Tax strategy · Advanced

The mega backdoor Roth:
an extra $47,500 of Roth space.

READ4 min·UPDATED·APPLIES TOUnited States·LEVELAdvanced
For high-income savers whose 401(k) allows after-tax contributions. Sources are listed at the bottom of the page; check their dates before acting.

If your 401(k) allows after-tax contributions and in-plan Roth conversions (or in-service rollouts), you can contribute tens of thousands more to a Roth account each year beyond the standard limits. This page covers exactly how it works and how to check whether your plan allows it.

This page covers US-specific accounts and tax law. Outside the US? The priority order is the same, the account names differ (ISAIndividual Savings Account (ISA)A UK tax-advantaged account where contributions are post-tax but all growth and withdrawals are tax-free.Full definition in the UK, TFSATax-Free Savings Account (TFSA)A Canadian tax-advantaged account where contributions are post-tax but all growth and withdrawals are tax-free.Full definition/RRSPRegistered Retirement Savings Plan (RRSP)A Canadian tax-deferred retirement account; contributions reduce taxable income and growth is tax-deferred until withdrawal.Full definition in Canada, Super in Australia, etc.).

Tax-year figures. Limits, brackets and thresholds on this page are for the tax year named next to them. They change every year.

READ FIRST

The mega backdoor Roth depends on your plan and how you carry out the steps. Convert promptly to avoid taxable growth. A CPA familiar with this strategy should review your first-year execution. Not financial or tax advice.

Short answer

Standard Roth IRAIndividual Retirement Account (IRA)A retirement account with tax advantages and eligibility rules. Traditional IRA contributions may be deductible; taxable withdrawals generally enter income. Roth contributions use after-tax money, and qualified Roth withdrawals are tax-free.Full definition limit 2026: $7,500. Mega backdoor Roth additional space: up to $47,500 more in after-tax 401(k) contributions that can be converted to Roth inside the plan or rolled to a Roth IRA. Total potential Roth contribution: $55,000 a year, plus any Roth 401(k) deferral. Plan-dependent. Most 401(k) plans do not allow it. Some do, especially at larger tech employers and in Solo 401(k) plans.

On this page
  1. The math
  2. The two requirements
  3. Step by step
  4. The Solo 401(k) advantage
  5. What can go wrong

The math

  • 2026 401(k) total contribution limit (all sources, IRC 415(c)): $72,000 ×DON'T TRUST, VERIFYClaim: 2026 IRC 415(c) total annual additions limit is $72,000 (2025 was $70,000), per IRS Notice 2025-67.Verify at: IRS Notice 2025-67 ↗ Source 1 in the list belowIndexed annually.
  • 2026 employee pre-tax / Roth deferral limit (402(g)): $24,500
  • Employer matchemployer matchAn employer contribution linked to qualifying employee contributions under a plan formula. Eligibility, vesting and plan terms determine what you receive and keep. The match ratio is not an annual investment return, and the invested balance can fall.Full definition: varies by plan

The gap between $24,500 and $72,000 (minus employer contributions) can be filled with after-tax contributions. That gap is potentially $47,500 before employer match, often $35,000 to $45,000 after.

After-tax contributions grow tax-deferred. If your plan allows conversion, those contributions can be converted to Roth immediately, which makes their growth tax-free forever. That is the mega backdoor Roth.

Where $47,500/yr more Roth space comes from
401(k)$72,000/yr, all sources
$24,500/yrUp to $47,500/yr after-tax
Roth IRA$7,500/yr, for comparison
$0$72,000/yr
$24,500/yr employee deferral, pre-tax or Roth$47,500/yr after-tax space before employer match$7,500/yr Roth IRA limit
Employer contributions also count toward the $72,000, so the after-tax space is often $35,000 to $45,000 a year.
If the plan allows both steps
After-tax contributionGrows tax-deferred; any gain is taxed on conversion
Convert to RothIn the plan, or roll out to a Roth IRA while still employed
Roth moneyGrowth tax-free forever
In 2026 the $24,500 deferral leaves up to $47,500 a year of the $72,000 401(k) limit for after-tax money, far more than the $7,500 Roth IRA limit. Converted in the plan or rolled to a Roth IRA promptly, it grows tax-free.

The two requirements

Your 401(k) plan must allow BOTH of these:

  1. After-tax contributions beyond the standard $24,500 deferral limit. Not the same as Roth 401(k) contributions.
  2. In-plan Roth conversionRoth conversionMoving eligible retirement money into a Roth account. Pre-tax amounts and earnings generally create taxable income; nondeductible basis is treated under the applicable aggregation rules. Conversions can affect tax brackets and income-based costs, and separate withdrawal rules apply.Full definition (convert after-tax money to Roth within the plan), OR in-service withdrawals of after-tax contributions (roll them out to a Roth IRA while still employed).

Many plans allow one but not both. Both are required for the strategy to work.

How to check

  • Read your Summary Plan Description (SPD) for the after-tax contribution section.
  • Call HR and ask directly: "Does our plan allow after-tax contributions beyond the standard employee deferral limit? Does it allow in-plan Roth conversions of after-tax contributions? Does it allow in-service distributions of after-tax contributions?"

Step by step

  1. Confirm your plan allows it. Do not assume.
  2. Elect after-tax contributions. In your plan portal, add an after-tax contribution percentage or dollar amount. This is separate from your pre-tax or Roth deferral election. Money comes from your paycheck after income tax.
  3. Convert to Roth promptly. As soon as each paycheck's after-tax contribution posts, convert it to Roth inside the plan. Some plans auto-convert (the ideal setup). Others require manual conversion each paycheck. Convert often to minimize taxable growth.
  4. If plan allows in-service rollout instead: roll the after-tax contributions directly to your Roth IRA. Earnings on those contributions go to a Traditional IRA to avoid immediate taxation. This split rollover is allowed under Notice 2014-54 ×DON'T TRUST, VERIFYClaim: IRS Notice 2014-54 allows splitting a distribution of after-tax contributions and their earnings into a Roth IRA and Traditional IRA respectively.Verify at: IRS Notice 2014-54 ↗ Source 2 in the list belowAuthoritative guidance enabling clean mega backdoor Roth execution..

The Solo 401(k) advantage

Solo 401(k) plans can be set up to allow mega backdoor Roth contributions from the start. If you are self-employed, open a Solo 401(k) at a provider that supports this feature. Not all do. Fidelity's standard Solo 401(k) has historically been conservative on this front; specialty providers like MySolo401k or Carry offer fuller mega backdoor support.

For a high-income self-employed person: employee deferral $24,500, employer contribution up to 20% of net SE income after half of SE tax, after-tax contribution fills the gap up to the $72,000 total limit. See Solo 401(k) for the base mechanics.

What can go wrong

The taxable-growth trap.

If the after-tax contribution earns investment gain before conversion, that gain is taxable income on conversion. Convert same day or same week.

Plan disallowance.

Many plans explicitly prohibit after-tax contributions or in-service distributions. Check before starting contributions you cannot convert.

IRS scrutiny and first-year execution.

This is a legal strategy explicitly addressed by Notice 2014-54 but execution matters. A CPA familiar with mega backdoor Roth should review your first year.

Where to go from here

Do next

Asset Location

Once the extra Roth space is filled, this decides which accounts hold which assets.

Read next

Backdoor Roth How-To

The simpler version for readers whose 401(k) does not allow the mega version.

Explore further

Sources

Verify badges in the text link to these sources. Figures, rates and rules change: check each source’s date before acting on it.

  1. Internal Revenue Service, IRS Notice 2025-67 ↩
  2. IRS Notice 2014-54 · irs.gov ↩
  3. IRS retirement plan limits · irs.gov

Last updated . Not financial or tax advice. Disclosures · How pages are reviewed

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