Mega Backdoor Roth: After-Tax 401(k) Limits, Conversion, and Five-Year Rule

The mega backdoor Roth is a way to move after-tax dollars through your 401(k) and into a Roth account, letting you save far more in Roth form than the regular Roth IRA rules allow. For 2026, the total that can go into a single 401(k) account from all sources is $72,000, while your own pre-tax and Roth 401(k) deferrals are capped at $24,500. The space between those two numbers, minus whatever your employer contributes, is the room the strategy uses. Contribute after-tax (non-Roth) dollars into that gap, then convert them to Roth quickly, and future growth on the money comes out tax-free.¹

Why the Strategy Exists

Direct Roth IRA contributions phase out at higher incomes. For 2026, the phase-out begins at $153,000 of modified adjusted gross income for single filers and $242,000 for joint filers, with contributions cut off entirely at $168,000 and $252,000. Even below those thresholds, the annual Roth IRA contribution cap is only $7,500.²

After-tax 401(k) contributions have no income limit, and the available room can run several times the Roth IRA cap. For someone earning well into six figures, this is often the only realistic way to get large sums into Roth treatment.

The Two Limits That Create the Room

Section 402(g) caps your elective deferrals, meaning the total of your pre-tax and Roth 401(k) contributions. For 2026 that limit is $24,500. Exceed it, and the excess is included in your gross income for the year.³⁴

Section 415(c) is the higher ceiling that covers everything landing in your account: your deferrals, employer match and profit-sharing, and after-tax deposits. For 2026 that total is $72,000.⁵ The mega backdoor Roth lives in the gap between the two.

Catch-up contributions for workers aged 50 and older sit at $8,000 for 2026, and a SECURE 2.0 provision raises that to $11,250 for participants who turn 60, 61, 62, or 63 during the year.³ Catch-up contributions are not counted toward the $72,000 Section 415(c) limit, so they do not shrink your after-tax room.⁵

Calculating Your After-Tax Room

The formula is $72,000 minus your elective deferrals minus your employer’s contributions for the year. What’s left is what you can put in after-tax.

Take someone earning $200,000 who maxes deferrals at $24,500 and gets a 5% employer match worth $10,000. Total allocated contributions are $34,500, leaving $37,500 for after-tax deposits.¹ Someone with a smaller match has even more room.

Get the employer match projection right. If the match is 50% on the first 6% of salary, run the exact numbers against your annual compensation rather than estimating. Overshooting the $72,000 cap triggers corrective distributions and additional paperwork.⁵ Also note that the plan can only count compensation up to $360,000 when calculating employer contributions for 2026.¹

What Your 401(k) Plan Must Allow

Two features have to be in your plan document. If either is missing, this strategy is not available to you, and there is no workaround at the participant level.

First, the plan has to permit after-tax contributions. These are legally separate from Roth contributions. Roth 401(k) deferrals go in after-tax and grow tax-free. After-tax non-Roth contributions go in after-tax, but their earnings are tax-deferred and become taxable at withdrawal unless you convert them. Many plans offer pre-tax and Roth options but skip the after-tax bucket entirely. Check your Summary Plan Description or ask HR.

Second, the plan has to let you move those after-tax dollars into a Roth account while you’re still working there. That means either in-service distributions to an external Roth IRA or in-plan Roth rollovers to the plan’s Roth sub-account.⁶ Without one of these, your after-tax money sits in tax-deferred limbo where only the principal comes out tax-free at retirement and the earnings are taxed as ordinary income. That mostly defeats the point.

How Nondiscrimination Testing Can Shrink the Room

Even when the plan offers after-tax contributions, annual testing can cut back what highly compensated employees are allowed to contribute. The Actual Contribution Percentage (ACP) test compares after-tax and matching contributions from highly compensated employees against those made by everyone else.⁷ If the gap is too wide, the plan fails, and the employer may need to refund contributions to highly compensated employees or restrict future deposits.

Some employers skip the after-tax feature to avoid this risk. Others adopt a safe harbor design that automatically passes testing by providing a minimum contribution to all eligible employees. Ask your plan administrator whether the plan is safe harbor or subject to annual testing; the answer determines how predictable your after-tax room actually is.

Converting Fast, and Why It Matters

Until after-tax dollars move into a Roth account, their earnings grow tax-deferred and will be taxed at withdrawal. The conversion is what locks in tax-free growth going forward, and only the earnings portion at the moment of conversion is taxable.

If you contribute $10,000 after-tax and it earns $15 before you convert, you owe income tax on $15. Wait months, and the earnings pile up, turning the conversion into a real tax bill. Same-day or same-week conversions keep the taxable slice tiny.

The cleanest setup is a plan that sweeps after-tax contributions into the Roth sub-account automatically. Turn it on and leave it. If automatic conversion isn’t available, submit conversion requests manually on a regular schedule, typically monthly or quarterly, through your plan administrator or benefits portal. When you make the request, specify that the source is the after-tax non-Roth bucket. Getting that designation wrong can trigger a conversion from your pre-tax balance instead, with a large and unexpected tax bill attached.

After the conversion, your plan administrator issues Form 1099-R for the tax year, breaking out the taxable portion, and you report it on your federal return.

Rolling Out to an External Roth IRA

Rather than converting inside the plan, some participants roll after-tax money out to an external Roth IRA. IRS Notice 2014-54 lets you split a single distribution so the after-tax contribution amount goes to a Roth IRA while any pre-tax earnings go to a traditional IRA.⁶ That keeps the pre-tax earnings out of taxable income for the conversion year and parks them in a traditional IRA until you withdraw them later.

Any single distribution has to include a proportional share of pre-tax and after-tax amounts; you can’t take only the after-tax dollars. The split rollover handles this by sending the two components to different receiving accounts at the same time. The plan administrator manages the mechanics.

The Five-Year Rule After Conversion

Once after-tax contributions land in a Roth account through conversion, the earnings on that money aren’t immediately available tax-free. A qualified distribution requires that at least five years have passed since the first contribution to that Roth account and that you have reached age 59½, become disabled, or the distribution goes to a beneficiary after your death.⁸

The clock starts January 1 of the tax year of the first contribution or conversion to that account. A first in-plan Roth conversion in March 2026 satisfies the five-year requirement on January 1, 2031. If you roll to an external Roth IRA that you’ve already had for years, that Roth IRA’s existing clock applies, which may already be satisfied.

This mostly matters if you might need the money before 59½. The converted principal (your original after-tax contributions) can be withdrawn without tax or penalty. It’s the earnings generated after conversion that are subject to the five-year rule and age requirement.

If You Overcontribute

Exceeding the $72,000 Section 415(c) limit is a plan-level compliance failure, and the plan risks losing its qualified status if not corrected.⁹ IRS correction programs allow the plan to distribute the excess and its attributable earnings; the participant reports the corrective distribution as income for the year it’s received.⁵

For elective deferrals over the $24,500 Section 402(g) limit, the deadline to remove the excess plus earnings is April 15 of the following year, and that deadline can’t be extended by a personal tax extension.¹⁰ Miss it, and the excess is taxed twice: once in the year you contributed it and again when you eventually withdraw it.

Run the Section 415(c) math before you set your after-tax contribution rate, and leave a small buffer. If your employer match moves around because of bonuses or variable pay, stay a few hundred dollars under the cap on purpose.

SECURE 2.0 Provisions That Touch This Strategy

Two SECURE 2.0 items are worth knowing about.

Starting in 2025, workers aged 60 through 63 can contribute $11,250 in catch-up rather than the standard $8,000.³ Because catch-up sits outside the Section 415(c) limit, this adds tax-advantaged savings on top of everything else without reducing after-tax room.

Under SECURE 2.0 Section 603, employees who earned $145,000 or more in FICA wages from the sponsoring employer in the prior year must make their catch-up contributions on a Roth basis. Final regulations generally apply to contributions in taxable years beginning after December 31, 2026.¹¹ This doesn’t change the mega backdoor Roth mechanics, but it does mean high earners lose the option to make their catch-up pre-tax, which shifts the year’s overall tax picture.

1
Internal Revenue Service. 2026 Amounts Relating to Retirement Plans and IRAs
2
Internal Revenue Service. Retirement Topics – IRA Contribution Limits
3
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026
4
eCFR. 26 CFR 1.402(g)-1 – Limitation on Exclusion for Elective Deferrals
5
Internal Revenue Service. Fixing Common Plan Mistakes – Failure to Limit Contributions for a Participant
6
Internal Revenue Service. Rollovers of After-Tax Contributions in Retirement Plans
7
Internal Revenue Service. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests
8
Internal Revenue Service. Roth Account in Your Retirement Plan
9
Office of the Law Revision Counsel. 26 U.S. Code 415 – Limitations on Benefits and Contribution Under Qualified Plans
10
Internal Revenue Service. Consequences to a Participant Who Makes Excess Annual Salary Deferrals
11
Internal Revenue Service. Treasury, IRS Issue Final Regulations on New Roth Catch-Up Rule, Other SECURE 2.0 Act Provisions