Mega-Backdoor Roth Calculator

You make after-tax (non-Roth) contributions to your 401(k), then convert them to a Roth IRA or Roth 401(k) — turning ~$30–50k/yr of additional space into tax-free retirement growth. The math is governed by IRC §415(c) (the $70k overall cap) and IRS Notice 2014-54 (the basis-isolation rule for the conversion).

The Backdoor Roth IRA is the $7,000/yr ($8k if 50+) Traditional-IRA-to-Roth-IRA workaround for high earners. The Mega-Backdoor uses your employer 401(k) and unlocks an order of magnitude more space ($30–46k/yr typically). They are independent — you can do both in the same year.

Read your Summary Plan Description (SPD), check your 401(k) provider portal under "Contribution Sources" (look for "After-Tax" alongside "Pre-Tax" and "Roth"), or ask your HR/benefits team directly. Common employers that allow it: Google, Microsoft, Meta, Netflix, Amazon (varies by group), Salesforce, most large tech. Many smaller employers do not. About 50% of plans offer after-tax per Vanguard's How America Saves 2024 report — but only a subset of those also allow conversion.

Two mechanisms to move after-tax money to Roth: (a) in-service distribution — pulling the after-tax balance out of your 401(k) while still employed, rolling it to a Roth IRA. (b) In-plan Roth Rollover (IRR) — converting the after-tax sub-account to a Roth 401(k) sub-account without leaving the plan. Either one works. Without one of them, your after-tax money grows tax-deferred (not tax-free), which is much worse than just contributing to a regular brokerage.

Convert immediately. If you let after-tax money grow before converting, the growth becomes taxable upon conversion (treated as pre-tax earnings under IRC §72). Most plans support automated periodic conversion (sometimes called "auto-conversion" or "Roth in-plan conversion every pay period") — turn it on if available. Otherwise convert manually after each contribution.

It always does. The $70k §415(c) limit includes: your elective deferral (pre-tax + Roth 401(k) combined, capped at $23,500), your employer match, employer non-elective / profit-sharing contributions, and your after-tax contributions. The calculator subtracts everything except the after-tax to give you the available room.

The catch-up contribution ($7,500 in 2025, or $11,250 super-catch-up if 60–63 under SECURE 2.0) is itself "elective deferral" that counts against your personal $23,500 limit BUT the §415 overall limit ($70k) is also raised by the catch-up amount. Net result: 50+ filers have roughly the same Mega-Backdoor room as under-50, with slightly different elective-deferral capacity.

The pro-rata rule (IRC §72(d)) only matters when you have BOTH pre-tax AND after-tax money in the same account at the time of a partial conversion. The Mega-Backdoor flow specifically isolates after-tax via the basis-isolation rule in IRS Notice 2014-54 — when you convert/distribute the after-tax sub-account separately, no pro-rata. The pro-rata rule does affect the Backdoor Roth IRA (different calculator), so do not confuse the two.

Yes — via a Solo 401(k) that allows after-tax contributions, but only specific Solo 401(k) providers offer this (Schwab and most banks DO NOT; specialty providers like My Solo 401k Financial, Carry, and a few others DO). Plan setup is more complex than a regular Solo 401(k) — the plan document needs explicit after-tax and Roth-conversion language.

Tax-free at withdrawal, assuming you follow the Roth qualification rules: (1) Roth IRA / Roth 401(k) must be open for 5+ years, (2) you must be 59½ or qualify under an exception. The "5-year clock" is per-account for Roth IRAs and per-conversion for Roth 401(k)s — be careful if you plan early withdrawal. The Mega-Backdoor principal can be withdrawn anytime (Roth contributions are always accessible); only the growth is locked until 59½.

No — it is an estimate based on published IRS rules and your inputs. Plan-specific rules vary (some plans have lower internal caps, ERISA non-discrimination testing limits, top-heavy plan restrictions). For your specific situation, talk to a licensed tax professional or a fee-only fiduciary financial planner.