By David L. Dunn, CPA/PFS, CFP®, CPWA®

Are you maxing out your 401k pre-tax contributions, firmly convinced you’ve captured every tax advantage your 401k has to offer? Your 401k may allow contributions far beyond that limit, and a little-known strategy could make every extra dollar grow tax-free.

You might be one of the few executives who max out their 401k pre-tax limit every year, but still contribute far less than your plan allows, and leave a significant tax advantage completely untouched.

Your 401k may allow you to contribute beyond the pre-tax limit, and those extra dollars go in as after-tax contributions. But what happens next determines everything.

Are those after-tax contributions being automatically converted to your Roth 401k before they generate taxable earnings? If not, you’re opening the door to unnecessary taxes, and failing to capture a high-leverage strategy for executives who’ve outgrown traditional Roth options.

The Mistake

Many executives who make this mistake are doing everything they’ve been told to do. They just don’t know there’s a next level, and that not reaching it could cost them hundreds of thousands in tax-free retirement wealth.

This mistake is deceptively simple: two steps already available inside your 401k that few executives know to take.

Step one: Making 401k after-tax contributions beyond the IRS pre-tax limit.

Step two: Automatically converting those after-tax contributions to a Roth 401k.

These two steps together make up what’s known as the Mega Backdoor Roth strategy.

This article covers 401k future contributions only. If after-tax dollars are already sitting in your 401k unconverted, read or watch 401k Critical Mistake #2

What’s at Stake

The cost of this mistake is invisible, until it isn’t. Every year it goes unaddressed is another year of tax-free growth that could have been captured.

Consequence 1: Missed Opportunity to Hyper-Fund

Here’s the first cost: stopping at the pre-tax limit, never knowing your plan may allow you to contribute significantly more. And every dollar you don’t contribute is a dollar that could have grown tax-free.

To understand how much more you can contribute, let’s start where many executives stop. These are the IRS pre-tax limits for 2026:

$24,500 if you’re under 50

$32,500 if you’re 50 to 59, or 64 or older: that’s the $24,500 base plus an $8,000 catch-up contribution

$35,750 if you’re 60 through 63: that’s the $24,500 base plus an $11,250 catch-up contribution

New for 2026: if your individual compensation exceeded $150,000 in 2025, those catch-up contributions must be made as Roth contributions.

But the IRS sets a second limit, one far higher than the pre-tax limit, and many executives don’t know it exists. For 2026, the total 401k contribution limit, including pre-tax, after-tax, and employer contributions, is:

$72,000 if you’re under 50

$80,000 if you’re 50 to 59, or 64 or older

$83,250 if you’re 60 through 63

That means, depending on your employer’s contribution, you could contribute up to $47,500 in after-tax dollars.

But here’s what changes everything: when those after-tax dollars are converted to a Roth 401k, the growth could be tax-free when distributed.

For executives who are phased out of Roth IRA eligibility, this is one of the few doors still open to build tax-free retirement wealth.

Consequence 2: The Tax Trap

Here’s what it costs when an executive takes step one–making 401k after-tax contributions above the pre-tax limit, and never knows step two exists.

Meet Jason, an executive at a leading tech firm. Over the years, he contributed $100,000 in after-tax dollars to his 401k. But he made one mistake: he never converted those contributions to a Roth 401k.

The investments grew. $200,000 in gains, sitting right alongside his original $100,000 contributions.

Jason reaches age 60, retires, and begins taking withdrawals.

The $100,000 in after-tax contributions? He can withdraw it tax-free.

The $200,000 in growth? Fully taxable as ordinary income.

At a 37% tax rate, that’s a $74,000 tax bill. Completely avoidable. But without step two, never avoided.

These figures are illustrative. Additional federal and state taxes may apply depending on your situation.

This is the hidden cost of inaction. Without converting those 401k after-tax contributions, the growth becomes taxable, turning a well-intentioned strategy into a mounting tax liability.

The Strategy: 3 Steps to Get It Right

Reaching the 401k pre-tax contribution limit can feel like the finish line. It isn’t.

Three steps. That’s all it takes.

Step 1: Once you reach the 401k pre-tax limit, keep going by making after-tax contributions. To estimate your maximum after-tax contribution amount, subtract your employer’s contribution from $47,500. Then contribute as much as you can, because every extra dollar you contribute is a dollar that could grow tax-free.

Step 2: If your 401k plan allows it, opt into daily automatic In-plan Roth conversions. This automatically converts your 401k after-tax contributions to your Roth 401k. This minimizes the time earnings can build up, reducing or even eliminating the tax triggered on each conversion.

Step 3: This strategy is powerful. But it’s also complex. The cost of getting this wrong can wipe out the advantage you’re trying to capture. A qualified advisor can make sure you get it right.

Two Warnings

Two warnings stand between this strategy and a costly mistake.

Warning #1: If you already have after-tax contributions sitting in your 401k, consider this action first: move those after-tax contributions to a new Roth IRA and the earnings on those after-tax contributions to a new Traditional IRA. Doing that before starting daily automatic In-plan Roth conversions in your 401k could eliminate taxes on future conversions.

Warning #2: If your 401k plan doesn’t allow daily automatic In-plan Roth conversions, don’t manually convert your after-tax contributions to the Roth 401k because you could trigger taxes you weren’t expecting. Instead, periodically transfer your 401k after-tax contributions to a new Roth IRA and the earnings on your after-tax contributions to a new Traditional IRA.

I cover both these scenarios in 401k Critical Mistake #2.

The Bottom Line

Making 401k after-tax contributions without Roth conversion is half a strategy, and half a strategy is a compounding tax problem waiting to happen. If you qualify, and it fits your situation, hyper-funding your 401k with the Mega Backdoor Roth could be one of the most powerful wealth-building opportunities hiding inside your 401k.

Don’t Miss the Next Critical Mistake

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If you found this article helpful, please share it with a colleague who takes their 401k seriously. It could save them tens or hundreds of thousands.

If you have after-tax contributions sitting unconverted in your 401k, you may be making 401k Critical Mistake #2 right now. That mistake covers the tax exposure building inside your 401k, and the cost of not knowing keeps growing. Read or watch it next.

Disclosures

David Dunn Wealth LLC (DDW) is a member firm of The Fiduciary Alliance LLC which is a Securities and Exchange Commission-registered investment adviser. See full disclosure HERE.