401(k) Roth Conversion: The Tax Math for High-Income California and Washington Families

By Kevin Luu, Co-Founder and Chief Learning Officer. Reviewed on 2026-09-30.

A 401(k) Roth conversion moves money from the pre-tax side of your 401(k) into the Roth side of the same plan (an “in-plan Roth rollover”), or into a Roth IRA when you leave a job. The whole converted amount is taxed as ordinary income in the year you convert, and in return future growth and qualified withdrawals are tax-free. For a California family already in the 37% federal bracket, converting pre-tax dollars in a normal earning year usually costs more than it saves; the conversions that tend to pay off at this income level are after-tax (mega backdoor) dollars and conversions done in a low-income year.

This guide shows the math the way a calculator would, with worked numbers for a $900,000 W-2 household, so you can see which of the three situations you are in.

What gets taxed when you convert a 401(k) to Roth

Your 401(k) can hold three kinds of money, and each converts differently:

• Pre-tax deferrals, employer match and their earnings. Fully taxable on conversion, at your ordinary federal and state rates.
• After-tax (non-Roth) contributions. Already taxed, so only the earnings on them are taxable when converted. This is the engine of the mega backdoor Roth.
• Roth deferrals. Nothing to convert; they are already Roth.

Your plan must offer in-plan Roth rollovers; many do, but it is a plan choice, not a right. The IRS confirms that no tax is withheld from an in-plan rollover of money you could not otherwise withdraw, and says you “may need to increase” withholding or make estimated payments to avoid an underpayment penalty (IRS Notice 2013-74, Q&A-4). In practice, that means the tax comes out of your paycheck or your savings, not out of the 401(k).

The rate you pay today: a $900,000 California household

Assumptions (illustrative): married filing jointly, $900,000 of W-2 wages in the Bay Area, each spouse deferring the 2026 maximum of $24,500 pre-tax, standard deduction of $32,200 for 2026.

• Federal taxable income: $900,000 minus $49,000 minus $32,200 = $818,800. For 2026, the 37% bracket starts at $768,700 for joint filers, so every extra dollar is taxed at 37%.
• California taxable income: assume about $840,000. The latest published California schedule (2025) taxes joint income between $742,958 and $891,542 at 10.3%. FTB had not published 2026 brackets when this was reviewed.

Now convert $50,000 of pre-tax 401(k) money:

Conversion income stays in the 10.3% band here because $840,000 plus $50,000 is still under $891,542. A larger conversion would move into 11.3%, and at more than $1,000,000 of California taxable income the 1% Behavioral Health Services Tax applies too, for a top rate of 13.3%.

The break-even test: today’s rate versus your rate in retirement

A conversion is a bet on tax rates. If the money grows at the same rate either way, a Roth conversion comes out ahead only when your tax rate at withdrawal would be higher than the rate you pay to convert. (Paying the tax from outside savings tilts things slightly toward converting, because it moves more money into the tax-free account, but it does not change the basic test.)

So the question for the $900,000 family is: will you withdraw this money at more than 47.3%? A few reference points using the same brackets:

Most families in the $750,000 to $3 million range spend their highest-earning years in their highest bracket. That is exactly why a blanket “convert everything” rule from a generic calculator can cost them money.

Where conversions do work at this income: after-tax dollars

For 2026, total contributions to one 401(k) (your deferrals, the employer’s money and any after-tax contributions) can reach $72,000, not counting catch-ups. If your plan allows after-tax contributions and in-plan conversion, the gap can be filled with after-tax money and converted to Roth right away.

Example: a Genentech or Amazon employee defers $24,500 and receives $10,000 of employer contributions (assumption). After-tax room for 2026 is $72,000 minus $24,500 minus $10,000 = $37,500.

• Converted within days: only the few dollars of earnings since contribution are taxable. The full $37,500 then grows tax-free.
• Left unconverted for 10 years at a hypothetical 6.5% a year: grows to about $70,400. The roughly $32,900 of earnings would be taxed as ordinary income when withdrawn, at whatever your rate is then.

This is the single most useful 401(k) Roth conversion for a high-income W-2 household. If your plan offers automatic daily in-plan conversion of after-tax contributions, it is usually worth turning on. See mega backdoor Roth vs brokerage for how this compares with investing the same cash in a taxable account.

The other window: a low-income year

High earners have lower-income years more often than they expect: a sabbatical, an extended parental leave, a gap between jobs, or the year one spouse steps back to be home with young children. That is when converting pre-tax money can make sense.

Example: one spouse leaves a biotech role for a year and household wages fall to $300,000 (assumption).

• Federal taxable income, assuming no pre-tax deferrals that year: $300,000 minus $32,200 = $267,800. The 2026 joint 24% bracket runs to $403,550, leaving $135,750 of room at 24%.
• Convert $135,000: federal 24% = $32,400. California at 9.3% (2025 schedule, joint income from $145,448 to $742,958; assume CA taxable income near $290,000) = $12,555.
• Total: $44,955, or 33.3%. That is 14 percentage points below the 47.3% marginal rate in the normal-year example, or at least $18,900 less tax on the same $135,000 (more in practice, because a conversion that large in a normal year would push part of it into California’s 11.3% band).

Two side effects to check before you pick the amount:

• Child Tax Credit. For 2026 the credit is $2,200 per child, reduced by $50 for each $1,000 of modified AGI over $400,000 for joint filers. At $300,000 of wages this family would normally get the full $4,400 for two children. A $135,000 conversion pushes MAGI to $435,000 and cuts the credit by $1,750 (35 times $50). Converting $100,000 instead keeps it intact.
• Net Investment Income Tax. Retirement plan distributions are not net investment income (IRS NIIT Q&A), but a conversion still raises MAGI. The 3.8% tax applies to the lesser of net investment income or MAGI over $250,000 for joint filers, so a larger conversion can expose more of that year’s interest, dividends and capital gains to it.

Changing jobs: 401(k) rollover to a Roth IRA

When you leave an employer, you can roll pre-tax 401(k) money directly into a Roth IRA. The tax math is identical to an in-plan conversion: the pre-tax amount is ordinary income that year. Rolling after-tax money to a Roth IRA is where the value is.

The common mistake at this income level is rolling a pre-tax 401(k) into a traditional IRA “to keep things simple”. For 2026, the Roth IRA income phase-out for joint filers is $242,000 to $252,000, so these families fund Roth IRAs through the backdoor Roth. Form 8606 figures the taxable part of a backdoor conversion using “the total value of all your traditional IRAs as of December 31” (Form 8606 instructions). A large traditional IRA makes most of each backdoor conversion taxable. 401(k) balances are not counted, so leaving pre-tax money in the old plan or moving it into the new employer’s plan usually keeps the backdoor clean.

California versus Washington

• California taxes conversions as ordinary income at up to 13.3% (2025 rates) and has no lower rate for capital gains, so there is no state-level trick. A family planning to leave California in retirement has a strong reason not to convert pre-tax money while resident.
• Washington has no individual income tax for 2026, so a Washington family converting in 2026 pays federal tax only. Washington’s capital gains tax exempts retirement accounts. Note the change coming: Washington has enacted a 9.9% tax on individual and joint AGI over $1,000,000, effective January 1, 2028 (WA DOR). Details of the income definition are pending, but a large conversion could push a high-earning household over that line from 2028. For Washington families near $1M of AGI, 2026 and 2027 may be cleaner years to convert.

Roth rules that matter after you convert

• Five-year clock. A qualified, tax-free withdrawal from a Roth 401(k) requires the account to have been held at least five taxable years and a trigger such as reaching age 59½, disability or death (IRS Roth comparison chart). If your first Roth money in the plan is an in-plan rollover, the clock starts on January 1 of that year (Notice 2013-74). More on the clock in when you can withdraw from a Roth.
• No lifetime required distributions. Designated Roth 401(k) accounts and Roth IRAs have no required minimum distributions for the owner (IRS RMD FAQs). Pre-tax accounts do, starting at age 73.
• Heirs. Roth money passes to children without income tax on qualified withdrawals, which matters if the account may outlast you. That is also why the beneficiary form and guardianship plan belong in the same conversation (see estate planning).

Common mistakes

1. Converting pre-tax money at 47% because a calculator assumed a flat future rate.
2. Forgetting that no tax is withheld on an in-plan conversion, then owing an underpayment penalty in April.
3. Leaving after-tax contributions unconverted for years, so their earnings become taxable.
4. Rolling an old 401(k) into a traditional IRA and breaking the backdoor Roth.
5. Ignoring the Child Tax Credit and NIIT side effects in a low-income year.

What to write down for your children

A conversion is a decision made once and felt for decades. Record the year, the amount, the rate you paid and why (for example, “Sabbatical year, converted $100,000 at 33%, stopped at $400,000 MAGI to keep the child credit”). Twenty years from now, your kids may inherit the Roth. The note tells them it was a deliberate trade, not an accident. Conversations fade. Documentation compounds.

Frequently asked questions

Is there a limit on how much I can convert to Roth in my 401(k)?

There is no annual dollar limit on conversions. The limit is practical: every pre-tax dollar converted is added to your taxable income for the year, so the real constraint is the bracket you are willing to fill. The $72,000 figure for 2026 limits contributions to the plan, not conversions of money already in it.

Can I pay the conversion tax from the 401(k) itself?

Not for an in-plan rollover of money you could not otherwise withdraw: Notice 2013-74 says no part of it may be withheld. For a rollover to a Roth IRA after leaving a job, any tax taken out of the rollover shrinks what reaches the Roth. Paying from savings or extra paycheck withholding keeps the full amount growing tax-free.

Should I convert my old 401(k) before moving from California to Washington?

Usually the reverse. Converting while a California resident adds up to 13.3% of state tax (2025 rates). Converting after becoming a Washington resident in 2026 or 2027 costs federal tax only. Make sure the move is real and documented before counting on it, because California can question a residency change that looks timed around a large taxable event.

Your next step

Check two things in your plan documents this week: whether the plan allows after-tax contributions with in-plan Roth conversion, and whether conversion can be set to run automatically. If a lower-income year is coming, run the numbers before January so the conversion lands in that year. If you want the full picture in one place, including RSUs and the kids’ accounts, our tax management and retirement and employee benefits work starts with a Tax & Wealth Assessment.

This is general education, and the right move depends on the family’s full picture.

Written by

Kevin Luu

Co-Founder and Chief Learning Officer, Bright Advisers

Kevin has advised high-income W-2 tech and biotech families since 2015, and leads the education-first Age Five Family Office from Brea, California.

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