Does the Pro Rata Rule Apply to 401(k) Plans?

The pro rata rule for 401(k) plans works differently than it does for IRAs, and the short answer is that the IRA pro rata rule does not apply to your 401(k). A 401(k) is a qualified employer trust, not an individual retirement plan, so the IRS does not blend its balance with your IRAs when deciding how much of a distribution is taxable. That separation is the reason the Mega Backdoor Roth exists, and it is also the reason a single wrong rollover can destroy years of tax planning.

What the IRA Pro Rata Rule Does

Under Internal Revenue Code Section 408(d)(2), every non-Roth IRA you own is treated as one account for tax purposes.¹ Traditional IRAs, SEP IRAs, and SIMPLE IRAs all get pooled. You cannot pick which dollars come out. The IRS forces a proportional split, so every dollar you withdraw or convert carries a mix of pre-tax and after-tax money that mirrors the pool as a whole.

The math is simple. Divide your total after-tax basis by the total year-end value of all your non-Roth IRAs. That fraction is the tax-free percentage of any distribution. With $10,000 of basis and $90,000 of pre-tax money, only 10% of a conversion escapes tax. Converting just the $10,000 you contributed after-tax does not exempt the other 90%.

The balance figure the IRS uses is the December 31 snapshot of the year you take the distribution, plus any distributions you already took during that year.¹ Emptying an IRA in January does not reset the ratio for a February conversion. You track your basis on Form 8606, filed with your return any year you make nondeductible contributions or take distributions from an IRA that contains them.²

What Gets Pulled In, What Stays Out

The aggregation net catches Traditional, SEP, and SIMPLE IRAs, all of which are individual retirement plans under Section 408.¹ It does not matter how many accounts you hold or at how many brokerages.

Several account types stay outside:

Roth IRAs, which are governed by Section 408A and never aggregated with pre-tax IRAs for pro rata purposes.
Inherited IRAs from a non-spouse, which under Treasury Regulation 1.408-8 are generally treated separately. A spousal IRA you elect to treat as your own joins your pool.
Employer plans — 401(k), 403(b), and 457(b) — which are qualified trusts, not individual retirement plans.

Why 401(k) Plans Are Outside the Rule

A 401(k) is a trust established under Internal Revenue Code Section 401(a), and distributions from it are governed by Section 402. Because Section 408(d)(2) only aggregates individual retirement plans, your 401(k) is simply invisible to the IRA pro rata calculation.¹ A 401(k) holding $500,000 in pre-tax money does not contaminate a Backdoor Roth conversion from a $7,000 nondeductible Traditional IRA contribution.

The exemption cuts the other way too. If large pre-tax IRA balances are making a Backdoor Roth expensive, and your 401(k) accepts incoming rollovers, moving those pre-tax IRA dollars into the 401(k) removes them from the IRA aggregation pool. The pre-tax money still exists, but it no longer inflates the denominator of your pro rata fraction.

The 401(k) Has Its Own Internal Proportionality Rule

Skipping this piece leads to real confusion, because 401(k) plans are not free from proportionality altogether. When a plan holds both pre-tax and after-tax dollars and you take a partial withdrawal (anything less than the entire account), the withdrawal itself has to include a proportional share of each type.³ If your 401(k) is 80% pre-tax and 20% after-tax, a $10,000 partial distribution contains roughly $8,000 pre-tax and $2,000 after-tax. You cannot pull out only the after-tax dollars and leave the pre-tax money behind.

What Notice 2014-54 changed is not that requirement. It changed what you can do with each component after it leaves the plan.

Split Rollovers Under Notice 2014-54

Notice 2014-54 established that when a 401(k) distribution contains both pre-tax and after-tax money, all disbursements scheduled at the same time are treated as a single distribution, and you can direct each component to a different destination. Instruct your plan administrator to process a direct rollover, and specify where each piece goes. Pre-tax money (elective deferrals, employer match, all earnings) rolls to a Traditional IRA or another employer plan. After-tax contribution dollars roll straight to a Roth IRA.⁴ Because you already paid income tax on those after-tax contributions, the Roth rollover produces no additional tax.

The Notice gives an explicit example: an employee with $80,000 pre-tax and $20,000 after-tax sends $80,000 to a Traditional IRA and $20,000 to a Roth IRA. Pre-tax amounts are assigned first to the direct rollover destinations, so the Traditional IRA receives entirely pre-tax money and the Roth IRA receives entirely after-tax basis.³ No tax is owed on the Roth piece.

One nuance is easy to miss. Earnings on your after-tax contributions inside the 401(k) are pre-tax money. Those earnings must go to the Traditional IRA (or stay in an employer plan) unless you are willing to pay income tax on them. The longer after-tax dollars sit in the plan before conversion, the more earnings pile up in the pre-tax bucket.

The Mega Backdoor Roth

The split rollover is the mechanism behind the Mega Backdoor Roth. If your 401(k) allows voluntary after-tax contributions above the elective deferral limit, you can contribute far more each year and then move those after-tax dollars into Roth savings, either through an in-plan conversion or a rollover to a Roth IRA.

For 2026, the total annual addition limit under Section 415(c) is $72,000.⁵ That cap covers everything going in: your elective deferrals, employer contributions, and any voluntary after-tax contributions. The elective deferral limit for 2026 is $24,500, with an $8,000 catch-up at age 50 or older, or $11,250 for ages 60 through 63 under SECURE 2.0’s enhanced catch-up.⁶

The gap between the $72,000 overall cap and your deferrals plus employer contributions is the room for after-tax contributions. Under 50, deferring $24,500 with a $10,000 employer match leaves $37,500 of headroom. That is far above the $7,500 Roth IRA contribution limit.

Not every plan supports this. The strategy needs three things: the plan must accept voluntary after-tax contributions (separate from pre-tax and Roth deferrals), it must permit either in-service withdrawals of after-tax money or in-plan Roth conversions, and it must pass the Actual Contribution Percentage test. The ACP test limits how much highly compensated employees (over $160,000 in prior-year pay for 2026) can contribute after-tax relative to other participants.⁵ A failed test can force refunds of excess contributions to those employees even if the plan document allows the contributions.⁷

The Trap: Rolling Pre-Tax 401(k) Money Into an IRA

The 401(k)’s exemption disappears the second pre-tax 401(k) money lands in a Traditional IRA. Those dollars become part of your IRA pool, and every future distribution or conversion from any non-Roth IRA is calculated against the whole pool.¹

This is where most Backdoor Roth conversions unravel. A taxpayer contributes $7,000 after-tax to a Traditional IRA and plans a clean conversion. They forget the $200,000 they rolled out of a prior employer’s 401(k) into a Traditional IRA years ago. Basis is now $7,000 out of $207,000. About 3.4% of the conversion is tax-free. The other 96.6% produces a tax bill they never planned for.

If you want to keep the Backdoor Roth option open, keep pre-tax money inside employer plans. Roll old 401(k) balances into your current employer’s plan rather than into an IRA. If pre-tax money already sits in a Traditional IRA, many 401(k) plans will accept a rollover in the other direction, which clears the balance that was poisoning your pro rata fraction.

Which Rule Applies When

Three scenarios cover most decisions:

Distribution or conversion from any non-Roth IRA: IRA aggregation under Section 408(d)(2) applies, and every dollar carries a proportional mix based on your December 31 balances.
Partial distribution from a 401(k) holding mixed money: the plan’s internal proportionality rule applies to the distribution itself, but Notice 2014-54 lets you route each component to a different destination once it leaves the plan.
Full distribution or split direct rollover from a 401(k): pre-tax and after-tax dollars can be fully separated, with pre-tax going to a Traditional IRA or another employer plan and after-tax basis going to a Roth IRA without tax. IRA aggregation only enters the picture if you later mix that pre-tax rollover with existing IRA basis.

Keeping after-tax and pre-tax retirement money inside a 401(k) gives you more control at distribution time than holding the same mix in IRAs. If your plan allows after-tax contributions and in-service distributions, you have the pieces for a Mega Backdoor Roth. If it does not, the most valuable move is often defensive: avoid rolling pre-tax 401(k) dollars into a Traditional IRA where they will complicate every future Roth conversion.

1
Office of the Law Revision Counsel. 26 USC 408 – Individual Retirement Accounts
2
Internal Revenue Service. About Form 8606 – Nondeductible IRAs
3
Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
4
Internal Revenue Service. Notice 2014-54 – Guidance on Allocation of After-Tax Amounts to Rollovers
5
Internal Revenue Service. Notice 25-67 – 2026 Amounts Relating to Retirement Plans and IRAs
6
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
7
Internal Revenue Service. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests