Your 401k plan can accept up to three different types of contributions. They all come out of your paycheck and they all go into your 401k account, but they're taxed completely differently. Most people only use one. Understanding all three lets you make a much more informed decision about where your money goes.

All Three Types at a Glance

Pre-Tax (Traditional) Contributions

Pre-tax contributions come out of your paycheck before income tax is calculated. If you earn $100,000 and contribute $10,000 pre-tax, the IRS sees $90,000 of taxable income that year. You pay less in taxes now.

The tradeoff: when you withdraw in retirement, every dollar (your original contributions and all the investment growth) is taxed as ordinary income. If your account grows from $500,000 to $2,000,000 over your career, the entire $2,000,000 is taxable when you take it out.

Pre-tax makes sense when you expect your tax rate in retirement to be lower than it is today. This is the case for many people. Retirement income is often lower than peak working income, which means a lower marginal tax bracket.

Roth Contributions

Roth contributions come out of your paycheck after income tax. You don't get a tax break today, so your taxable income stays the same. But in exchange, qualified withdrawals in retirement are completely tax-free. Your contributions, your growth, all of it.

Roth also has a unique advantage: you can withdraw your contributions (not earnings) at any time without taxes or penalties. Pre-tax money is locked until age 59½ with limited exceptions.

Roth makes sense when you expect your tax rate to stay the same or increase. Early-career employees often benefit most: if you're at a lower tax bracket now and expect promotions and raises to push you higher, paying taxes now at the lower rate is a good deal.

Pre-Tax vs Roth: Let's Compare

Say you have $10,000 of pre-tax income to contribute, you're in the 24% bracket today, and you expect to be in the 24% bracket in retirement. Assume 7% growth over 25 years.

At the same tax rate, you keep the same amount either way. The math is symmetric: paying 24% now or 24% later on the same growth rate yields the same result. The decision comes down to whether you think your future rate will be higher or lower.

But there's a subtlety: with Roth, you can contribute the full $24,500.00 limit. With pre-tax, you also contribute the full limit, but the government has a claim on those dollars. Roth lets you effectively shelter more money because the limit is on the contribution amount, not the pre-tax equivalent.

After-Tax Contributions

After-tax contributions are the least understood type, and most 401k plans don't even offer them. Like Roth, they come from after-income-tax dollars. But unlike Roth, the earnings on after-tax contributions grow tax-deferred, not tax-free. You'll owe ordinary income tax on those earnings when you withdraw.

So why would anyone use them? Because after-tax contributions can be converted to Roth. When you do an in-plan Roth conversion (or roll over to a Roth IRA), the contributed amount moves to Roth tax-free, and from that point forward, all growth is tax-free too. This is the mega backdoor Roth strategy.

After-tax contributions don't share the $24,500.00 elective limit with pre-tax and Roth. Instead, they count toward the per-employer 415(c) limit of $72,000.00, which also includes your elective contributions and employer match. The gap between what you and your employer contribute and the $72K cap is your after-tax space.

What About Employer Match?

Your employer match is always pre-tax, no matter what type of contribution you make. If you contribute Roth and your employer matches 4%, that match goes into a separate pre-tax bucket in your 401k. You'll owe income tax on the match and its earnings when you withdraw in retirement.

This is important: switching from pre-tax to Roth does not affect your match. Your employer matches based on how much you contribute as a percentage of your pay, not what type of contribution it is. If your employer matches 100% of the first 4%, they match 4% whether you contribute pre-tax or Roth.

How Do I Invest Once the Money Is In?

All three contribution types go into the same 401k account and are invested the same way. Your plan offers a menu of funds - typically index funds, target-date funds, and sometimes individual stocks. You choose how to invest, and that allocation usually applies to all your money in the plan.

If your plan offers a total stock market index fund or an S&P 500 index fund (like a Fidelity 500 Index Fund, which tracks the same thing as VOO), that's a popular default choice. The key insight is that the investment choice (what fund you pick) is separate from the contribution type (pre-tax, Roth, or after-tax). You can invest in the same index fund regardless of which bucket your money goes into.

One thing to note: you generally can't buy ETFs like VOO directly inside a 401k. Instead, look for the equivalent mutual fund in your plan's fund menu. For Fidelity plans, this would be something like FXAIX (Fidelity 500 Index Fund), which tracks the same S&P 500 index as VOO. For a deeper dive on the different fund types, ETF-to-mutual-fund equivalents, and how to choose, see the guide to 401k fund types.

Our Simple Decision Framework

The Most Important Thing: Get the Match

Regardless of which type you choose, the single most impactful thing you can do is contribute enough to get your full employer match. If your employer matches 100% of the first 4%, that's an instant 100% return on your money. No investment in the world consistently beats that.

Whether you contribute that 4% as pre-tax or Roth is a secondary decision. Either way, you're capturing free money from your employer. Don't leave it on the table.

Beyond the Basics

Once you're contributing enough to get your full match, the natural next steps are:

1. Max out your elective contributions: try to reach the full $24,500.00 across pre-tax and/or Roth
2. Fund a Roth IRA via the backdoor Roth IRA if your income is too high for direct contributions
3. Explore the mega backdoor Roth: if your plan supports after-tax contributions with in-plan conversion, you can contribute up to the full $72,000.00 415(c) limit. Read the complete mega backdoor Roth guide