Mega Backdoor Roth runs entirely in your browser using JavaScript (browser). Your data never leaves your device.
Free Mega Backdoor Roth Calculator
Estimate how much you could move into Roth via the Mega Backdoor Roth strategy — if your 401(k) plan allows after-tax contributions and in-plan Roth conversions. Enter your age, compensation, elective deferrals already made, and employer contributions to see the after-tax space available under the IRC §415(c) annual additions limit ($70,000 for 2025, $72,000 for 2026).
Use 2026 for current-year planning ($72,000 §415(c) limit); switch to 2025 for the year you contributed at the $70,000 limit.
Your 401(k) this year
2026 limitsDrives catch-up eligibility: +$7,500 at 50+, +$11,250 at ages 60–63 (SECURE 2.0).
Annual additions can never exceed your compensation (IRC §401(a)(17) caps the figure used at $350,000).
The 2026 elective deferral limit is $24,500 (plus catch-up if eligible).
Profit sharing or nonelective contributions, if any.
Try a scenario
After-tax space for Mega Backdoor Roth
$42,500
Estimated after-tax 401(k) contributions you could make this year and convert to Roth.
After-tax space for Mega Backdoor Roth
$42,500
⚠️ This number assumes your 401(k) plan supports the strategy. The Mega Backdoor Roth requires that your plan allows (a) after-tax contributions beyond the elective deferral limit, AND (b) in-service distributions or in-plan Roth conversions. Many plans allow one or neither. Read your Summary Plan Description, or ask your plan administrator, before counting on this space.
After-tax Roth space
$42,500
📐 Open methodology, sources & limitations
Formula
catch-up = $11,250 (age 60-63, both years)
| $7,500 (age 50-59, 2025) / $8,000 (2026)
| $0 (under 50)
IRC §415(c) annual additions ceiling = $70,000 (2025) / $72,000 (2026) + catch-up
Applied ceiling = min(§415(c) ceiling, W-2 compensation)
After-tax space = max(0, Applied ceiling
− elective deferrals already made
− employer match
− other employer contributions)Assumptions
- IRS retirement limits — 2025: elective deferral $23,500, §415(c) annual additions $70,000, age-50 catch-up $7,500 (IRS Notice 2024-80). 2026: $24,500, $72,000, $8,000 (IRS Notice 2025-67). Age 60–63 catch-up is $11,250 in both years.
- Catch-up contributions are modeled as raising both the elective deferral limit and the §415(c) annual additions ceiling for the eligible age.
- Annual additions can never exceed the participant’s compensation; the compensation figure is itself capped under IRC §401(a)(17) at $350,000 (2025) / $360,000 (2026).
- Assumes a single-employer plan — no aggregation of the §415(c) limit across multiple employers.
- Assumes the elective deferral entered is the full pre-tax + Roth 401(k) amount; after-tax contributions are what this tool sizes.
Sources
- IRC §415(c) — Limitation on annual additions to a defined contribution plan — §415(c)
- IRC §402(g) — Elective deferral limit — §402(g)
- IRC §401(a)(17) — Annual compensation limit — §401(a)(17)
- IRC §414(v) & SECURE 2.0 Act §109 — Catch-up contributions, incl. ages 60–63 — §414(v)
- IRS Notice 2024-80 — 2025 retirement plan cost-of-living adjustments
- IRS Notice 2025-67 — 2026 retirement plan cost-of-living adjustments
This tool does NOT model:
- Plan-specific restrictions — your plan may permit neither after-tax contributions nor in-plan Roth conversion
- Highly-compensated-employee (HCE) ADP/ACP nondiscrimination testing, which can force refunds
- Aggregation of the §415(c) limit across multiple unrelated employers
- The pro-rata rule on earnings that accrue on after-tax money before it is converted
- The difference in tax treatment between in-service distributions and in-plan Roth conversions
- Tax years other than 2025 and 2026 — limits adjust annually for inflation
Last reviewed: 2026-06-12 · Tax year modeled: 2025 & 2026 (selectable)
This methodology section exists so you can verify the math. We show our formulas because you deserve to know how a number was calculated. This is calculation transparency, not financial advice.
What changed for 2026
IRS Notice 2025-67 raised every limit that drives the mega backdoor Roth for 2026. The §415(c) annual additions ceiling — the number this whole strategy is built on — rose from $70,000 to $72,000, and the elective deferral limit rose from $23,500 to $24,500. Those two increases offset, so for someone maxing the deferral with the same employer match, the after-tax space grows by about $1,000. The age-50 catch-up rose to $8,000, while the SECURE 2.0 enhanced catch-up for ages 60–63 stays at $11,250. The §401(a)(17) compensation cap rose to $360,000. Pick the year in the calculator above to size your space with the right limits.
| Limit | 2025 | 2026 |
|---|---|---|
| §415(c) annual additions ceiling | $70,000 | $72,000 |
| Elective deferral limit (§402(g)) | $23,500 | $24,500 |
| Catch-up, age 50+ (§414(v)) | $7,500 | $8,000 |
| Enhanced catch-up, ages 60–63 | $11,250 | $11,250 |
| Compensation cap (§401(a)(17)) | $350,000 | $360,000 |
What the Mega Backdoor Roth actually is
Most people know the 401(k) elective deferral limit — $23,500 for 2025, $24,500 for 2026 — as the cap on what they can put into a 401(k). It isn't. It's only the cap on elective deferrals (your pre-tax and Roth contributions). The real ceiling on total contributions to the plan is the IRC § 415(c) annual additions limit: $70,000 for 2025, $72,000 for 2026, counting your deferrals, all employer money, and after-tax contributions together.
The gap between those two numbers — § 415(c) minus your deferrals minus employer contributions — is space that can be filled with after-tax 401(k) contributions. The “Mega Backdoor Roth” is the move of filling that space and then promptly converting the after-tax money to Roth, either through an in-plan Roth conversion or an in-service distribution to a Roth IRA. Done well, it can move tens of thousands of dollars per year into Roth treatment that the income limits on direct Roth IRA contributions would otherwise block. (Whether Roth treatment is even the right call for you is its own question — the Roth vs Traditional calculator runs that comparison.)
The two things your plan must allow
This is the part no calculator can answer for you. The strategy only works if your 401(k) plan document permits both of the following:
- After-tax contributions beyond the elective deferral limit (a distinct contribution type — not pre-tax, not Roth deferral).
- In-service distributions or in-plan Roth conversions so the after-tax money can move to Roth before it accumulates much taxable earnings.
Many plans allow one and not the other; many allow neither. Your Summary Plan Description (SPD) is the authority. Call your plan administrator and ask specifically about “after-tax contributions” and “in-plan Roth conversions” — these are precise terms, and a vague yes is not enough.
Getting the mechanics right
- Max your regular deferral first. The after-tax space is what's left after your $23,500 (2025) or $24,500 (2026) deferral and all employer contributions. Front-loading the regular deferral does not change the total but keeps the buckets clear.
- Convert as soon as the plan allows. After-tax contributions grow, and any earnings on them are taxable when converted (the pro-rata rule). Converting immediately — ideally with automatic in-plan conversion — keeps the taxable earnings near zero.
- Watch the compensation cap. Annual additions can never exceed your compensation, and the compensation figure is itself capped at $350,000 (2025) / $360,000 (2026). Low earners are limited by pay, not by § 415(c).
- Mind HCE testing. If you are a highly-compensated employee, after-tax contributions are subject to ACP nondiscrimination testing. A plan that fails testing can refund part of your contribution.
- This is a strategy estimate, not a recommendation. Whether the Mega Backdoor Roth fits your situation depends on your full tax picture, cash flow, and your plan's rules. Confirm the mechanics with your plan administrator and a CPA before relying on the number.
Frequently Asked Questions
What is the Mega Backdoor Roth?+
It is contributing after-tax dollars to a 401(k) beyond the elective deferral limit ($23,500 for 2025, $24,500 for 2026), then converting that money to Roth. The total is bounded by the IRC §415(c) annual additions limit — $70,000 for 2025, $72,000 for 2026.
How do I know if my 401(k) plan allows it?+
Read your Summary Plan Description, or ask your plan administrator two specific questions: does the plan accept after-tax contributions, and does it allow in-plan Roth conversions or in-service distributions. The strategy needs both. No calculator can determine this — it is plan-specific.
What are the 2025 and 2026 limits?+
The IRC §415(c) total annual additions limit is $70,000 for 2025 ($77,500 with the age-50 catch-up, $81,250 at ages 60–63) and $72,000 for 2026 ($80,000 with the age-50 catch-up, $83,250 at ages 60–63), per IRS Notice 2025-67. After-tax space is that ceiling minus your elective deferrals and all employer contributions.
How is this different from a regular Backdoor Roth?+
A regular Backdoor Roth moves up to the IRA limit ($7,000 for 2025, $7,500 for 2026) through a nondeductible IRA contribution and conversion. The Mega Backdoor Roth works inside a 401(k) and can move far more — often $30,000–$45,000 — because it uses the much larger §415(c) ceiling.
Does the pro-rata rule apply?+
Yes, to earnings. After-tax contributions themselves convert tax-free, but any investment earnings on them before conversion are taxable at conversion. Converting promptly — ideally automatically — keeps those earnings, and the tax, near zero.
When should I convert?+
As soon as the plan allows. The longer after-tax money sits before conversion, the more taxable earnings accumulate. Some plans offer automatic in-plan conversion of after-tax contributions, which is ideal.
What if my compensation is above the limit?+
Annual additions are capped at your compensation, and the compensation figure used is itself capped under IRC §401(a)(17) — $350,000 for 2025, $360,000 for 2026. High earners are limited by §415(c) ($70,000 / $72,000), not by pay; lower earners can be limited by pay.
Does this work with a Solo 401(k)?+
Some Solo 401(k) plans can be structured to allow after-tax contributions and Roth conversions, but many off-the-shelf Solo 401(k) plan documents do not. It depends entirely on the specific plan document. This calculator models a standard employer plan.
What is the deadline?+
After-tax contributions are payroll contributions, so they must generally be made through payroll by the end of the calendar year (or the plan’s contribution deadline). They cannot be made after year-end the way IRA contributions can.
Are there state tax implications?+
Converting after-tax contributions to Roth is generally not a federally taxable event (earnings aside). Most states follow the federal treatment of Roth conversions, but a few differ. This tool models the federal §415(c) mechanics only.
You might also like
Browse all 8 Retirement & Investing tools →401(k) Calculator with Employer Match
Project your 401(k) balance with employer match, IRS limits, catch-up contributions, and Traditional vs Roth tax modeling
Roth vs Traditional IRA Calculator
Should you contribute to a Roth or Traditional IRA? Fair side-account comparison with LTCG drag
RSU Tax Calculator
See the RSU under-withholding gap — supplemental withholding vs your true marginal rate, plus FICA, Medicare, and state