The Pro Rata Rule and the Mega Backdoor Roth: How It Applies in 2026

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

The IRA pro rata rule does not apply to a mega backdoor Roth that stays inside your 401(k). When you convert after-tax 401(k) money to Roth, only the earnings on those after-tax dollars are taxable, and your IRA balances play no part. The pro rata rule bites in two other places: the regular backdoor Roth IRA (where every pre-tax dollar in your traditional, SEP and SIMPLE IRAs counts) and the earnings that ride along with after-tax money when it leaves the plan.

This matters because the two strategies usually run side by side in the same household. A dual-income family earning $900,000 in the Bay Area or Seattle is well above the 2026 Roth IRA income limit, so both spouses often use the backdoor Roth IRA, and one or both may also have a plan at Apple, Amazon, Genentech or a hospital system that allows after-tax contributions. A single forgotten rollover IRA can turn a tax-free conversion into a taxable one. This guide shows where the rule applies, where it does not, and how to set up both so the conversion stays clean.

Can you put $20,000 in a Roth IRA? No, but you can get more into Roth money

For 2026 the IRA contribution limit is $7,500 per person, or $8,600 if you are 50 or older ($7,500 plus a $1,100 catch-up). That limit covers traditional and Roth IRAs combined. Direct Roth IRA contributions also phase out for married couples filing jointly with modified AGI between $242,000 and $252,000 (IRS Notice 2025-67), so a household at $750,000 or more cannot contribute directly at all.

That leaves two routes for this family:

• Backdoor Roth IRA: contribute $7,500 to a traditional IRA with no deduction, then convert it to a Roth IRA. A married couple can do this once each, $15,000 in total for 2026.
• Mega backdoor Roth: make after-tax (not Roth, not pre-tax) contributions to your 401(k), then convert them to Roth inside the plan or roll them to a Roth IRA. Along with Roth 401(k) deferrals (capped at $24,500 for 2026), it is one of the few ways a W-2 earner in this income range can move $20,000 or more a year into Roth money.

The 2026 cap on total additions to one person’s 401(k), counting your deferrals, the employer match and after-tax money, is $72,000. Catch-up contributions sit on top of that limit. Your mega backdoor room is $72,000 minus your own deferrals minus what your employer puts in.

What the pro rata rule actually says

When you take money out of a traditional IRA, including by converting it, the IRS does not let you choose which dollars leave. Every distribution is treated as a proportional slice of all your non-Roth IRAs added together. The tax-free share equals your after-tax basis divided by the total value of all your traditional, SEP and SIMPLE IRAs, measured on December 31 of the year you convert, plus anything converted or distributed during the year. The calculation lives on Form 8606, and each spouse files a separate one, because IRAs are individual accounts.

Three details trip people up:

• The balance that counts is the one on December 31, not the day you convert. Opening a new rollover IRA in November still spoils a conversion you did in February.
• 401(k), 403(b) and 457(b) balances do not count. Neither do Roth IRAs, or your spouse’s IRAs.
• A SEP IRA from a past consulting gig counts in full, even though it feels like a work plan.

Worked example: a forgotten rollover IRA in a $900,000 household

Assumptions (illustrative): Priya and Dan live in San Jose, file jointly, and earn $900,000 in combined W-2 pay. Their federal taxable income is above $768,700, so their top federal rate for 2026 is 37%. Their California taxable income is about $860,000, which falls in the 10.3% bracket under the 2025 schedule (the latest FTB has published). Priya has a $140,000 rollover IRA from a job she left in 2021, all pre-tax.

In March 2026 she contributes $7,500 to a traditional IRA, takes no deduction, and converts the $7,500 to Roth a week later. On December 31 the rollover IRA is still worth $140,000.

Left alone, the “tax-free” backdoor costs about $3,370 in tax this year, and the same drag returns every year the rollover IRA sits there. The unused basis (about $7,120) is not lost; Form 8606 carries it forward. But it comes back only a small slice at a time, pro rata, for as long as the pre-tax balance remains.

The usual fix is a roll-in: move the pre-tax rollover IRA into Priya’s current 401(k) before December 31, if the plan accepts incoming rollovers. Only pre-tax IRA money can go into a workplace plan, which is exactly what you want here, because the $7,500 of basis stays behind in the IRA and converts cleanly. Dan runs his own Form 8606 and is not affected by Priya’s IRA.

Where a pro rata effect shows up inside the mega backdoor Roth

The IRA rule stays out of the 401(k), but the plan has its own version. After-tax contributions are your basis. Any growth on them before conversion is pre-tax earnings. When you convert or withdraw after-tax money, the earnings come along in proportion, and that part is taxable.

Continuing the example: Priya defers the 2026 maximum of $24,500 and her employer adds an assumed $12,000 match. Her after-tax room is $72,000 minus $24,500 minus $12,000, or $35,500.

• Automatic in-plan conversion each paycheck: the after-tax money converts within days, earnings are close to zero, and the tax cost is usually a few dollars.
• Convert once in December: if the $35,500 grew to $38,000 over the year, the $2,500 of earnings is taxable. At 37% federal plus 10.3% California, that is about $1,180. In Washington, the same conversion costs about $925, federal only.

Many families think the answer is to hold the after-tax money until year-end and convert it once. The opposite is true. Frequent conversion is what keeps the mega backdoor close to tax-free.

Rolling after-tax money to a Roth IRA instead

If your plan allows in-service withdrawals of after-tax money, you can send it to a Roth IRA rather than converting inside the plan. Under IRS Notice 2014-54, when you take the after-tax dollars and their earnings out together, you can direct the after-tax part to a Roth IRA tax-free and the earnings to a traditional IRA to keep deferring tax.

That split creates the trap. Earnings parked in a traditional IRA are counted in your IRA pro rata calculation on December 31, so they make your next backdoor Roth IRA partly taxable. Either convert the earnings as well and pay tax on that small amount, or roll them into your 401(k) before year-end if the plan accepts roll-ins.

Does the plan track after-tax money separately?

Many plans keep after-tax contributions in their own source, so a conversion pulls only from after-tax money and its earnings. If a plan does not separate them, a withdrawal can be allocated across your whole account, pre-tax balances included. Ask the plan administrator, or check the summary plan description, before you start.

Plan features to confirm before you contribute

1. After-tax contributions allowed. Not the same as Roth 401(k) deferrals, which count toward the $24,500 limit.
2. In-plan Roth conversion or in-service withdrawal of after-tax money. Without one of these, after-tax dollars sit and grow taxable earnings. IRS Notice 2013-74 covers in-plan Roth rollovers of amounts not otherwise distributable.
3. Automatic conversion. Some plans convert every paycheck on request; others need you to request each conversion.
4. After-tax “spillover”. Some plans let deferrals spill into after-tax once you hit the $24,500 limit. Check that the spillover is converted too.
5. Nondiscrimination testing. After-tax contributions and matches are tested under the ACP test. If the plan fails, highly compensated employees can have after-tax money refunded after year-end (IRS 401(k) Fix-It Guide).
6. Match timing. If you reach the $24,500 deferral limit early in the year, some plans stop matching when your deferrals stop unless they make a year-end true-up. And if after-tax contributions fill the $72,000 cap before the match is credited, there may be no room left for the match.

California versus Washington

In California, the taxable part of any conversion is ordinary income for state purposes, at rates up to 13.3% (2025 schedule), and California has no lower rate for gains. That makes stray IRA balances and year-end conversions more expensive for California families. The conversion itself is not wages, so CA SDI (1.3% of wages for 2026) does not apply to it.

Washington has no tax on wage or conversion income in 2026, and its capital gains tax (7%, or 9.9% on gains above $1 million) exempts retirement accounts. But Washington has enacted a 9.9% income tax on AGI over $1,000,000 starting January 1, 2028 (WA Department of Revenue). A Seattle household that plans a large conversion, such as an old pre-tax balance, has a reason to consider finishing it before 2028, while it pays federal tax only. DOR has not yet issued guidance on exactly which income the new tax will reach.

Common mistakes we see

• Doing a backdoor Roth IRA in a year when a new rollover IRA or SEP IRA will still exist on December 31.
• Choosing Roth 401(k) deferrals and thinking that is the mega backdoor. It is not; after-tax is a separate contribution type.
• Letting after-tax money sit for months before converting.
• Sending the earnings from an in-service withdrawal to a traditional IRA, then running a backdoor Roth IRA the same year.
• Skipping Form 8606, or filing one for only one spouse. Without it, the IRS has no record of your basis.
• Withdrawing converted money within five years before age 59½. Each conversion has its own five-year clock for the 10% penalty on converted amounts; see when you can withdraw from a Roth IRA.

The decision for your family

For many households in this income range the usual order looks like this. First, clear pre-tax IRA balances out of both spouses’ names, usually by rolling them into a current 401(k). Second, run both backdoor Roth IRAs each year. Third, if either plan allows after-tax contributions with automatic conversion, decide how much of the remaining room to fund, weighed against liquidity needs and what else you save for, such as 529 plans. If you are unsure whether Roth dollars beat a taxable account for your goals, our comparison of the mega backdoor Roth versus a brokerage account walks through the tradeoffs, and whether to max out your 401(k) covers the pre-tax side.

What to document for the kids

Roth money is often the account parents most want to pass on: a Roth IRA has no lifetime required distributions for the owner, and neither does a designated Roth 401(k). Write down which accounts are Roth, where the Form 8606 filings live, and why you chose Roth over taxable savings. A child who inherits a Roth IRA at 30 will make better choices if they know how it was built and what it was for. Conversations fade. Documentation compounds.

Frequently asked questions

Does my 401(k) balance affect the backdoor Roth pro rata calculation?

No. Form 8606 counts only traditional, SEP and SIMPLE IRAs, valued on December 31 of the conversion year. Pre-tax money in a 401(k), 403(b) or governmental 457(b) is ignored. That is why rolling an old IRA into a current employer plan is the standard fix for a backdoor Roth that would otherwise be partly taxable. Your spouse’s IRAs are also ignored, because each person’s calculation is separate.

How much can we put into a mega backdoor Roth in 2026?

Up to $72,000 per person in total annual additions, minus your own deferrals and employer contributions. A spouse who defers $24,500 and receives a $12,000 match has $35,500 of after-tax room. If both spouses have plans that allow it, each has a separate limit. Catch-up contributions for those 50 and over are on top of the $72,000.

Can I undo a conversion if the pro rata rule made it taxable?

No. Recharacterizing a Roth conversion back to a traditional IRA has not been allowed since 2018 (IRS Publication 590-A). Once a conversion is done, the tax follows the Form 8606 calculation for that year. The only lever left is the December 31 balance: moving pre-tax IRA money into a 401(k) before year-end still reduces the taxable share of a conversion made earlier that year.

If you have an old rollover IRA, or you are not sure whether your plan allows after-tax contributions, a short review of both spouses’ accounts before December 31 is the practical next step. Our retirement and employee benefits and tax management work at Age Five Family Office™ covers this as part of the annual plan. This is general education, and the right move depends on your 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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