Once you've decided how to contribute to your 401k (pre-tax, Roth, or after-tax), the next question is what to invest in. Your 401k plan gives you a menu of funds to choose from. This guide explains what each type is, how they differ, and how to think about picking one.

Start Here: The Big Picture

A “fund” is just a basket of investments bundled together. Instead of buying 500 individual stocks yourself, you buy one fund that holds all 500. Your 401k plan selects which funds to offer, and you choose how to allocate your money across them.

The most important things to understand about any fund are: what it holds (stocks, bonds, or both), how it's managed (does someone pick the investments, or does it follow a formula), and what it costs (the expense ratio).

Index Funds

An index fund tracks a specific market index by holding all (or most) of the securities in that index. Nobody is actively deciding what to buy or sell. The fund just mirrors the index.

Common indexes you'll see in 401k plans:

• S&P 500 - the 500 largest US companies. This is the most common index fund in 401k plans.
• Total Stock Market - all US publicly traded companies (large, mid, and small cap)
• Total International - companies outside the US
• Total Bond Market - US investment-grade bonds

Index funds are popular for a few reasons. They're cheap (low expense ratios, often 0.01% to 0.10%), they're diversified (you own hundreds or thousands of companies), and historically, most actively managed funds fail to beat their benchmark index over long periods.

Common 401k Index Funds

Target-Date Funds

A target-date fund (also called a lifecycle fund) is designed to be the only fund you need. You pick the fund with a year closest to when you plan to retire, and it handles everything else.

Early on, the fund holds mostly stocks for growth. As the target year approaches, it automatically shifts toward bonds for stability. This gradual shift is called a “glide path.”

For example, a “Target 2060” fund today might be 90% stocks and 10% bonds. By 2055, it might be 50/50. By 2065, it might be 30% stocks and 70% bonds.

Pros and Cons

If your plan offers target-date funds from Vanguard or Fidelity (Freedom Index series), they tend to have lower expense ratios because they use index funds internally. Target-date funds from other providers sometimes use actively managed funds underneath, which raises the cost.

Actively Managed Mutual Funds

An actively managed fund has a portfolio manager (or team) who decides what to buy and sell, trying to beat a benchmark index. You're paying for their expertise through a higher expense ratio.

The data on active management is not encouraging for most investors. Over a 15-year period, roughly 90% of actively managed large-cap US funds underperform the S&P 500 index. The fees compound over time: a fund charging 0.80% instead of 0.03% costs you tens of thousands of dollars over a career on a $100,000 balance.

That said, some 401k plans only offer actively managed options in certain asset classes (like international or small-cap). If that's the case, compare the expense ratios and pick the cheapest option that gives you the exposure you want.

ETFs vs Mutual Funds

You may have heard of ETFs (exchange-traded funds) like VOO, VTI, or SPY. ETFs and mutual funds can hold the exact same investments. The difference is how they trade:

In practice, this distinction mostly doesn't matter inside a 401k. Most 401k plans use mutual funds, and you generally can't buy ETFs directly. If you see VOO recommended online and your plan has VFIAX or FXAIX, those are the mutual fund equivalents. They track the same index and perform nearly identically.

Common ETF to Mutual Fund Equivalents

If you have side income from contract work, these same fund types apply to a solo 401(k) or SEP IRA. See the guide to retirement strategies for W2 employees with side income for how to set those up.

Company Stock (ESPP / Stock Fund)

Some employers offer a company stock fund in the 401k, or let you buy company stock at a discount through an Employee Stock Purchase Plan (ESPP). While buying company stock at a discount can be a good deal, holding a large portion of your retirement savings in a single company is risky.

Your salary already depends on your employer. If the company struggles, you could face layoffs and a drop in your retirement savings at the same time. A general guideline is to keep no more than 5-10% of your portfolio in any single stock, including your employer.

If your company offers discounted stock purchases, consider buying and then selling (after any required holding period) and reinvesting the proceeds into a diversified index fund.

Stable Value / Money Market Funds

These are the most conservative options in a 401k. They aim to preserve your principal and pay a small, steady return (currently around 4-5%). They're appropriate for money you need soon (within a few years) or as a small stabilizing allocation, but not for long-term retirement savings where you need growth to outpace inflation.

What About Expense Ratios?

The expense ratio is an annual fee expressed as a percentage of your balance. It's deducted automatically from the fund's returns. You never see a line item for it - it just reduces your performance.

The difference between 0.03% and 1.00% doesn't sound like much, but over 30 years it costs you tens of thousands of dollars. Always check the expense ratio before choosing a fund.

So, How Do You Choose?

For most people, a simple approach works best:

1. If you want simplicity: pick a target-date fund matching your expected retirement year. Make sure it's an index-based target-date fund if your plan offers one (look for “Index” in the name). You're done.
2. If you want lower fees and more control: build a simple portfolio with 2-3 index funds. A common starting point is a total US stock market fund (or S&P 500) plus a total international fund. Add a bond fund if you want less volatility.
3. Avoid high-expense actively managed funds unless there's no index alternative in your plan for a particular asset class. Check the expense ratio of every fund before investing.

The specific allocation between US, international, and bonds depends on your age, risk tolerance, and how many years you have until retirement. A common rule of thumb is to hold your age in bonds (e.g., 30% bonds at age 30), though many investors tilt more aggressively toward stocks when they're young.

One More Thing: Rebalancing

If you build your own portfolio from multiple funds, check it once a year and rebalance back to your target allocation. If stocks had a great year and your 80/20 stock/bond split is now 88/12, sell some stock fund and buy more bond fund to get back to 80/20. Many plans let you set up automatic rebalancing.

If you're in a target-date fund, this is handled for you automatically.