each year below the top brackets and manages Medicare IRMAA. A one-year conversion can fit narrow cases, such as a one-time income drop, but a three to seven year window suits the typical household.

How much tax will I pay if I convert my 401(k) to a Roth IRA?

You pay ordinary income tax on the converted amount at your 2026 marginal rate, somewhere from 10% to 37%. The conversion stacks on your other income, so a large one can span several brackets. There is no separate conversion tax and no cap on the amount. A $200,000 conversion for a joint filer often lands in the 22% to 24% range.

How much can I convert from a 401(k) to a Roth IRA without paying taxes?

Very little is truly tax-free, because a conversion is taxable ordinary income. You can offset some of it with deductions: the 2026 standard deduction is $16,100 for single filers and $32,200 for joint filers, plus $2,050 (single) or $1,650 per spouse at age 65, and a $6,000 per-person senior deduction for 2025 through 2028. Converting in a low-income year lowers the effective rate.

At what age does it no longer make sense to convert to a Roth IRA?

There is no age cutoff. Conversions remain available at any age, but the math changes over time. Once you reach 73 (or 75 if born in 1960 or later), you must take your required minimum distribution before converting, since an RMD cannot be converted. Conversions also affect Medicare premiums starting at age 63 because of the two-year IRMAA lookback.

How do I avoid taxes on a 401(k)-to-Roth conversion?

You cannot avoid the tax entirely, but you can manage it. Common approaches include converting across several years to stay in a target bracket, converting during low-income years before Social Security or RMDs begin, using Qualified Charitable Distributions from an IRA at age 70½ to offset income, and paying the tax from outside cash so the full balance stays in the Roth.

Will converting my 401(k) to a Roth IRA raise my Medicare premiums?

It can, temporarily. Because a conversion adds to modified adjusted gross income, it can trigger Medicare’s IRMAA surcharge, which begins above $109,000 for single filers and $218,000 for joint filers in 2026. IRMAA uses a two-year lookback, so a conversion at 63 affects premiums at 65. A paced plan can keep each year under a chosen tier.

Can a Roth conversion be reversed?

No. The 2017 Tax Cuts and Jobs Act eliminated recharacterization of conversions, effective for 2018 and later. Once a balance is converted, it cannot be moved back into a traditional 401(k) or IRA, and the tax is owed for that year. That permanence is why modeling the amount and timing before converting matters.