A 45-year-old software engineer earning $250,000 walks into open enrollment having already done the obvious things. The full $24,500 pre-tax deferral is going in. The employer match (assumed here at $11,250, or roughly 4.5% of salary) is captured. A direct Roth IRA is off the table because single-filer income blew past the $153,000 to $168,000 phase-out years ago. That is where most high earners stop. It is also where the biggest legal Roth funding lever in the U.S. tax code is sitting unused on their plan document.
The Number Most High Earners Miss: $72,000
The 401(k) employee deferral cap gets all the press. The IRS Section 415(c) cap, which governs total annual additions to a defined contribution plan, gets almost none. For 2026 the 415(c) limit is $72,000 for participants under 50, and it includes employee deferrals, employer match, and any after-tax employee contributions stacked on top.
Run the subtraction in the scenario above. Start at $72,000. Take out the $24,500 pre-tax deferral. Take out the $11,250 employer match. What is left is $36,250 of unused space inside the plan. If the plan document permits after-tax employee contributions and in-service Roth conversions, that $36,250 can be funded with after-tax dollars and immediately rolled to the Roth side of the 401(k) or to a Roth IRA. The earnings then grow tax-free forever.
That is the mega backdoor Roth, and the binding constraint sits with the employer’s plan administrator, who must turn both switches on.
Why This Matters More in 2026 Than It Did Last Year
Two things changed the calculus this year. First, SECURE 2.0’s high-earner catch-up rule is now live. Starting in 2026, employees 50 and older who earned more than $150,000 in 2025 must deposit any catch-up contributions into a Roth 401(k) rather than the pre-tax bucket. Our 45-year-old is not there yet, but the rule signals the direction of policy: Washington is steering high earners into Roth whether they like it or not.
Second, the household savings cushion has thinned. The personal savings rate fell to 3.7% in the first quarter of 2026, down from 5.2% a year earlier, while per capita disposable income rose to $68,359. High earners with the cash flow to fill the after-tax bucket have a narrowing window before lifestyle creep eats the capacity.
The Tax Math Over 20 Years
Funding $36,250 per year of after-tax contributions and converting promptly to Roth means roughly $725,000 of principal moved into a tax-free bucket between age 45 and 65. At a 7% compounded return, the same contributions grow to a balance well into seven figures, none of which generates a 1099 in retirement, counts toward the IRMAA Medicare surcharge calculation, or pushes Social Security benefits into the 85% taxable zone.
Compare that to leaving the $36,250 in a taxable brokerage account. Every dividend is taxed annually. Every rebalance triggers capital gains. At withdrawal, the basis is recovered tax-free but every dollar of growth is taxed at long-term capital gains rates, and the income still counts toward IRMAA thresholds. The Roth version sidesteps all of it.
The friction point is the conversion timing. After-tax contributions that sit too long before conversion accrue earnings, and those earnings are taxable on conversion. A plan that supports daily or per-paycheck automatic in-plan Roth conversions eliminates the problem. A plan that requires a manual quarterly call to the recordkeeper does not.
Three Actions Worth Taking This Week
- Pull the summary plan description and search for two phrases: “after-tax contributions” and “in-service conversion” or “in-plan Roth rollover.” Both must exist. Roth in-plan conversions were offered by 36% of plans in 2024, with 10% offering automatic conversion of after-tax contributions, so coverage is real but not universal.
- Confirm your match and 415(c) math with HR. Verify the actual employer contribution dollars for the year, subtract them and your $24,500 deferral from $72,000, and set the after-tax payroll election to the remainder.
- Set conversions to automatic if the plan allows it. If it does not, calendar a quarterly conversion request to minimize taxable earnings on the after-tax balance before it moves to Roth.
The mega backdoor Roth is a plan-design feature already sitting inside most large-employer 401(k)s. The only question is whether you funded the $36,250 of space your employer already gave you.
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