The Mega Backdoor Roth: How After-Tax 401(k) Contributions Can Supercharge Retirement Savings

Most people who contribute to a 401(k) think of two contribution types: pre-tax traditional and Roth, both subject to the same annual employee elective-deferral limit set by the IRS. But a subset of 401(k) plans allow a third type of contribution, after-tax, that sits outside that deferral limit and opens the door to a strategy commonly called the mega backdoor Roth.

How the Strategy Works, Step by Step

The mega backdoor Roth relies on the fact that the IRS caps 401(k) savings in two separate ways. First, there is an annual limit on employee elective deferrals (the pre-tax or Roth dollars withheld from a paycheck). Second, there is a much larger overall limit under Internal Revenue Code Section 415(c) that caps the combined total of employee deferrals, employer matching or profit-sharing contributions, and after-tax employee contributions. According to the IRS, for 2024 the employee elective-deferral limit was $23,000, while the overall Section 415(c) limit was $69,000 ($76,500 including catch-up contributions for those 50 and older). For 2025, the IRS raised the elective-deferral limit to $23,500 and the overall limit to $70,000, as detailed in IRS retirement plan contribution limit guidance published on IRS.gov.

That gap between the elective-deferral limit and the overall Section 415(c) limit is the space the mega backdoor Roth strategy occupies. If a plan permits after-tax contributions, an employee who has already maxed out pre-tax or Roth deferrals can contribute additional after-tax dollars, up to the point where total contributions from all sources hit the overall limit. Those after-tax dollars can then be moved into a Roth account, either through an in-plan Roth conversion within the 401(k) or, if the plan allows in-service withdrawals, rolled directly into a Roth IRA.

Why Plan Design Is the Gatekeeper

Not every 401(k) plan offers this feature. Employers are not required to allow after-tax contributions, and even when they do, they are not required to allow in-plan conversions or in-service withdrawals that make the Roth conversion step possible. Without a conversion mechanism, after-tax contributions simply sit in the plan and their future earnings will be taxed as ordinary income when withdrawn, which defeats much of the purpose. Savers who want to use this strategy need to check their plan's summary plan description or ask their plan administrator about three specific features.

• Whether the plan allows after-tax (non-Roth, non-deductible) employee contributions beyond the standard deferral limit.
• Whether the plan allows in-plan Roth conversions, in-service distributions, or automatic "Roth in-plan conversion" sweeps of after-tax balances.
• Whether any employer matching formula or profit-sharing contribution reduces the amount of room left under the overall Section 415(c) limit.

The Tax Mechanics That Make Timing Matter

After-tax contributions are made with money that has already been taxed, so the contribution itself is not taxed again on conversion. However, any investment earnings that accumulate on after-tax contributions before they are converted or rolled over are treated as taxable income at conversion. For this reason, many savers who use the mega backdoor Roth try to convert or roll over after-tax contributions as quickly as possible, often through automatic periodic conversions built into the plan, to minimize the taxable earnings component. Plans differ in how frequently conversions can occur, ranging from daily automatic sweeps to periodic manual elections.

Who Tends to Benefit

This strategy is most relevant for higher earners who have already maxed out their standard 401(k) elective deferrals and have additional cash flow available to save, and whose employer plan happens to support the necessary features. It is less useful for savers who have not yet maxed out ordinary contributions, since standard pre-tax or Roth deferrals, along with any employer match, are typically a more straightforward first priority. It is also irrelevant for savers in plans that do not permit after-tax contributions or in-plan conversions.

Key Limits to Verify Each Year

Because the IRS adjusts contribution limits annually for inflation, the specific dollar figures governing this strategy change from year to year. Savers should confirm the current elective-deferral limit, the overall Section 415(c) limit, and any age-based catch-up contribution amounts directly on IRS.gov before calculating how much after-tax room they have available in a given plan year.