The mega backdoor Roth is one of the most powerful — and most misunderstood — moves available to people with access to a generous 401(k). While a regular Roth IRA caps contributions at a few thousand dollars a year, the mega backdoor Roth lets you pour far more after-tax money into a Roth account by first routing it through your workplace plan, then converting it to Roth. This calculator estimates exactly how much after-tax room you have under the annual additions limit, how much you can convert to a Roth IRA, and how much that money could grow tax-free until retirement.
The key idea is simple. A normal 401(k) has two buckets: your own pre-tax or Roth elective deferrals (capped at $23,500 in 2025) and your employer's match. But the IRS also sets a separate, much higher cap on the total annual additions to your account — $70,000 in 2025, excluding catch-up contributions. Whatever room is left between that $70,000 ceiling and your deferrals plus employer match can often be filled with after-tax contributions, which you then roll or convert into a Roth IRA. Once the money is in Roth, every dollar of future growth and every qualified withdrawal is 100% tax-free.
Not every plan offers this feature. You need a plan that (1) allows voluntary after-tax contributions and (2) permits either in-plan Roth conversions or a so-called "in-service rollover" of after-tax money. Check with your benefits office before assuming your plan qualifies — the math below assumes it does.
How to Use
- Enter your Elective Deferrals — the pre-tax and Roth dollars you already choose to defer from each paycheck. In 2025 this is usually up to $23,500 if you are under age 50.
- Add your Employer Contributions — the match and any nonelective contributions your company deposits on your behalf.
- Confirm the Annual Additions Limit. The default is the 2025 ceiling of $70,000; adjust it if you are using a later year or know your plan's specific limit.
- Type the Desired After-Tax Contribution — the extra after-tax money you would like to put in and convert to Roth.
- Set Years Until Withdrawal and Expected Annual Return to project the tax-free future value. These are optional; leave them at the defaults if you only want the contribution-room math.
The result card shows your after-tax contribution room (the gap between the $70,000 cap and what you and your employer already put in), the allowable after-tax contribution clamped to that room, any excess that would push you over the limit, the amount converted to Roth, and the projected tax-free Roth balance at withdrawal.
The Math Behind the Numbers
Three limits interact, and conflating them is the most common mistake:
- Elective deferral limit — $23,500 in 2025 (plus a $7,500 catch-up at age 50+). This governs only your own pre-tax and Roth salary deferrals.
- Annual additions limit (§415(c)) — $70,000 in 2025. This is the total of everything flowing into your account in a year: your deferrals, the employer match, and your after-tax contributions combined.
- Roth IRA direct contribution limit — only a few thousand dollars, and it phases out at high incomes. The mega backdoor Roth exists precisely to bypass this last limit.
The formula the calculator uses is:
After-tax room = Annual additions limit − Elective deferrals − Employer contributions
Then Allowable after-tax contribution = min(Desired after-tax, After-tax room), and the converted balance grows tax-free as:
Future Roth value = Converted amount × (1 + annual return)^years
Worked example: someone defers $23,500, receives a $7,000 employer match, and wants to add $30,000 after-tax. The used portion is $30,500, leaving $39,500 of room under the $70,000 ceiling. Their desired $30,000 fits entirely, so the full $30,000 converts to Roth. At a 7% return over 20 years, that $30,000 grows to about $116,090 — roughly $86,090 of completely tax-free earnings. If they instead tried to put that same $30,000 into a taxable brokerage account, a meaningful slice of those gains would be lost to dividends and capital-gains taxes every year.
Why After-Tax Money Is Different
After-tax 401(k) contributions are made with dollars on which you have already paid income tax. That is the whole point: you get no deduction today, but when you convert the after-tax balance to Roth, your already-taxed basis comes out tax-free, and only the (usually small) investment earnings on the after-tax portion are taxed at conversion. Because conversions are typically done frequently — ideally every quarter — the earnings inside the after-tax sub-account stay minimal, which is why the mega backdoor Roth is nearly tax-free in practice.
Two plan features make or break it. In-plan Roth conversion lets you convert after-tax dollars inside the plan to a Roth 401(k) balance. In-service withdrawal/rollover lets you roll the after-tax money directly into a Roth IRA while you are still working. The pro-rata rule complicates things if you also have pre-tax balances, but pure after-tax sub-accounts with regular conversions avoid most of that complexity.
Examples
Example 1 — The textbook mega backdoor. A software engineer defers $23,500 pre-tax, gets a $10,500 match, and contributes $30,000 after-tax. Used funds total $34,000, leaving $36,000 of room. The full $30,000 converts; projected at 7% for 25 years it becomes about $162,000 tax-free.
Example 2 — Room already used up. A worker defers $23,500 and receives a generous $46,500 employer contribution, hitting the full $70,000 ceiling. After-tax room is $0, so any desired after-tax contribution — say $20,000 — shows up as excess and cannot be done without breaching the limit.
Example 3 — A saver who wants to fill every dollar. With $23,500 deferred and a $5,000 match, the room is $41,500. Contributing the maximum after-tax amount of $41,500 pushes total additions to exactly $70,000. That is the full mega backdoor Roth in action, and at 6% over 30 years the converted balance compounds to roughly $238,000 tax-free.
FAQ
What is a mega backdoor Roth?
A mega backdoor Roth is a strategy that lets you contribute far more than the normal Roth IRA limit by first making after-tax contributions to your workplace 401(k), then converting that money to a Roth IRA or Roth 401(k). It relies on the gap between the $23,500 elective deferral limit and the much higher $70,000 total annual additions limit.
How much can I contribute with the mega backdoor Roth?
Your room is the annual additions limit ($70,000 in 2025) minus your elective deferrals and employer contributions. For someone deferring $23,500 with a $7,000 match, that leaves about $39,500 of after-tax room — far more than the standard Roth IRA limit.
Does the mega backdoor Roth have an income limit?
No. Unlike direct Roth IRA contributions, which phase out at higher incomes, the mega backdoor Roth is not restricted by modified adjusted gross income. That is why it is especially valuable to high earners who cannot contribute to a Roth IRA directly.
Is the money taxed when I convert?
Your already-taxed after-tax basis converts tax-free. Only the investment earnings on the after-tax sub-account are taxable at conversion. Converting frequently (for example, quarterly) keeps those earnings tiny, so the conversion is nearly tax-free.
What plan features do I need?
Your 401(k) must allow voluntary after-tax contributions and either in-plan Roth conversions or in-service rollovers of after-tax money to a Roth IRA. Confirm both with your plan administrator — not every plan offers them.
When can I withdraw the money tax-free?
Qualified Roth withdrawals are tax-free once you are at least 59½ and the Roth account has been open for at least five years. Earnings that grew inside the converted Roth balance come out completely tax-free in retirement.
What happens if I contribute more than the room?
If your desired after-tax contribution exceeds the available room under the $70,000 annual additions limit, the excess is an over-contribution that must be corrected. This calculator flags that excess so you do not accidentally breach the cap.
Disclaimer
This mega backdoor Roth calculator is provided for informational purposes only and does not constitute financial, tax, or investment advice. Contribution limits, plan rules, and tax treatment change by year and by employer plan, and the pro-rata rule and conversion timing can affect your actual tax outcome. Projected future values are hypothetical and assume a constant return; results will vary. Always confirm your plan's specific features and consult a qualified tax professional before executing a mega backdoor Roth conversion.