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.
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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.
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.
Frequently Asked Questions
What is the 2026 § 415(c) overall 401(k) contribution limit?
For 2026 the IRS overall defined contribution limit under IRC § 415(c) is $70,000 — combining employee elective deferrals, employer match, employer profit-sharing, and after-tax employee contributions. Workers age 50+ add a $7,500 catch-up (raising the personal cap to $77,500) and SECURE 2.0 § 109 lets workers age 60-63 use a $11,250 super catch-up (raising the cap to $81,250). The $70,000 base is indexed annually; verify the exact 2026 figure with the IRS Cost of Living Adjustment Revenue Procedure.
How does the Mega Backdoor Roth differ from the regular Backdoor Roth IRA?
The regular Backdoor Roth IRA uses non-deductible IRA contributions converted to Roth, but is hampered by IRC § 408(d)(2) pro-rata aggregation across all traditional IRA balances. The Mega Backdoor Roth uses 401(k) after-tax contributions converted via in-plan Roth rollover (IRS Notice 2014-54) or in-service distribution. Because the 401(k) tracks money sources separately under § 72(d), there is no pro-rata problem — clean basis-only conversions are possible regardless of pre-tax IRA balances elsewhere.
What is an in-plan Roth rollover (IPRR)?
An in-plan Roth rollover under IRS Notice 2014-54 is a transaction where the participant directs the 401(k) plan to convert pre-tax or after-tax money into the designated Roth account within the same plan. The basis (after-tax principal) converts tax-free; any earnings convert taxable as ordinary income. The conversion is reported on Form 1099-R with code G (in-plan rollover) and code H (after-tax to Roth). Must be performed immediately after each after-tax contribution to keep earnings near zero.
Does my plan support the Mega Backdoor Roth?
About half of large-company 401(k) plans support both required features: (1) after-tax (non-Roth) contribution source, and (2) either in-plan Roth rollover or in-service distribution. Big Tech companies (Microsoft, Google, Meta, Amazon since 2024, Salesforce) are well-known for supporting it. Request the plan document or summary plan description from HR or the recordkeeper. If only after-tax contributions are allowed without conversion, the strategy is much weaker because earnings remain taxable.
How much extra Roth space does this strategy create?
For someone earning $200,000 with a 5% match maxing the $23,500 elective deferral, after-tax headroom in 2026 is $70,000 − $23,500 − $10,000 = $36,500 of additional Roth principal. Over 30 years at 7% growth, a single-year $36,500 contribution becomes ~$278,000 tax-free. Doing it for 10 years builds roughly $1.5 million of tax-free Roth on top of standard 401(k) and IRA Roth contributions.
Are after-tax 401(k) contributions the same as Roth deferrals?
No. After-tax (non-Roth) contributions and designated Roth deferrals are different money sources within the same plan. Designated Roth deferrals count against the $23,500 § 402(g) elective deferral cap; after-tax contributions count only against the § 415(c) overall limit. Earnings on after-tax contributions are taxable at distribution unless converted to Roth — earnings on Roth deferrals are already tax-free if held 5+ years. The Mega Backdoor strategy targets after-tax money and converts it to Roth.