The Mega-Backdoor Roth, Step by Step

If you are already putting the maximum allowed into your 401(k) as an elective deferral and still want to shelter more of your income from tax, a "mega-backdoor Roth" may be the next lever — but only if your specific plan document supports the two extra features it depends on. This article walks through what the strategy actually is, how much room you might have once your own contributions and your employer's are accounted for, how to find out whether your plan allows it at all, and the pro-rata mechanics that determine how much of it ends up truly tax-free.

What is a mega-backdoor Roth?

"Mega-backdoor Roth" is not a term the tax code uses anywhere — it is industry shorthand for a two-step move: making after-tax, non-Roth contributions to your 401(k) beyond your regular elective deferral, and then moving that after-tax money into a Roth account, either through an in-plan Roth conversion that stays inside the same 401(k) or through an in-service rollover into a Roth IRA. The after-tax contribution in the first step is a specific, separate bucket that many 401(k) plans do not even offer: it is not the same as a Roth 401(k) contribution, even though the word "Roth" shows up in both. An after-tax contribution goes in without any current tax deduction, exactly like a Roth contribution would, but unlike a Roth contribution it has not yet had the conversion step applied, which means its future earnings are still taxable until you do that second step. The whole point of converting promptly is to let very little in the way of earnings accumulate in that after-tax bucket before it becomes Roth money, since earnings that build up before conversion are the one part of this strategy that is not tax-free, as the section on the pro-rata rule below explains. Whether either the after-tax contribution or the conversion step is even available to you is entirely a function of your specific plan's design, not of the tax code generally allowing or disallowing it — which is the subject of the next two sections.

How much can actually go in?

Your after-tax contribution room is not a separate limit of its own — it is whatever is left of the overall annual limit on total contributions to your 401(k) from every source combined, once your own elective deferrals and everything your employer puts in on your behalf are subtracted out. That overall limit caps the sum of three things: your elective deferrals, traditional and Roth combined; any employer match or profit-sharing contribution; and your after-tax contributions — all counted against one shared ceiling for the year, unlike the elective-deferral limit, which counts only your own paycheck contributions. If you are already contributing the maximum elective deferral and your employer is contributing a meaningful match or profit-sharing amount, the after-tax room left under the overall limit can be substantially smaller than the headline mega-backdoor number people mention, because employer money eats into the same shared ceiling before you get to add anything after-tax. The exact dollar figures for the overall limit, the elective-deferral limit, and the age-based catch-up amounts are set annually and adjusted for inflation, so the precise room you have this year is a calculation worth doing freshly rather than assuming last year's number still applies — the current-year figures are named in Key Numbers below. Because the after-tax bucket is whatever is left over, the calculation has to start from your actual elective deferral and your actual employer contribution for the year, not from the overall limit alone.

Does my plan even allow this?

Nothing about the tax code requires a 401(k) plan to offer after-tax contributions or to allow an in-plan Roth conversion or an in-service distribution — both features are optional plan design choices, and a large share of 401(k) plans offer neither one. Making the mega-backdoor strategy work requires your plan to allow after-tax contributions in the first place, and separately to allow either an in-plan Roth conversion of that after-tax money or an in-service withdrawal you can roll into a Roth IRA yourself; a plan that allows after-tax contributions but has no path to convert or roll them out leaves that money sitting in a taxable after-tax bucket indefinitely, which defeats the purpose. The place to check is your plan's summary plan description, which should describe both features by name if they exist, or a direct question to your plan administrator or HR benefits contact if the summary plan description is unclear or silent on either point. Even where a plan technically allows after-tax contributions, nondiscrimination testing required for certain plan types can limit how much highly compensated employees are permitted to contribute, or can force a refund of amounts already contributed after the fact if the plan fails its test for the year — a real possibility for exactly the kind of employee likely to be reading this article. Confirming both the plan terms and the plan's testing history before you commit a full year of after-tax contributions is worth the extra step, rather than finding out at tax time that some of what you contributed is coming back to you as a taxable refund.

What about the pro-rata rule and earnings?

Once your after-tax contributions are sitting in the plan, any investment earnings they generate before you convert them are themselves taxable on conversion — only your original after-tax contribution amount converts tax-free, because that money was never deducted from your income in the first place, while the earnings on top of it have never been taxed at all yet. This is why converting promptly matters more here than the word "eventually" suggests: the longer after-tax money sits before conversion, the more earnings accumulate on it, and the larger the taxable slice of an otherwise mostly tax-free move becomes. Some plans support converting on a very short cycle, even automatically shortly after each after-tax contribution hits the account, which keeps the earnings portion small almost by design; plans that only allow occasional or manual conversions require more attention from you to avoid letting a taxable balance build up. Within the 401(k) itself, your after-tax contributions and any pre-tax money are tracked in separate sub-accounts precisely so the plan can identify which dollars are already-taxed contributions and which are earnings or pre-tax money when a conversion or distribution happens — this separate accounting inside a single 401(k) is different from, and simpler than, the pro-rata calculation you may have heard about for a traditional IRA holding both deductible and nondeductible contributions, which blends everything together across all of your IRAs rather than tracking it account by account.

What order should I fund my accounts in?

Our policy is to think about every tax-advantaged dollar in a fixed order, rather than deciding account by account each year: first, contribute enough to get the full employer match, because turning down a match is turning down guaranteed money before anything else is considered; second, fund a health savings account if you are enrolled in an eligible health plan, for reasons we cover in our article on the HSA's triple tax advantage rather than repeating here; third, continue funding your elective deferral toward its annual limit; and only after those three are satisfied do we look at after-tax contributions and a mega-backdoor conversion as the next dollar. We put after-tax contributions after the match, the HSA, and the elective deferral deliberately: the first two come with a guaranteed return or a more favorable tax treatment than after-tax 401(k) money gets, and the elective deferral itself is simpler to administer and does not depend on plan features that, as the earlier sections here cover, not every employer plan actually offers. None of this means a mega-backdoor Roth is not worth doing — for someone who has already maxed the first three and still has room to save, it is often the best next dollar available, particularly if converting promptly keeps the taxable earnings slice small. It does mean we would rather confirm the first three are actually maxed, and confirm your plan supports the mega-backdoor mechanics at all, before building a plan around a strategy that, for a meaningful share of employer plans, is not on the menu to begin with.

Key numbers (2026)

• Elective deferral limit (§402(g)) — $24,500 for 2026, up from $23,500 in 2025 (IRS Notice 2025-67).
• Overall annual addition limit (§415(c)) — $72,000 for 2026, up from $70,000; this is the figure your after-tax room is calculated against, since it caps your elective deferrals, employer contributions, and after-tax contributions combined (IRS Notice 2025-67).
• Catch-up contributions — $8,000 for 2026 if you are 50 or older, up from $7,500; if you turn 60, 61, 62, or 63 in 2026, the enhanced SECURE 2.0 catch-up remains $11,250 instead (IRS Notice 2025-67).
• Annual compensation limit (§401(a)(17)) — $360,000 for 2026, up from $350,000 (IRS Notice 2025-67).
• Earnings on after-tax contributions — always taxable when converted or distributed, regardless of the year (IRS Publication 575).

Current as of 2026-09

Sources

• IRS Newsroom, "401(k) limit increases to $24,500 for 2026, IRA limit increases to $7,500" (IR-2025-111, Nov. 13, 2025) — https://www.irs.gov/newsroom/401k-limit-increases-to-24500-for-2026-ira-limit-increases-to-7500
• IRS Notice 2025-67, "2026 Amounts Relating to Retirement Plans and IRAs, as Adjusted for Changes in Cost-of-Living" — https://www.irs.gov/pub/irs-drop/n-25-67.pdf
• IRS Publication 575, Pension and Annuity Income — https://www.irs.gov/publications/p575
• IRS, "Retirement topics — 401(k) and profit-sharing plan contribution limits" — https://www.irs.gov/retirement-plans/plan-participant-employee/retirement-topics-401k-and-profit-sharing-plan-contribution-limits

Sample content for demonstration purposes — not financial advice.