The Most Underused Feature in Your 401(k)

You may already have access to one of the most powerful tax-free savings tools available to a W-2 employee. Most people who do never fully use it. Here's what it is, what it's costing you, and what to do about it.

If you work at Google, Intuit, Apple, Salesforce, Stripe, or a number of other major tech employers, there's a good chance your 401(k) offers something called voluntary after-tax contributions.

You may have seen it when you set up your 401(k) elections, and probably get periodic company emails about it. But if you're like most people with access to this feature, one or more of the following is probably true:

• You're not contributing the maximum amount available every year

• You're contributing but not immediately converting the money to tax-free Roth

• You may understand the mechanics in theory but haven't connected them to what this actually means for your financial future over ten or twenty years

This post is here to help you close that gap.

What are Voluntary After-tax contributions?

Your 401(k) has two separate contribution limits that most people treat as one.

The first is the employee elective deferral limit — $24,500 in 2026. This is the limit on your pre-tax 401(k) or Roth 401(k) contributions that come directly out of your paycheck. This is you telling the company to direct some of your pay into your 401(k) plan into one or both of those categories of retirement savings.

The second is the overall defined contribution limit — $72,000 in 2026. This covers everything going into the plan: your elective deferrals, your employer's match, and — if your plan allows it — voluntary after-tax contributions you make on top of the standard limit.

Voluntary after-tax contributions fill the gap between those two numbers. After you've maxed your $24,500 elective deferrals and your employer has contributed their match, there may be $30,000 or more of remaining space in that $72,000 overall limit. Voluntary after-tax contributions let you fill that gap.

The contributions go in after tax — with no upfront deduction, just like a Roth IRA. But unlike a Roth IRA, there's no income limit and the contribution ceiling is much higher.

The step many people skip — and why you shouldn’t

Here's the problem.

Contributing voluntary after-tax dollars to your 401(k) and then leaving them there as after-tax contributions is almost never the right move. The earnings on those contributions will grow and eventually be taxable as ordinary income when you withdraw them.

The step that makes voluntary after-tax contributions genuinely powerful is converting them promptly to Roth — either through an in-plan Roth conversion (keeping them in the 401(k) but in the Roth bucket) or via an in-service distribution to your Roth IRA (moving them out of the employer plan entirely while you're still employed).

Once converted to Roth, those dollars grow tax-free. No taxes on the gains. No Required Minimum Distributions on your Roth IRA. No income tax bill when you withdraw them in retirement.

This combination — voluntary after-tax contributions plus immediate conversion to Roth — is what's commonly called the Mega Backdoor Roth. The "mega" is because the amounts dwarf what you can move through the regular Backdoor Roth strategy in your IRA. A standard Backdoor Roth in 2026 moves $7,500 per year (plus another $1,100 if aged 50+). The Mega Backdoor Roth can move $30,000, $35,000 or more — depending on your employer match and how much of the overall limit remains available.

Most people with access to this feature are doing one of three things wrong: not contributing the after-tax dollars at all, contributing them but not converting, or converting inconsistently. Each of those is leaving money on the table — not in a theoretical sense, but in a compounding, decades-long, very real sense.

What does it actually cost you to ignore this?

Let's make it concrete.

As an example, say you have $35,000 of available after-tax contribution room each year. You contribute it and convert to Roth immediately. Assuming 7.5% annual growth over 20 years, that $35,000 per year becomes over $1.5 million — all of it tax-free.

Now say you contribute the same $35,000 each year but leave it in the after-tax bucket without converting. Over 20 years, you will have contributed $700,000 in after-tax contributions and the additional approximately $815,000 of growth will be fully taxable. Painful! Or say you simply don't bother contributing it at all — the money sits in your bank account or a taxable brokerage where interest and/or dividends hit your tax return each year and any realized gains are subject to capital gains tax every year.

The difference isn't small. It's the difference between building a very substantial tax-free pool of wealth, or not. For a tech employee with 15 or 20 working years ahead of them, the compounding gap between a fully utilized Mega Backdoor Roth and an ignored one can be massive.

The other thing worth understanding: you're already paying California state income tax on your RSU income, your bonus, and your salary at rates that are among the highest in the country. The Mega Backdoor Roth doesn't reduce today's tax bill — but it does ensure that the money you save from here forward grows in an environment where the IRS and the California FTB have no further claim on it.

Does your plan actually allow it?

Most major tech employers offer plans that permit voluntary after-tax contributions and in-plan Roth conversions or in-service distributions. But plan designs vary, and the only way to know for certain is to check.

Three ways to verify:

Review your Summary Plan Description (SPD). This is the governing document for your 401(k). Ask HR for a copy if you don't have one. Look for language about "voluntary after-tax contributions" and "in-plan Roth conversions" or "in-service distributions."

Check your benefits portal. Most large tech employers surface this in their online HR systems. Search for "after-tax" in the contribution elections section — if you can set a contribution percentage for "after-tax" separately from your pre-tax or Roth elections, your plan likely supports it.

Ask your plan administrator directly. "Does our plan allow for voluntary after-tax contributions and in-plan Roth conversions?"

The two conversion methods

If your plan permits this, you'll typically have two options for moving the after-tax contributions into Roth:

In-plan Roth conversion — the after-tax dollars stay inside the 401(k) but are moved into the Roth 401(k) bucket. That works if you’re happy with the plans investment fund line-up.

In-service distribution to a Roth IRA — the after-tax dollars leave the 401(k) entirely and roll directly into your Roth IRA while you're still employed. Many clients prefer this when available due to a broader suite of investment options and especially if they have their standard 401(k) investment elections set to a target date retirement fund, but want their tax-free money invested more aggressively for a higher expected long-term return.

One critical timing point: convert in-plan or directly rollover the after-tax contributions as quickly as possible after making them to really “mega” your Mega Backdoor Roth.

A note on the 1099-R you'll receive

When you do a Mega Backdoor Roth conversion or in-service distribution, your plan administrator will issue a Form 1099-R. This can look alarming — it shows a large gross distribution amount. But because you're converting after-tax contributions, the taxable amount in Box 2a is zero, or very close to it.

Make sure your CPA or tax preparer knows this transaction occurred and understands how to report it correctly. Done right, there's no tax due and no penalty. Done wrong — or handed to a preparer who isn't familiar with it — it can look like a taxable distribution. It isn't, but as with all tax-related items, a little clear communication with your tax preparer goes a long way, especially during their busy tax prep season.

How this fits into your bigger picture

The Mega Backdoor Roth is most valuable when it's part of a coordinated approach.

For tech employees specifically, a few things are worth thinking through together:

Your RSU vesting creates ongoing tax impacts. Every year, RSUs vest and are taxed as ordinary income. You can't defer that. What you can control is where your retirement savings accumulate. Pushing more dollars into Roth — through the Backdoor Roth and the Mega Backdoor Roth — gives you a growing tax-free pool that helps balance the tax-deferred weight of your regular 401(k) and the taxable nature of company stock you decide to retain.

The tax-free bucket is the hardest to fill. Your taxable brokerage is easy to add to. Your pre-tax 401(k) fills automatically with every paycheck. The Roth bucket — particularly for high earners above the direct Roth IRA income limit — is harder to build unless pursued without clear intention.

Inertia is expensive. The most common reason people with access to this feature don't maximize it isn't necessarily ignorance — it's that changing your 401(k) contribution elections feels like a small administrative task that keeps getting pushed off. Over time though it isn't small. It's one of the highest-value financial decisions available to you. If you haven't revisited your after-tax contribution elections recently, don’t put it off.

If you work at Alphabet, Intuit, Apple, Salesforce, or another major tech employer and you're not sure whether you're maximizing your plan's after-tax contribution features — or if you know you're not and want to understand what it would actually mean for your financial picture — get in touch. This is one of the first conversations we have with almost every tech employee client we work with.