401(k) to Roth IRA Conversion: The Complete 2026 Guide

Most Roth conversion guides focus on traditional IRAs, but the majority of Americans' retirement savings sit in 401(k) plans — often left behind at former employers. This guide covers the mechanics, tax implications, and strategy for converting a 401(k) balance to a Roth IRA, including the one rollover mistake that costs thousands of people real money every year.

Why convert a 401(k) to a Roth IRA?

A traditional 401(k) and a traditional IRA behave identically from a tax standpoint: contributions were pre-tax, the account grows tax-deferred, and every dollar withdrawn in retirement is ordinary income. If you expect to be in a higher tax bracket later — or if required minimum distributions will push income back up — converting that pre-tax balance to Roth now locks in today's lower rate.

The math is the same whether the source is an IRA or a 401(k). What differs is the mechanics. A 401(k) is an employer-sponsored plan governed by ERISA; to convert it, you first need to move it out of the plan (unless your plan allows an in-plan Roth conversion — see below). That rollover step introduces a key decision point and a trap many people fall into.

Common situations where converting a 401(k) makes sense:

• You recently retired and have a former employer's 401(k) that's been sitting idle
• You're in the golden window — retired but not yet taking RMDs — and want to use low-bracket years efficiently
• You have a 401(k) with a traditional IRA that has after-tax basis; converting the 401(k) first avoids the pro-rata rule (see below)
• Your 401(k) plan has limited investment options or high fees, and you prefer the flexibility of an IRA

Direct rollover vs. indirect — and the 20% withholding trap

There are two ways to move money from a 401(k) to a Roth IRA. One of them silently costs people thousands of dollars in lost tax-free growth every year.

The solution is simple: always elect a direct rollover. When you contact your 401(k) plan administrator, explicitly request that the funds be transferred directly to your Roth IRA at your new custodian. No 20% withholding. No 60-day clock. The money moves institution-to-institution, and you pay taxes at filing.

Step-by-step: how to do a direct 401(k) → Roth IRA conversion

1. Open a Roth IRA at your chosen custodian (Fidelity, Vanguard, Schwab, etc.) if you don't already have one. You can open one regardless of income — there is no income limit on conversions.²
2. Get the receiving account details. Your Roth IRA custodian will give you an account number and DTC/ABA routing information for the incoming rollover.
3. Contact your 401(k) plan administrator. Request a direct rollover to a Roth IRA. Use the word "direct" — this tells them to make the check or wire payable to the Roth IRA custodian, not to you. Ask specifically whether pre-tax 401(k) money can be rolled directly to a Roth IRA (it can, under IRC § 402(c) and IRS Notice 2014-54).³
4. Specify Roth IRA as the destination. This is a taxable conversion, not a tax-free rollover to a traditional IRA. Make the instruction explicit. Some plan administrators need the receiving account to be clearly identified as a Roth IRA to process it correctly.
5. Plan for the tax bill. The full converted amount will be included in your taxable income for the year. Increase your estimated tax payments or have extra withholding taken from other income sources — do not withhold from the conversion itself (that reduces the amount going into the Roth IRA).
6. Track it on Form 8606. Your custodian will send a 1099-R. Code 2 (or G for direct rollover) indicates the distribution is rollover-eligible. Your tax preparer or tax software will handle the Form 8606 that reports the conversion.

Tax implications: what you'll owe and when

Converting a pre-tax 401(k) to a Roth IRA is a taxable event. The full converted amount — minus any after-tax contributions you made (see below) — is added to your ordinary income in the tax year the conversion completes.

Key points:

• No special rate. Converted 401(k) funds are taxed at your marginal ordinary income rate, exactly like IRA conversions or W-2 income. No long-term capital gains treatment, no flat rate.
• Pay taxes from outside funds. This is critical. If you withhold taxes from the 401(k) itself, you're reducing the amount that enters the Roth IRA — and possibly triggering a penalty if you're under 59½. Pay the tax bill from a taxable brokerage account or savings.
• After-tax contributions convert tax-free. If you made after-tax (non-deductible) contributions to your 401(k), those dollars are not taxed again on conversion. They're already basis. You'll receive the breakdown on your 1099-R — Box 5 shows the employee contribution amount not subject to tax.
• The size matters for IRMAA. A large 401(k) conversion in 2026 raises your 2026 MAGI, which Social Security uses to set your 2028 Medicare Part B and Part D premiums. See the IRMAA-Aware Conversion Calculator for the specific tier thresholds. This is the same dynamic as IRA conversions — the source of the funds doesn't change the IRMAA calculation.
• Social Security taxation interaction. If you're receiving Social Security, conversion income can push more of your SS benefit into the taxable zone. A $100K conversion can trigger an effective marginal rate of 27–37% in the phase-in zone, not just your stated bracket rate. Details: Roth Conversions and Social Security.

2026 federal tax brackets (ordinary income)

Source: IRS Rev. Proc. 2025-32.⁴ Thresholds apply to taxable income after standard deduction ($16,100 single / $32,200 MFJ in 2026) and other adjustments.

The goal for most conversions is to fill the 22% or 24% bracket without spilling into 32%. The tax bracket calculator shows exactly how much room you have given your other income sources.

Timing: the golden window still applies

The same logic that makes traditional IRA conversions valuable applies to 401(k) conversions. The Roth conversion golden window — roughly age 60 to 72 for most retirees — is when:

• Earned income has stopped (or sharply reduced), lowering your base taxable income
• Social Security may not have started yet (or is only partially taxable)
• Required minimum distributions haven't begun (RMD age is 73 for those born 1951–1959; 75 for those born 1960 or later⁵)

This combination creates a window where your effective tax rate on conversion income may be the lowest it will ever be. Once RMDs start, they add mandatory income that consumes bracket space you'd otherwise use for conversions. Converting the 401(k) before that point — whether by converting it directly to a Roth IRA or rolling to a traditional IRA first and then converting — is typically more tax-efficient than waiting.

Full timing analysis: The Roth Conversion Golden Window: A Phase-by-Phase Strategy.

The pro-rata advantage: 401(k)s don't pollute the calculation

This is one of the most overlooked benefits of converting a 401(k) rather than an IRA, and it matters specifically when you have a traditional IRA with after-tax (non-deductible) basis.

The pro-rata rule (IRC § 408(d)(2)) requires that every IRA conversion include a proportional share of pre-tax and after-tax money. If you have $1.5M in traditional IRAs with $75,000 in after-tax basis, you can't convert just the $75,000 basis tax-free — 95% of every dollar converted is taxable, regardless of intent.

But 401(k) balances don't count in the pro-rata calculation.⁶ Only traditional IRAs, rollover IRAs, SEP IRAs, and SIMPLE IRAs are aggregated. A $600,000 traditional 401(k) sitting at your former employer doesn't affect the pro-rata fraction on your IRA at all.

For people with significant non-deductible IRA contributions tracked on Form 8606, the order of operations matters. Converting the 401(k) directly to Roth — before rolling it into the traditional IRA — keeps the pools separate and preserves the IRA's basis ratio.

Full pro-rata rule explanation with worked examples: The Pro-Rata Rule and Roth Conversions.

In-service conversions: can you convert while still employed?

Most people think of 401(k)-to-Roth conversions as something you do after leaving an employer. But there are two mechanisms available while still working:

In-plan Roth conversion

If your employer's plan offers this feature, you can convert traditional 401(k) balances to a designated Roth account within the same plan — without leaving your employer or taking a distribution. The converted amount is taxable in the year of conversion, just like an external rollover.

Not all plans offer in-plan Roth conversions. Check your Summary Plan Description or ask your HR or benefits administrator.

In-service distribution for external rollover

Some plans allow participants who have reached age 59½ to take a distribution while still employed and roll it to an external Roth IRA. This is plan-specific — not all plans permit in-service distributions of pre-tax contributions. After-tax contributions (if you've been making them) are more commonly available for in-service distribution at any age under IRS Notice 2014-54.³

If you're still working and want to convert, the first step is to check your plan documents. If neither in-plan conversion nor in-service distribution is available, you'll need to wait until you separate from service (retirement, job change, etc.) before rolling the 401(k) out for conversion.

Partial vs. full conversion: which is better?

There's no requirement to convert an entire 401(k) balance at once. For most people, partial annual conversions during the golden window produce better lifetime tax outcomes than a single large conversion, for several reasons:

• Bracket management. A $600,000 conversion in one year pushes income into the 32–37% brackets. Converting $60,000–$80,000 annually for 8–10 years can keep the entire conversion at 22–24%.
• IRMAA management. Staying below IRMAA tier thresholds ($106,000 single / $212,000 MFJ in 2026⁷) avoids Medicare premium surcharges that stack on top of regular tax cost.
• No recharacterization safety net. Since TCJA 2017, Roth conversions cannot be undone. Smaller annual conversions reduce the risk of overpaying in a year when markets decline or unexpected income arrives.
• RMD pressure offset. Each year of partial conversion reduces the traditional IRA/401(k) balance, which reduces future RMDs — compounding the benefit over time.

The lifetime tax savings calculator on this site models the full multi-year picture: convert vs. no-convert, year-by-year tax projections, breakeven age, and total lifetime tax savings.

Common mistakes with 401(k) to Roth conversions

1. Taking the indirect rollover. The 20% withholding trap is the most expensive and most avoidable mistake. Always elect a direct rollover. There is almost no reason to use the indirect method.
2. Rolling to traditional IRA first to "simplify," then converting. This works, but it can permanently worsen your pro-rata ratio if you have after-tax IRA basis. If you have Form 8606 basis in an existing IRA, consider converting the 401(k) directly to Roth before commingling the pools.
3. Converting a large amount in the year you retire. The retirement year often includes W-2 income from the first part of the year. Adding a large 401(k) conversion on top of that final W-2 can push total income into unexpectedly high brackets. Model the full-year income picture before converting.
4. Withholding taxes from the converted funds. Paying tax out of the conversion means less goes into the Roth IRA and potentially triggers an early distribution penalty. Pay the tax bill from outside funds.
5. Forgetting IRMAA's two-year lookback. A large conversion in 2026 affects 2028 Medicare premiums — not 2026 or 2027. Factor this into multi-year planning, especially if Medicare enrollment is approaching.
6. Assuming the 401(k)'s investment options are as good as an IRA. This isn't a conversion mistake per se, but leaving a 401(k) unconverted — just to avoid the hassle — may mean paying higher fund expenses for years. The conversion creates an opportunity to consolidate into lower-cost index funds.