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Mega Backdoor Roth

The mega backdoor Roth is an advanced strategy for high earners to contribute over $100,000 per year to a Roth account by making after-tax contributions to a 401(k) plan, then converting those contributions to a Roth IRA.

For the simpler backdoor Roth strategy, see backdoor Roth; for Roth conversion strategies, see Roth conversion; for 401(k) contribution mechanics, see 401(k) plan.

How it works

A standard 401(k) plan has a combined annual contribution limit of roughly $69,000 (2024) across employee deferrals, employer match, and employer nonelective contributions. Most people max out at $30,500 ($23,500 employee + ~$6,000 match).

But the law allows after-tax contributions beyond this limit, up to the $69,000 total. The mega backdoor Roth exploits this by:

  1. Making after-tax contributions to your 401(k) beyond the standard limits.
  2. Converting those after-tax contributions to a Roth account (either in-plan Roth or external Roth IRA).
  3. The earnings on those after-tax contributions may or may not convert, depending on plan rules.

For example: you defer $23,500, get $7,000 in match (total $30,500). Your plan’s limit is $69,000. You then contribute $38,500 in after-tax money, hitting the $69,000 limit. You immediately convert that $38,500 to your Roth IRA, paying no taxes (the contribution was after-tax; no earnings yet).

The mechanics in detail

Step 1: Check plan rules. Not all 401(k) plans allow after-tax contributions or in-plan conversions. You must confirm your plan document allows both.

Step 2: Make after-tax contributions. Some plans allow this via payroll; others via lump-sum contributions. Your HR or plan administrator can guide you.

Step 3: Convert immediately. Convert the after-tax balance to Roth (either in-plan or roll to external Roth IRA) as soon as contributions are received. The faster the conversion, the less earnings accumulate (which would complicate taxes).

Step 4: Manage the tax impact. If the after-tax contributions generate earnings before conversion, those earnings are taxable. To minimize this, convert immediately.

The pro-rata rule complication

If you have a traditional IRA with pre-tax money, the pro-rata rule can make a mega backdoor Roth inefficient. When converting after-tax 401(k) money to Roth, the IRS considers your total traditional and after-tax IRA balances. If you have $100,000 in a traditional IRA and convert $40,000 of after-tax 401(k) money, the pro-rata rule treats 71% of the conversion as pre-tax (and thus taxable).

Solutions:

  • Roll traditional IRA to 401(k). If your employer plan allows, this removes the traditional IRA from the pro-rata calculation.
  • Time conversions carefully. Perform mega backdoor conversions in years when you have minimal traditional IRA balance.

Who should consider it

  • High earners above Roth IRA income limits. If your income is too high to contribute directly to a Roth, this strategy gets around that limit.
  • People with high savings rates. If you have $50,000+ per year to save beyond retirement account limits, a mega backdoor Roth is a tax-efficient home.
  • Those with 30+ year time horizon. The longer the Roth grows tax-free, the more valuable.

Risks and limitations

Plan elimination. Employers can change or eliminate plans, though mega backdoor provisions are relatively stable.

Earnings tax. If the after-tax contributions sit in the account for months before conversion, earnings accumulate and are taxable.

Pro-rata rule. If you have a traditional IRA, the pro-rata rule can significantly reduce the tax benefit.

Complexity. This strategy requires understanding 401(k) mechanics, conversions, and tax implications. Mistakes can be costly.

See also

Wider context