Mega Backdoor Roth Contributions: Powerful Opportunity or Testing Trap?

What Is a Mega Backdoor Roth? 

Most retirement plan participants are familiar with 401(k) deferrals, both pre-tax and Roth, and the standard (combined) limit. Participants can contribute up to the limit annually, plus any applicable catch-up contributions. 

For 2026, subject to any specific plan limitations, 401(k) participants can elect to put away $24,500*, plus: 

• $8,000 (age 50+ catch-up contribution), or 
• $11,250 (age 60-63 catch-up contribution) 

(*Deferral limits change each year. Click here for the most recent limits.) 

Above, beyond, and including the annual 401(k) deferral limit, there is a lesser- known limit, called the Annual Additions Limit; that dollar figure is the maximum that can be allocated to a participant's account each year from all sources combined. For 2026, that amount is $72,000. 

Generally, the space between the $24,500 employee deferral figure mentioned above and the $72,000 Annual Additions Limit is for employer contributions, including employer matching, Safe Harbor, and profit-sharing contributions. However, there is one exception when participants can contribute if their Plan allows: voluntary after-tax contributions. 

Depending upon how much their employer has contributed, participants may contribute additional after-tax dollars, up to the Annual Additions limit. They can then convert those contributions to Roth status, if the plan allows for in-plan Roth conversions. 

This has become known as a "Mega Backdoor Roth": “Mega”, because the amount that can be saved through this strategy is much higher than in a similar IRA strategy. “Backdoor”, due to conversion from after-tax contributions to Roth. 

An Example 

Suppose one year a participant puts away $24,500 in 401(k) deferrals and receives $4,500 in employer matching contributions, totaling $29,000 in retirement savings for the year. 

Under the Total Defined Contribution limit rules, they would still be able to put away an additional $43,000 in after-tax contributions before reaching the $72,000 overall limit. Those after-tax contributions can then be converted to Roth via in-plan Roth conversions, allowing future earnings to potentially grow tax-free. 

Why Roth Contributions Are So Attractive 

The appeal is straightforward: Once assets are successfully converted to Roth status, future qualified distributions—including investment earnings—generally can be received tax-free. 

For participants who expect substantial investment growth over many years, the long-term tax savings can be significant. 

Many high-income earners are especially interested in Roth accumulation because they may already be maximizing their regular 401(k) contributions and are looking for additional savings opportunities. 

However, there is a critical issue that needs to be considered: after-tax employee contributions are subject to Actual Contribution Percentage (“ACP”) testing requirements. 

This is where many participants encounter problems. 

The Testing Trap 

The ACP test is one of nondiscrimination, designed to ensure that highly compensated employees (HCEs) do not receive disproportionate benefits compared to non-highly compensated employees (NHCEs).401(k) plans with Safe Harbor provisions generally eliminate the need for non-discrimination tests for deferral and matching contributions. However, Safe Harbor provisions do not automatically exempt voluntary after-tax employee contributions from ACP testing.  

This is important to keep in mind, especially as interest in Mega Backdoor Roth strategies continues to grow. So, while this strategy can be highly effective for certain plan participants in the right circumstances, their after-tax contributions remain subject to ACP testing at the Plan Sponsor level. 

Why Smaller Plans Often Struggle 

This is where theory and reality often diverge. 

The employees most interested in making large after-tax contributions are frequently those that are considered HCEs. For 2026, an HCE is defined as making more than $160,000 in the prior year (2025), and/or is more than a 5% owner. (Just as with annual plan limits, the annual salary changes from year-to-year.) 

In a typical small or mid-sized business, the owner(s)/executives are often HCEs, and also are interested in making large, after-tax contributions. “Rank-and-file" employees are less likely to do so. When annual ACP testing is performed, comparing contribution rates between HCEs and NHCEs, and the HCEs are making substantial after-tax contributions while NHCEs are not, there is a good chance the plan will fail the ACP test. When that happens, excess after-tax contributions may need to be refunded to employees. 

The result can be frustrating, for both the participants - who discover that their contributions could not remain in the plan - and for the plan sponsor, who must issue the refunds. 

This generalization is highly dependent on employee demographics and plan design, of course. For example, in a small engineering firm, where all (or most) employees are making over $160,000 per year and contributing above and beyond the maximum, advanced plan design options exist that can help the testing be less likely to fail. 

As a note, all of the scenarios that we’ve listed depend upon using a Third-Party Administrator fluent in Advanced Plan Design scenarios, and who works proactively to make sure that the plan stays in compliance. 

Why Large Companies Often Have Better Results 

Mega Backdoor Roth strategies tend to work more effectively in large employers, for several reasons: 

• Special rules can be employed with advanced plan design to manage the number of employees who are considered HCEs 
• Larger companies may have more employees with higher salaries who are interested in saving additional amounts 
• Employer matching contributions may be more substantial 
• Varying workforce structures can result in broader groups of employees participating at higher contribution levels 

All of these conditions create more favorable testing demographics. Larger employers often have a broader mix of employees contributing at different levels to the plan, which can improve ACP testing results. Even if only a small percentage of employees makes after-tax contributions, a larger workforce creates more opportunities for NHCEs to participate, helping balance the testing between HCEs and NHCEs. 

What Employers Should Consider 

Employers occasionally receive requests from participants to add Mega Backdoor Roth capabilities to their retirement plan. Before making plan design changes to enable this, however, employers should discuss the following with their advisor or third-party administrator: 

• Whether the plan demographics are likely to support ACP testing 
• The administrative complexity involved 
• The potential for participant refunds if testing fails 
• Whether plan amendments are needed to support after-tax contributions and Roth conversions 

Adding the option for Mega Backdoor Roth Contributions may make sense in some plans, but not all. 

The Bottom Line 

A Mega Backdoor Roth can be an excellent retirement savings tool for those participants who have already maximized their traditional 401(k) contributions and want additional savings opportunities. 

However, the strategy is not as simple as it may appear. Plan design and workforce demographics need to support its viability.