After-tax 401(k) contributions: the mechanic behind both backdoor Roths

A client asks why their $46,000 mega backdoor Roth conversion came out essentially tax-free, while their spouse's $7,500 backdoor Roth IRA conversion was taxable on about 92.5% of the amount. Both strategies have the same name and the same destination account, and the tax results couldn't be more different.

The difference comes down to two separate sets of rules. IRC Section 408(d)(2) pools every IRA a taxpayer owns, while the allocation rules for plan distributions in Section 402(c)(2) and IRS Notice 2014-54 let a participant direct where the pretax portion of a plan distribution lands. Once you see that, the two strategies stop looking like variations on a theme and start looking like two different statutory pathways that end at the same account type.

This guide walks through both side by side: the pro-rata trap that catches the regular version, why after-tax 401(k) money sits outside it, what Notice 2014-54 does and doesn't change, and where Form 8606 does and doesn't belong.

Key takeaways

• The pro-rata rule under IRC Section 408(d)(2) aggregates every traditional, SEP, and SIMPLE IRA a taxpayer owns. Form 8606 uses their December 31 value plus the year's distributions and conversions, and any conversion is taxed proportionally across pretax and after-tax dollars.
• A regular backdoor Roth is fully tax-free only if the taxpayer holds no other pretax IRA money. One old rollover IRA is enough to make most of the conversion taxable.
• A 401(k) balance isn't part of that IRA aggregation, because Section 408(d)(2) reaches only IRAs. After-tax plan money never dilutes an IRA conversion.
• Plan distributions have their own pro-rata rule: under Section 72(e)(8), each distribution carries a proportional share of pretax and after-tax amounts. Section 402(c)(2) then treats a rollover as consisting first of the pretax portion.
• Notice 2014-54 lets a participant who takes one distribution and directs it to several destinations assign the pretax portion to a traditional IRA or plan and the after-tax portion to a Roth IRA, if the allocation is timely elected. It doesn't change how much of the distribution is pretax in the first place.
• Form 8606 belongs to the IRA side of the ledger. A direct rollover of after-tax plan money to Roth generally doesn't trigger it, but it can still matter for other IRA activity and for later Roth IRA distributions.
• For 2026, the Section 415(c) annual additions limit is $72,000 and the Section 402(g) deferral limit is $24,500. The gap between them is the ceiling on after-tax room, before employer contributions and plan-specific limits.

Why "backdoor" Roth strategies exist at all

Roth IRAs have an income ceiling. Direct contributions phase out under IRC Section 408A(c)(3), which for 2026 means modified AGI between $153,000 and $168,000 for single filers and between $242,000 and $252,000 for joint filers, per Notice 2025-67.

Nondeductible traditional IRA contributions have no such income ceiling (see Sections 219(b) and 408(o)). The 2026 IRA limit is $7,500, or $8,600 at age 50 and over. So the workaround writes itself: contribute nondeductible dollars to a traditional IRA, then convert them to Roth.

The conversion is a taxable event. Section 408A(d)(3) treats it as a distribution, and only the after-tax basis comes out untaxed. If the nondeductible contribution is the only money in the IRAs and it converts before earnings accrue, the tax bill is close to nothing.

Close to nothing changes fast the moment another dollar of pretax IRA money enters the picture.

The rule that wrecks the simple version

Section 408(d)(2) doesn't let a taxpayer choose which dollars convert. It treats all of a taxpayer's traditional, SEP, and SIMPLE IRAs as one pool and takes pretax and after-tax amounts out of that pool proportionally.

On Form 8606, the pool is the December 31 value of those IRAs plus the year's distributions and conversions. Here's the arithmetic for a spouse who makes a $7,500 nondeductible contribution, holds a $92,500 pretax rollover IRA, and converts only the $7,500:

The spouse converted "their" nondeductible contribution, technically. The IRS sees one pool with a 7.5% after-tax share and taxes the conversion at that ratio, no matter which account the money physically sat in.

A workaround exists, and it's mechanical rather than clever. Section 408(d)(2) reaches only IRAs, so moving pretax IRA money into an employer plan takes it out of the pool. Only the pretax portion of an IRA can go into a 401(k), and only if the plan accepts incoming rollovers, so nondeductible basis stays behind.

Because Form 8606 measures the IRAs at December 31, the rollover has to be completed by year-end of the conversion year. Doing it first keeps the sequence clean. Confirm that no pretax money remains in any traditional, SEP, or SIMPLE IRA on December 31, and that the rollover was completed properly under Section 408(d)(3).

Form 8606: what it actually tracks

The form has three parts. Part I tracks cumulative nondeductible contributions to traditional IRAs and runs the pro-rata calculation, Part II reports a conversion to Roth and its taxable amount, and Part III figures the taxable portion of certain distributions from Roth IRAs.

Line 6 of Part I asks for the December 31 value of every traditional, SEP, and SIMPLE IRA the taxpayer owns, which is the aggregation figure Section 408(d)(2) requires. Lines 7 and 8 add the year's distributions and conversions. The instructions for Form 8606 walk through the full sequence.

Skip the form in a year a nondeductible contribution is made, and there's no IRS record that the money was already taxed. A distribution years later can then be treated as fully taxable, even though part of it never should have been.

Enter the mega backdoor Roth

A 401(k) plan can allow up to three kinds of employee contributions: pretax elective deferrals, Roth elective deferrals, and a third bucket that's neither, generally called non-Roth after-tax contributions. That bucket is the raw material for a mega backdoor Roth, and it runs on different statutes than anything on the IRA side.

Where the room comes from

IRC Section 415(c) caps the combined annual additions to a defined contribution plan: employee deferrals, employer match, profit-sharing, and after-tax contributions, all summed together. For 2026 that ceiling is $72,000, up from $70,000 in 2025, under Notice 2025-67. The limit is also capped at 100% of compensation when that is lower.

Section 402(g) sets the separate, smaller limit on employee deferrals: $24,500 for 2026. After-tax room is what remains of the $72,000 after the participant's own deferrals, employer contributions, and any other annual additions. Age-50 catch-up contributions sit outside the $72,000 limit under Section 414(v).

A participant who defers the full $24,500 and receives no employer contributions would have up to $47,500 of after-tax room. That's an illustrative maximum, not a typical figure. Most plans leave far less once employer contributions are counted, and plan terms or nondiscrimination testing can cap after-tax contributions lower still.

The mechanic behind both backdoor Roths

Two rules do the work here, and most explanations blur them together.

The plan's own pro-rata rule

Under IRC Section 72(e)(8), each distribution from a plan account holding both pretax and after-tax money carries a proportional share of each. That's why a participant generally can't take a payment of only after-tax contributions: the payment brings an allocable slice of pretax dollars along, including earnings on the after-tax money. It's the same proportional idea as Section 408(d)(2), applied to one plan account instead of a pool of IRAs.

Where the dollars land: Section 402(c)(2) and Notice 2014-54

Section 402(c)(2) says that when only part of an eligible rollover distribution is rolled over, the amount rolled over is treated as consisting first of the portion that is includible in income, meaning the pretax portion.

Earlier guidance in Notice 2009-68 treated each disbursement to a separate destination as carrying its own pro rata share of pretax and after-tax amounts. Notice 2014-54 changed that for distributions made from 2015 on: all disbursements scheduled at the same time are treated as one distribution. If that distribution's pretax amount is less than the amount directly rolled over, the entire pretax amount is assigned to the direct rollovers, and the participant chooses how it is allocated among the destinations by telling the plan administrator before the rollovers are made.

What the Notice doesn't change is the size of the pretax and after-tax portions of the distribution itself, which still come from the plan's pro rata calculation. It changes where those portions are assigned. It also applies only to distributions from qualified plans, 403(b) plans, and governmental 457(b) plans, not to IRAs.

The Notice's Example 4

Example 4 builds on Example 1 and shows the mechanic with real numbers:

• Employee C's plan account holds $250,000: $200,000 pretax and $50,000 after-tax
• C takes a $100,000 distribution, which under Section 72(e)(8) contains $80,000 pretax ($100,000 x $200,000 / $250,000) and $20,000 after-tax
• C directs $80,000 in a direct rollover to a traditional IRA and $20,000 in a direct rollover to a Roth IRA
• C is permitted to allocate the $80,000 pretax amount to the traditional IRA, so the $20,000 reaching the Roth IRA is entirely after-tax

Those figures work because of the stated balances: the distribution's pretax portion equals the amount headed to the traditional IRA. Change the account balances, the distribution size, or the destination split, and the result changes. The example also involves an employee who has separated from service, so whether an in-service distribution is available depends on the plan's terms.

No comparable allocation exists on the IRA side. Section 408(d)(2) aggregates the pool and taxes the blend, without regard to which account a dollar sits in or where it's headed.

Two roads from after-tax 401(k) to Roth

Plans that support after-tax contributions typically offer one or both of the following.

In-service distribution split

Under Notice 2014-54, a participant takes a distribution and routes it to two destinations at once: the pretax portion, which includes earnings on the after-tax contributions, to a traditional IRA, and the after-tax basis to a Roth IRA. The IRS's own guidance on rollovers of after-tax contributions confirms that earnings on after-tax contributions are pretax amounts, and that after-tax contributions can go to a Roth IRA without those earnings when the allocation is structured correctly. The plan has to support the split, so confirm that before requesting it.

In-plan Roth rollover

IRC Section 402A(c)(4) lets the conversion happen without the money leaving the plan. The Small Business Jobs Act of 2010 created the option for amounts already eligible for distribution, and the American Taxpayer Relief Act of 2012 extended it to any vested amount under Section 402A(c)(4)(E), as the IRS describes in its Notice 2013-74 guidance.

An in-plan rollover is taxable to the extent it includes pretax amounts, such as earnings on the after-tax money. The rollover itself generally isn't hit with the 10% additional tax, but Notice 2010-84 applies a five-year recapture rule to later distributions attributable to it. Plans that convert after-tax contributions automatically and frequently keep the taxable earnings slice small.

Where Form 8606 usually stays out of it

For Form 8606 purposes, a conversion means money moving from a traditional, SEP, or SIMPLE IRA to a Roth IRA. A direct rollover of after-tax plan money to a Roth IRA, or an in-plan rollover to a designated Roth account, isn't that, so it doesn't by itself trigger the form.

Instead, the payer reports the transaction on Form 1099-R. For a direct rollover from a plan to a Roth IRA, the IRS instructions call for the total in Box 1, the taxable amount in Box 2a, any basis recovery in Box 5, and code G in Box 7.

The form can still come into play in three situations:

• The participant rolls after-tax plan money into a traditional IRA instead of a Roth IRA. The basis then sits inside the IRA, subject to the same Section 408(d)(2) aggregation, and Part I needs to track it from that year forward.
• The participant has separate IRA activity, such as a regular backdoor Roth or IRA distributions while holding basis.
• A later nonqualified Roth IRA distribution requires Part III, which asks for basis in conversions and in rollovers from qualified plans to a Roth IRA.

Side by side: the 2026 numbers

One more wrinkle: the Roth catch-up mandate

Starting in 2026, SECURE 2.0 Section 603 requires catch-up contributions to be made as Roth for participants whose prior-year FICA wages from the plan sponsor exceeded the Roth catch-up wage threshold. For 2026 catch-ups, the test compares 2025 wages to $150,000, up from $145,000, under Notice 2025-67.

The mandate covers catch-up contributions, which are up to $8,000 for 2026 ($11,250 at ages 60 to 63), and not the non-Roth after-tax contributions that fund a mega backdoor Roth. Catch-up dollars are also separate from the $72,000 annual additions limit. A participant caught by the mandate loses the option to make those dollars pretax, and they go in as Roth elective deferrals that count toward the participant's five-year period under Section 402A(d)(2), which starts with the first designated Roth contribution to that plan.

Why the paper trail matters

Everything here comes down to sequence and documentation: confirming that no pretax IRA money remains at year-end before converting, confirming the plan separately accounts for after-tax contributions before requesting a split, and confirming which statute governs a specific client's situation. Writing that reasoning down is what makes a position defensible later, and tools built for drafting tax memos make that faster.

Bizora AI traces answers like these back to the Code section, Treasury regulation, or IRS notice actually behind them, with a View Steps reasoning path showing how the conclusion was reached. Report-based planning software can model a conversion's bracket effects, as this comparison with Holistiplan explains, but which rule governs a plan distribution is a research question that needs a citation.

Before the money moves

Both strategies rest on the same idea: get money to Roth through a route the Code permits instead of the route the income limits would otherwise block. The regular version needs a clean IRA balance sheet at year-end. The mega version needs a cooperative plan and a distribution allocated correctly.

Confirm the aggregation number before converting. Confirm the plan's after-tax accounting and the pro rata split before directing a distribution. File Form 8606 in the years it applies, and expect a Form 1099-R, not Form 8606, when after-tax plan money goes straight to Roth.

Research your next backdoor Roth or after-tax 401(k) question in Bizora AI, with a 7-day free trial, no credit card required.

Sources

• IRC Section 408, including Section 408(d)(2), the IRA pro-rata aggregation rule, and Section 408(d)(3), rollovers
• IRC Section 408A, Roth IRAs, including the Section 408A(d)(3) conversion rules
• IRC Section 402, including Section 402(c)(2), the rollover ordering rule
• IRC Section 402A, designated Roth accounts and in-plan Roth rollovers
• IRC Section 72, including Section 72(e)(8), pro rata treatment of plan distributions
• IRC Section 415, the annual additions limit
• IRC Section 414, including Section 414(v), catch-up contributions
• IRS Notice 2014-54, allocation of after-tax amounts to rollovers
• IRS Notice 2010-84, in-plan Roth rollovers, including the five-year recapture rule
• IRS: deadline extended to add new in-plan Roth rollover provisions, describing Notice 2013-74 and Section 402A(c)(4)(E)
• IRS Notice 2025-67, 2026 retirement plan and IRA limits, including the Roth catch-up wage threshold
• IRS news release IR-2025-111, 2026 limit announcement
• IRS: rollovers of after-tax contributions in retirement plans, plain-language confirmation of split-rollover mechanics
• Instructions for Form 8606, nondeductible IRAs
• Instructions for Forms 1099-R and 5498, reporting of direct rollovers to a Roth IRA