Converting a 401(k) to a Roth IRA means paying income tax on the transferred balance now so that every future dollar of growth and every withdrawal in retirement comes out federally tax-free. The trade makes sense when your current tax bracket is lower than the one you expect to face in retirement, when you have years or decades for the money to compound, and when you can pay the resulting tax bill from savings outside the retirement account itself. It rarely makes sense when the reverse is true.
What You Get in Exchange for the Tax Bill
The core benefit is permanent. Once money lands in a Roth IRA, all future investment growth is shielded from federal income tax, and qualified withdrawals cost you nothing.1Office of the Law Revision Counsel. 26 USC 408A Roth IRAs In a traditional 401(k), by contrast, you owe ordinary income tax on the full balance as you withdraw it, including every dollar of gains the account earned along the way.
The longer the time horizon, the bigger the payoff. A $100,000 conversion at 45 that grows to $400,000 by 65 delivers $300,000 in gains that never get taxed. Kept in the traditional 401(k), that same $400,000 would be taxable as it came out.
A second benefit shows up later in life. Traditional 401(k) accounts force required minimum distributions starting at age 73 (or 75 for those born in 1960 or later).2Office of the Law Revision Counsel. 26 USC 401a9 – Required Distributions Those forced withdrawals stack on top of Social Security and other income, often pushing retirees into higher brackets or making Social Security benefits taxable. Roth IRAs have no RMDs during the original owner’s lifetime, so the money can keep compounding for as long as you want to leave it there.3Internal Revenue Service. Retirement Plan and IRA Required Minimum Distributions FAQs
One boundary worth knowing before you go through the paperwork: designated Roth accounts inside employer 401(k) plans are also exempt from lifetime RMDs after SECURE 2.0. If dodging RMDs is your only reason to convert and your employer already offers a Roth 401(k) option, check that first.3Internal Revenue Service. Retirement Plan and IRA Required Minimum Distributions FAQs
The third benefit is estate-related. The SECURE Act of 2019 requires most non-spouse beneficiaries to empty an inherited retirement account within ten years of the original owner’s death. An inherited traditional 401(k) or IRA generates ordinary income tax on every withdrawal, which lands hardest on adult children in their peak earning years. An inherited Roth IRA follows the same ten-year rule but pays out federally tax-free. Converting during your lifetime shifts the tax burden from your heirs to you.4Internal Revenue Service. Retirement Topics – Beneficiary
When a Conversion Actually Saves Money
A Roth conversion saves money when the rate you pay today is lower than the rate you would have paid on future withdrawals. It costs money when the reverse is true. Everything else is secondary to that comparison.
For 2026, federal income tax brackets range from 10% to 37%, with the lower rates from the Tax Cuts and Jobs Act extended by legislation passed in 2025.5Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026 Rates can still change through future legislation, so locking in a known rate today removes some of the guesswork about the next two or three decades.
The strongest conversion opportunities show up in specific low-income windows:
- Early retirement gap years, when you have stopped working but delayed Social Security, often leave you in the 10% or 12% bracket.
- Job transitions, sabbaticals, or a year of reduced earnings drop your marginal rate on the converted amount.
- Years with large deductions — heavy medical expenses, charitable gifts, or business losses — create bracket room to convert into.
If you expect to retire into a lower bracket than you sit in today, converting a large 401(k) at your current rate is usually the wrong direction. If you expect the opposite, or if you have a long horizon and money you will not need to touch, the conversion tends to come out ahead.
Why Partial Conversions Beat One Big Move
You do not have to convert the entire balance in a single year, and for most people you should not. Converting a slice at a time — sometimes called a Roth conversion ladder — lets you fill up a lower bracket each year without spilling into the next one.
Consider a married couple filing jointly in 2026 with $150,000 in other taxable income. They sit in the 22% bracket and could convert roughly $61,400 before crossing into the 24% bracket at $211,400.5Internal Revenue Service. IRS Releases Tax Inflation Adjustments for Tax Year 2026 Spreading $300,000 in conversions across five years at 22% beats converting it all at once and pushing half the amount into the 32% bracket.
There is a second reason partial conversions are safer. Every Roth conversion made after December 31, 2017 is irrevocable. The Tax Cuts and Jobs Act eliminated the recharacterization option that used to let you undo a conversion if the market fell.6Office of the Law Revision Counsel. 26 US Code 408A – Roth IRAs If you convert $200,000 and the market drops 30% the next month, you still owe tax on the full $200,000. Smaller, staggered conversions limit the damage of any single unlucky year.
Costs Beyond the Federal Income Tax
Medicare Premium Surcharges
A large conversion can quietly raise your Medicare premiums two years later. Part B and Part D premiums are set from your modified adjusted gross income from two years prior, and once you cross certain thresholds you pay Income-Related Monthly Adjustment Amounts on top of the standard premium.
For 2026, the Part B IRMAA tiers begin at these income levels:7Centers for Medicare & Medicaid Services. 2026 Medicare Parts A and B Premiums and Deductibles
- Single filers above $109,000: Part B rises to $284.10/month, an extra $81.20.
- Single filers above $171,000: Part B rises to $527.50/month.
- Single filers above $500,000: Part B reaches $689.90/month.
- Joint filers: thresholds are roughly double, starting at $218,000.
Part D carries a parallel surcharge at the same income tiers. Combined, the extra cost can exceed $12,000 in a single year for high-income conversions. A conversion in 2026 hits your Medicare premiums in 2028. Keeping each year’s converted amount below the first IRMAA threshold avoids the surcharge entirely, which is another argument for spreading conversions across multiple years.
State Income Tax
Most states with an income tax treat the conversion amount as taxable income, so the effective cost is your combined federal and state rate. A handful of states have no income tax at all. If you are planning a move to a lower-tax or no-tax state within the next couple of years, waiting until after the move to convert can meaningfully change the math.
How To Execute the Conversion
While You Are Still Employed
Whether you can move money out of a current employer’s 401(k) depends on the plan. Most plans restrict in-service distributions to participants who have reached age 59½, and some do not allow them at all. Ask your plan administrator before you make any assumptions. Once you leave the employer, the restriction goes away and you can roll over any amount.
Direct Transfer, Not an Indirect Rollover
Ask for a direct trustee-to-trustee transfer. The 401(k) administrator sends the funds straight to your Roth IRA custodian, nothing is withheld, and you never touch the money.
If the administrator instead mails you a check, you are in an indirect rollover, and the rules turn hostile. You have 60 days to deposit the full amount into a Roth IRA, or the whole distribution becomes taxable, plus a 10% early withdrawal penalty if you are under 59½.8Internal Revenue Service. Rollovers of Retirement Plan and IRA Distributions The administrator is also required to withhold 20% for taxes from a 401(k) indirect rollover, so to move the full pre-tax amount into the Roth you have to make up that 20% out of pocket. Depositing only the 80% you received leaves the withheld portion treated as a taxable distribution.9Internal Revenue Service. Topic No. 413, Rollovers From Retirement Plans
Tax Reporting
Two forms document the move. The 401(k) administrator issues Form 1099-R reporting the distribution, and the Roth IRA custodian issues Form 5498 confirming receipt of the rollover. Keep both for your records and your return.10Internal Revenue Service. Instructions for Forms 1099-R and 5498
Paying the Tax Without Triggering Penalties
The single most important rule: pay the tax bill from money outside the retirement account. If you let the plan withhold taxes from the distribution, that withheld portion never makes it into the Roth IRA, and if you are under 59½ it counts as an early distribution subject to the 10% penalty on top of the income tax.11Internal Revenue Service. Topic No. 557, Additional Tax on Early Distributions From Traditional and Roth IRAs
A large conversion can also trigger an underpayment penalty if your regular wage withholding does not cover the extra tax. You avoid the penalty by meeting one of these safe harbors:12Internal Revenue Service. Underpayment of Estimated Tax by Individuals Penalty
- Your total tax due after withholding and credits is under $1,000.
- You paid at least 90% of the current year’s tax.
- You paid at least 100% of last year’s tax, or 110% if your prior-year adjusted gross income was over $150,000 ($75,000 if married filing separately).
The simplest fix is a quarterly estimated tax payment in the quarter the conversion occurs. Estimated tax deadlines fall on April 15, June 15, September 15, and January 15 of the following year. Converting earlier in the year gives you more room to spread payments across the remaining quarters.
One Boundary: Highly Appreciated Employer Stock
If a meaningful part of your 401(k) is your employer’s stock and it has appreciated substantially, converting the whole account to a Roth IRA may not be the cheapest path. A separate election called Net Unrealized Appreciation lets you take a lump-sum distribution of the actual shares, pay ordinary income tax only on the stock’s original cost basis, and then pay long-term capital gains rates on the appreciation when you sell. Capital gains rates top out at 20%, well below the 37% top ordinary rate.
NUA has strict rules. You must distribute your entire vested balance from all plans with that employer in a single tax year, take the stock as shares rather than cash, and meet a triggering event such as separation from service or reaching 59½. Miss any requirement and the whole amount defaults back to ordinary income treatment. If your 401(k) is mostly mutual funds with little or no company stock, NUA does not apply and a standard conversion is the simpler answer.