The bottom line

The "mega backdoor Roth" lets you stuff up to $46,500 (2026) of after-tax money per year into a Roth account through your 401(k) — far more than the regular Roth IRA limit of $7,000 or the regular Roth 401(k) elective deferral of $23,500. The trick: contribute beyond the elective-deferral cap into an after-tax sub-account of your 401(k), then immediately convert those after-tax dollars to Roth treatment via either an in-plan Roth conversion or an in-service distribution to a Roth IRA. This works only if your employer's plan supports both (1) after-tax contributions beyond the elective limit and (2) regular conversions of those contributions. Tech companies and large financial firms often have it; many small plans don't. When available, it's the highest-leverage Roth tax move in the IRC.

How the contribution math actually works

The IRS sets two separate ceilings on 401(k) plan inflows under IRC §402(g) and §415:

  • §402(g) — the elective deferral cap on employee pre-tax + Roth combined: $23,500 for 2026 ($31,000 if 50+).
  • §415(c) — the total annual additions cap, including employee elective + employer + after-tax: $70,000 for 2026 ($77,500 if 50+).

The gap between these caps is the mega backdoor space:

$70,000  total annual additions (§415)
−$23,500  employee elective (pre-tax + Roth)
−$X       employer match / profit-sharing / nonelective
─────────
=mega backdoor space (after-tax bucket)

If your employer's match adds up to $11,000, your remaining mega backdoor space is $70,000 − $23,500 − $11,000 = $35,500 that you can contribute to the after-tax bucket.

Maximum theoretical: $70,000 − $23,500 − $0 employer = $46,500.

After-tax dollars are post-tax payroll deductions — you've already paid income tax on them. Putting them in the after-tax sub-account doesn't get you an additional deduction. The value comes from the next step.

Step 2: convert to Roth treatment

After-tax dollars sitting in the 401(k) earn pre-tax growth like any other 401(k) dollars. If left in the after-tax sub-account, the eventual distributions are partial-basis returns — your contributions come out tax-free, but the growth on those after-tax dollars is taxed as ordinary income.

That's no good. The mega backdoor's value comes from converting those after-tax dollars to Roth treatment immediately, before they grow. Two paths:

Path A: In-plan Roth conversion

Some plans offer an in-plan Roth conversion (also called "Roth in-plan rollover"). You move the after-tax balance to the Roth sub-account of the same 401(k). Once in the Roth bucket, growth is permanently tax-free.

The conversion itself is taxable on the gain portion only — if you convert quickly after contribution, the gain is near-zero, so the conversion is effectively tax-free. If you let the after-tax dollars sit and grow for years before converting, the gain becomes taxable at conversion at ordinary rates.

Best practice: convert as quickly as possible. Some plans automate this with a "daily Roth conversion" feature.

Path B: In-service distribution to Roth IRA

Some plans allow in-service withdrawals (no separation from employment required) of after-tax contributions and their earnings. You distribute the after-tax balance and roll it directly to your Roth IRA. The earnings portion is taxable; the basis portion goes into Roth tax-free.

Per IRS Notice 2014-54, you can split a single distribution into two destinations: basis to Roth IRA, earnings to Traditional IRA. This is the "after-tax allocation rule" — clean separation, no taxable conversion at all.

Practical comparison

  • In-plan conversion: simpler, all stays in the 401(k), Roth sub-account growth is tax-free thereafter. Available less often.
  • In-service distribution to Roth IRA: more flexibility (Roth IRA has more investment choice and no RMDs at age 73 unlike Roth 401(k)), uses the §401-§408 portability rules, more common.

The plan-provision dependency

Both legs of the mega backdoor require specific plan features. Check your plan's Summary Plan Description for:

  1. After-tax contributions allowed: a sub-account separate from pre-tax and Roth, with the §415 cap as the limit.
  2. In-plan Roth conversions allowed OR in-service withdrawals of after-tax + earnings allowed.
  3. Frequency of conversions / withdrawals: ideally automatic / daily / monthly. Annual is OK; "once on separation only" is no good.

Companies with the strongest mega backdoor support: Google, Meta, Amazon, Microsoft, Apple, Bloomberg, Goldman Sachs, McKinsey, Citadel, Two Sigma, Bridgewater, large law firms with big 401(k) plans.

Companies typically without: small-business plans (under ~50 employees), most retail / hospitality plans, public-sector 403(b)s.

If your plan doesn't allow it, you cannot do the mega backdoor, full stop. Talk to HR — sometimes there's a path to advocate for the feature; sometimes not.

The pro-rata problem (yes, again)

If your after-tax sub-account has earned anything between contribution and conversion, the conversion is partly taxable per IRC §72(e)(8). Specifically:

basis fraction = after-tax basis ÷ (after-tax basis + earnings)
tax-free portion of conversion = conversion × basis fraction
taxable portion = conversion × (1 − basis fraction)

Example: contribute $20,000 after-tax, it grows to $20,500 over 3 months, then convert. Basis $20,000, earnings $500. 97.6% basis, 2.4% earnings.

  • Tax-free: $20,000 × 97.6% = $19,520. Plus $20,500 − $19,520 = $980 of converted earnings... wait, this isn't right.

Let me redo: if you convert the entire $20,500, your basis is $20,000 and earnings are $500. The conversion is $20,500. Of that, $20,000 is basis (no tax), $500 is earnings (taxed as ordinary income at marginal rate, no penalty).

So at 32% federal + 5% state, you owe $185 of tax on the $500 of earnings. The $20,000 of contribution converts tax-free.

Convert quickly to minimize this.

What about IRC §401(a)(17) compensation cap?

The §401(a)(17) cap on compensation that can be considered for retirement-plan purposes is $355,000 for 2026. If you earn $500k, only the first $355k can be the basis for employer match, profit-sharing, and after-tax contributions in your plan.

This affects very-high earners disproportionately: at $500k base, your match might be capped at 5% × $355k = $17,750 (rather than 5% × $500k = $25,000). Your mega backdoor space is correspondingly compressed.

The §415(c) total-additions cap is $70,000 regardless of salary, but the after-tax space inside it is constrained by your effective compensable comp.

Worked example

You earn $300,000 base + $50,000 bonus. Employer 401(k) match: 6% on first $345k of comp = $21,000 employer contribution.

Your contribution plan:

  • Pre-tax 401(k) elective: $23,500.
  • Mega backdoor after-tax: $70,000 − $23,500 − $21,000 = $25,500.

You set payroll deduction at $25,500 / 26 pay periods = $980/period after-tax.

Each pay period, $980 lands in the after-tax sub-account. Plan auto-converts daily to Roth sub-account. Conversion happens with $0–$5 of earnings; near-zero tax impact.

Year-end:

  • Pre-tax 401(k): $23,500.
  • Roth (via mega backdoor): $25,500.
  • Employer match: $21,000.
  • Total in 401(k): $70,000.

Plus you do the regular backdoor Roth IRA: $7,000.

Annual Roth contributions: $32,500. ($25,500 mega + $7,000 backdoor.)

Compare to a friend with the same income but no mega backdoor: they get $7,000 Roth IRA only. The difference compounds.

Over 25 years at 7% real, $32,500 annual Roth lands at ~$2.4M tax-free. Friend's $7,000/year lands at ~$525k. Mega backdoor delta: ~$1.9M of tax-free retirement money.

Common pitfalls

  1. Plan doesn't allow it. Many people assume their 401(k) supports it; check the SPD.

  2. Conversion timing. Converting infrequently lets after-tax dollars accrue meaningful earnings, which then convert as taxable income. Convert at least monthly; daily auto-conversion is gold.

  3. Hitting the §415 cap with bonus deferrals. If you elect a high % of bonus to pre-tax, your year-end pre-tax contribution may already be near $23,500, eating into your mega backdoor space. Coordinate elections.

  4. Mistaking match for after-tax. Some plans show "non-Roth contributions" in a single bucket. Confirm your after-tax election is going into the correct sub-account.

  5. Not splitting after-tax distribution properly. If you do an in-service rollover to Roth IRA + Traditional IRA, ensure the basis goes to Roth and earnings go to Traditional (per Notice 2014-54). Many custodians can't handle the split — consolidate to a single Roth IRA destination or do an in-plan conversion instead.

  6. Forgetting to elect the after-tax bucket each year. Some plans require annual re-election. Check enrollment timing.

Practical checklist

  • ✅ Confirm plan supports after-tax contributions beyond the §402(g) cap.
  • ✅ Confirm plan supports either in-plan Roth conversions or in-service distributions of after-tax + earnings.
  • ✅ Set up automatic / daily conversion to minimize earnings-on-after-tax.
  • ✅ Calculate your remaining §415 space after pre-tax + match.
  • ✅ Coordinate bonus deferrals to avoid pushing pre-tax to the §402(g) cap mid-year.
  • ✅ Pair with the regular backdoor Roth IRA for a total of $32k+ Roth/year.
  • ❌ Don't leave after-tax dollars unconverted; their growth is taxable on conversion.
  • ❌ Don't assume your plan has it. Read the SPD.
  • ❌ Don't max the mega backdoor without first hitting employer match (free money, then mega).

The mega backdoor Roth is the largest single Roth contribution any individual can legally make. For high-income tech, finance, and consulting employees with the right plan, it's the difference between a $1M Roth balance and a $3M Roth balance at retirement. Read the SPD.