After-Tax 401(k) → Mega Backdoor Roth Calculator 2026

Calculate exactly how much after-tax 401(k) headroom your 2026 plan allows under the IRC § 415(c) overall limit, then estimate the Mega Backdoor Roth conversion (in-plan Roth rollover or in-service distribution to a Roth IRA) and 30-year tax-free growth versus a taxable brokerage equivalent. Uses the IRS 2026 limits: $70,000 § 415(c) total ($77,500 with age 50+ catch-up, $81,250 with age 60-63 super catch-up under SECURE 2.0). Free, private, runs entirely in your browser.

Your 2026 Profile
Determines elective-deferral catch-up: 50+ adds $7,500; 60-63 super catch-up adds $11,250 (SECURE 2.0).
Gross wages — caps after-tax contributions to compensation minus pre-tax/Roth.
Plan Inputs
Pre-tax + designated Roth. 2026 cap: $23,500 (under 50) / $31,000 (50+) / $34,750 (60-63).
Common: 5% match (Big Tech often 50% on first 6% = effective 3-6%).
~50% of large-company plans support this. Distinct from designated Roth deferrals.
Convert IMMEDIATELY to avoid earnings becoming taxable on conversion (basis-only rollover ideal).
Growth Assumptions
Typical retirement horizon for a 40-year-old.
7% nominal = long-run S&P 500 average after inflation-style adjustment.
Used to compare tax-free Roth vs taxable brokerage at retirement.
Used for taxable-brokerage comparison: 0%, 15%, or 20% federal LTCG.
After-Tax Headroom (2026)
$0
§ 415(c) Total Limit
$0
30-Yr Tax-Free Roth Value
$0
Taxable Brokerage Equivalent
$0
2026 § 415(c) Calculation
Step Amount
2026 IRS limits used: § 415(c) overall limit $70,000 (under 50) / $77,500 (age 50-59 + catch-up) / $81,250 (age 60-63 super catch-up under SECURE 2.0 § 109). § 402(g) elective deferral $23,500 / $31,000 / $34,750. The Mega Backdoor Roth strategy depends on your specific plan document allowing both (a) after-tax contributions and (b) in-plan Roth rollover or in-service distribution. Verify plan availability before contributing.

Source: IRC § 415(c) + IRS Notice 2014-54 (allocation of pre-tax and after-tax amounts) + SECURE 2.0 Act § 109. Last updated: May 3, 2026.
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What Is a Mega Backdoor Roth?

The Mega Backdoor Roth is a tax strategy that lets high-income employees move tens of thousands of after-tax dollars into a Roth account each year — far beyond the $7,000 standard Roth IRA contribution limit. The mechanic exploits the gap between the elective deferral cap (IRC § 402(g): $23,500 for 2026) and the overall defined contribution limit (IRC § 415(c): $70,000 for 2026). Employees who max their pre-tax deferrals and receive an employer match still have headroom up to the § 415(c) ceiling, which can be filled with after-tax contributions and then immediately converted to Roth via an in-plan Roth rollover (Notice 2014-54) or in-service distribution to a Roth IRA. Source: IRS Notice 2014-54.

The strategy is only available if the plan document explicitly allows after-tax (non-Roth) contributions AND permits either an in-plan Roth rollover or an in-service distribution. Roughly half of Fortune 500 401(k) plans support both. Before contributing, request a copy of the plan document or summary plan description and confirm both features are available. Last updated: May 3, 2026.

2026 Contribution Limits and Headroom Math

For 2026, the IRS overall defined contribution limit under IRC § 415(c) is $70,000 (the dollar amount is indexed annually by Revenue Procedure). Workers age 50+ add a $7,500 catch-up to elective deferrals, raising the personal § 415(c) cap to $77,500. SECURE 2.0 § 109 added a "super catch-up" of $11,250 (instead of $7,500) for workers aged 60, 61, 62, and 63 — pushing the personal § 415(c) cap to $81,250. After-tax headroom is calculated as: § 415(c) personal limit minus elective deferrals minus employer match minus employer profit-sharing. The headroom can then be filled with after-tax contributions, subject to the lesser of headroom and 100% of compensation under § 401(a)(17).

Example: a 40-year-old with $200,000 salary maxes elective deferral at $23,500 and gets a 5% employer match ($10,000). Total to date: $33,500. § 415(c) headroom: $70,000 − $33,500 = $36,500. They can contribute up to $36,500 in after-tax 401(k) and immediately convert it to Roth — adding $36,500 of Roth basis on top of their existing $23,500 of Roth deferrals (if elected). Over 30 years at 7%, that single-year $36,500 grows to about $277,800 of tax-free Roth.

In-Plan Roth Rollover vs In-Service Distribution to Roth IRA

Two conversion mechanisms exist after the after-tax contribution lands. The first is an in-plan Roth rollover (IPRR) under IRS Notice 2014-54: the employee directs the plan to move the after-tax amount and any earnings to the Roth 401(k) source. The basis (after-tax principal) converts tax-free; any earnings accrued before conversion are taxed as ordinary income at conversion. The second is an in-service distribution: the plan rolls the after-tax sub-account directly to an outside Roth IRA, again with basis converting tax-free and earnings taxed. The key to either method is converting immediately — daily or weekly automated conversions prevent earnings from accruing inside the after-tax bucket and keep the conversion entirely tax-free.

Choose IPRR if you want to keep the money in the 401(k) plan (loan availability, ERISA creditor protection, ability to delay RMDs while still working under § 401(a)(9)(C)). Choose in-service to a Roth IRA if you want a wider investment menu, no plan administrative fees, or to consolidate Roth assets. Both routes are equivalent for the basis-only conversion math — the choice is procedural.

Pro-Rata Rule Does NOT Apply (Different from Backdoor Roth IRA)

Unlike the regular Backdoor Roth IRA — which is plagued by the IRC § 408(d)(2) pro-rata rule that aggregates all traditional IRA balances when calculating the taxable portion of a conversion — the Mega Backdoor Roth uses the 401(k) plan's own basis-tracking rules under § 72(d). Because each money source (pre-tax deferral, Roth deferral, after-tax, employer match) is tracked separately within the plan, an after-tax contribution can be cleanly converted without dragging in pre-tax balances. This makes the Mega Backdoor Roth far cleaner than the IRA version for participants who hold large pre-tax IRA or rollover IRA balances elsewhere.

This calculator assumes immediate conversion (zero earnings inside the after-tax bucket). If you cannot convert immediately, the small amount of earnings that accrue between contribution and conversion will be taxable at ordinary rates, and you may receive two 1099-Rs for that conversion event. Confirm plan procedures with your recordkeeper (Fidelity, Vanguard, Schwab, Empower) before assuming "automatic" conversions.