What is the mega backdoor Roth?
The mega backdoor Roth is an advanced retirement savings strategy that lets you contribute after-tax dollars to your 401(k) and then convert them to a Roth account, either within the plan (in-plan Roth conversion) or by rolling them out to a Roth IRA. This allows contributions far beyond the normal \$7,000 annual Roth IRA limit.
The strategy takes advantage of the gap between the employee deferral limit (\$23,500 in 2025) and the total 401(k) plan limit (\$70,000 in 2025). That gap, minus employer contributions, can be filled with after-tax contributions that you then convert to Roth.
How the mega backdoor Roth works
**Three-step process:**
1. **Make after-tax 401(k) contributions** — contribute to your 401(k) above the pre-tax or Roth contribution limit, using after-tax (non-deductible) dollars 2. **Convert or roll over to Roth** — if your plan allows in-service withdrawals or in-plan Roth conversions, convert the after-tax balance immediately to Roth 3. **Tax-free growth** — the converted funds now grow tax-free, and qualified withdrawals in retirement are tax-free
The key requirement is that your 401(k) plan must allow: (a) after-tax contributions, and (b) either in-service withdrawals or in-plan Roth conversions. Not all plans support this. Check your Summary Plan Description or ask your HR department.
2025 and 2024 contribution limits
The IRS sets annual limits that determine how much after-tax space is available:
- **2025:** Employee deferral limit \$23,500 (\$31,000 if 50+), total plan limit \$70,000 (\$77,500 if 50+) - **2024:** Employee deferral limit \$23,000 (\$30,500 if 50+), total plan limit \$69,000 (\$76,500 if 50+)
**After-tax space = Total limit minus Employee deferrals minus Employer contributions**
For example, if you contribute \$23,500 in 2025 and your employer adds \$10,000, you have up to \$36,500 in after-tax contribution space (\$70,000 minus \$23,500 minus \$10,000).
Tax advantage over taxable investing
When you convert after-tax 401(k) contributions to Roth, those funds grow tax-free. The same annual investment in a taxable brokerage account would face capital gains taxes on all growth, typically 15% or 20% for long-term gains.
Over decades, this difference compounds significantly. A \$36,500 annual Roth contribution over 25 years at 7% annual return would grow to approximately \$2.4 million, all tax-free. The same contribution in a taxable account, assuming 15% capital gains tax on gains, would net roughly \$2.1 million after taxes. The Roth advantage in this scenario is over \$300,000.
Risks and limitations
- **Plan must support it:** Many employer 401(k) plans do not allow after-tax contributions or in-service withdrawals. This strategy is not available at all employers. - **Pro-rata rule:** If you have pre-tax funds in the same after-tax account, conversions may be partially taxable. Converting immediately after contributing minimizes this risk. - **Income does not matter:** Unlike the Backdoor Roth IRA, the mega backdoor Roth has no income limits. High earners who are phased out of direct Roth IRA contributions can still use this strategy. - **Not the same as backdoor Roth IRA:** The backdoor Roth IRA is a separate strategy using traditional IRA non-deductible contributions. The mega backdoor Roth works through your employer 401(k).
How to use this calculator
1. Enter your age and select the tax year 2. Enter your planned employee 401(k) contributions (pre-tax or Roth) 3. Enter your employer match or profit sharing contribution 4. Set your years to retirement and expected annual return 5. See your available after-tax space and projected Roth balance at retirement
FAQs
Q: How do I know if my 401(k) plan supports the mega backdoor Roth? A: Check your plan's Summary Plan Description (SPD) for "after-tax contributions" and "in-service distributions" or "in-plan Roth rollovers." You can also ask your HR department or plan administrator directly.
Q: Can high earners use the mega backdoor Roth? A: Yes. Unlike Roth IRA direct contributions (which phase out above approximately \$161,000 for single filers in 2024), the mega backdoor Roth has no income limits. It is especially valuable for high earners who are ineligible for direct Roth IRA contributions.
Q: What is the difference between the backdoor Roth IRA and the mega backdoor Roth? A: The backdoor Roth IRA involves making a non-deductible traditional IRA contribution and converting it to Roth, limited to \$7,000 per year. The mega backdoor Roth uses after-tax 401(k) contributions and is limited only by the total plan limit minus employee and employer contributions, potentially \$40,000 or more per year.
Q: Do I pay taxes when converting after-tax 401(k) contributions to Roth? A: The after-tax contributions themselves are not taxable again since you already paid tax on that money. However, any earnings on those contributions between when you contributed and when you converted are taxable. Converting immediately after contributing minimizes this tax exposure.
Q: Does converting to Roth count as income? A: Only the taxable portion (earnings) counts as ordinary income. If you convert shortly after contributing, the earnings are minimal and the taxable amount is small.