You buy index funds through a brokerage account, the same way you'd buy individual stocks

To invest in an index fund, you open an account at a brokerage firm, deposit money, search for the fund by its ticker symbol, and place a buy order. The fund then holds a basket of stocks or bonds that track a market index — like the S&P 500 or the total stock market — so you own a piece of hundreds or thousands of companies with a single purchase. The whole process takes minutes once your account is set up.

The real work is choosing which brokerage to use and which index fund to buy. Different brokerages charge different fees, offer different funds, and have different minimum deposits. Index funds themselves vary by what they track, their expense ratios (the annual cost to own them), and whether they're mutual funds or exchange-traded funds (ETFs). Both types track an index, but they trade and work slightly differently.

Key Takeaways

  • You need a brokerage account to buy index funds; common brokerages include Fidelity, Vanguard, Charles Schwab, and Robinhood, and most have no minimum deposit or trading fees.
  • Index funds come as mutual funds (priced once daily) or ETFs (traded throughout the day like stocks), and both track the same indexes but have different tax and trading mechanics.
  • An expense ratio is the annual percentage fee the fund charges; lower is better, and ratios for index funds typically range from 0.03% to 0.20% per year.
  • You can buy index funds inside a regular taxable brokerage account or inside a retirement account like a 401(k) or IRA, and the account type affects how taxes work.
  • Once you buy an index fund, you own it until you sell it; you don't have to do anything else, though many investors add money regularly over time.

Choosing a brokerage and opening an account

A brokerage is a company that lets you buy and sell investments. You'll need to pick one, create an account, and link a bank account to deposit money. Most major brokerages — Fidelity, Vanguard, Charles Schwab, E-Trade, and Robinhood — have no account minimums, no monthly fees, and no trading commissions. The differences are usually in the user interface, customer service, and which index funds they offer at the lowest cost.

When you open an account, you'll choose between a taxable brokerage account (money you invest is taxed as you earn gains) or a retirement account like a traditional IRA, Roth IRA, or 401(k) through your employer. Retirement accounts have tax advantages but come with rules about when you can withdraw money without penalty. Most people start with a taxable account if they're investing outside of retirement savings, or use their employer 401(k) if one is available.

The application takes 10 to 15 minutes. You'll provide your name, Social Security number, address, and employment information. Once approved — usually instantly — you can link your bank account and transfer money in. Most transfers take one to three business days to appear in your brokerage account.

Mutual funds versus ETFs: which type to buy

Both mutual funds and ETFs track an index, but they work differently. A mutual fund is priced once per day, after the market closes. You place an order during the day, but the price you pay is determined at 4 p.m. Eastern time. An ETF trades throughout the day like a stock, so the price changes every minute the market is open, and you see the exact price before you buy.

For most people starting out, the difference doesn't matter much. ETFs are slightly more tax-efficient in taxable accounts because of how they're structured, and they're easier to buy in small dollar amounts. Mutual funds require a minimum initial investment at some brokerages (often $1,000 to $3,000), though that minimum is waived if you set up automatic monthly contributions. If you're investing small amounts regularly, an ETF removes that friction.

The real difference is in cost. Look at the expense ratio — the percentage you pay annually to own the fund. For a broad market index fund, expense ratios typically range from 0.03% to 0.20% per year. On a $10,000 investment, that's $3 to $20 per year. Vanguard, Fidelity, and Schwab all offer index funds with expense ratios below 0.10%, so shop around but don't overthink it; the difference between 0.05% and 0.10% is negligible.

Finding and buying a specific index fund

Once your account is funded, log into your brokerage and look for a search or "buy" function. You'll search by the fund's ticker symbol — a short code like SPY, VOO, or VTI for S&P 500 funds, or BND or VTI for bond funds. The brokerage will show you the fund's current price, expense ratio, and a brief description of what it tracks.

Click the fund, enter the dollar amount or number of shares you want to buy, and review the order. For mutual funds, you'll see a note that the price will be set at the market close. For ETFs, you'll see the current market price. Once you confirm, the order is placed. If you're buying a mutual fund during the trading day, the order executes at that day's closing price. If you're buying an ETF, it executes immediately at the price shown.

After the order settles (usually one to two business days), the fund appears in your account holdings. You now own a fractional share of every stock or bond in that index. You don't receive certificates or statements in the mail; everything is digital and visible in your account dashboard.

Understanding expense ratios and why they matter over time

An expense ratio is a percentage of your investment that the fund company charges each year to manage the fund and cover administrative costs. It's deducted automatically from the fund's value, so you don't write a check — you just see slightly lower returns. For index funds, which are passively managed (they simply hold the stocks in the index), expense ratios are very low, usually between 0.03% and 0.20% annually.

The difference compounds over decades. If you invest $10,000 in a fund with a 0.05% expense ratio versus one with a 0.20% ratio, and both earn 7% per year, the lower-cost fund will have roughly $1,000 more after 30 years. That's not a reason to obsess — all index funds are cheap compared to actively managed funds — but it's a reason to avoid funds with expense ratios above 0.30% if you're building a long-term portfolio.

Investing in a retirement account versus a taxable account

If you're investing through an employer 401(k), your brokerage choices are limited to what your employer's plan offers. Most 401(k) plans include at least one S&P 500 index fund and a total market index fund. You contribute pre-tax money (reducing your taxable income that year), and you don't pay taxes on gains until you withdraw in retirement. You can contribute up to a set limit each year; for 2024, that limit is $23,500 for people under 50.

An IRA (individual retirement account) is for people without an employer plan or who want to save additional retirement money. A traditional IRA works like a 401(k): contributions may be tax-deductible, and you pay taxes on withdrawals in retirement. A Roth IRA is the opposite: you contribute after-tax money, but withdrawals in retirement are tax-free. You can contribute up to $7,000 per year (for 2024) to an IRA, regardless of type.

A taxable brokerage account has no contribution limits and no withdrawal restrictions, but you pay capital gains tax on profits when you sell. For most people, maxing out a 401(k) or IRA first makes sense because of the tax advantages, then investing additional money in a taxable account. But if you're just starting and don't have much to invest, a taxable account is simpler and has no rules.

Setting up automatic investments and staying the course

Many investors set up automatic monthly transfers from their bank account to their brokerage, then buy the same index fund each month. This is called dollar-cost averaging, and it removes the pressure of trying to time the market. Whether the market is up or down that month, you buy the same dollar amount, which over time smooths out the ups and downs.

Most brokerages let you automate this with a few clicks. You set a transfer amount and frequency (weekly, monthly, quarterly), and the money moves automatically. Then you can set up an automatic purchase of your chosen index fund on the same day the money arrives. After that, you don't have to think about it. The fund buys more shares when prices are low and fewer when prices are high, which is exactly what you want.

The hardest part of index fund investing isn't buying — it's not selling during a market downturn. Index funds go up and down, sometimes sharply. If you panic and sell during a drop, you lock in losses and miss the recovery. If you leave the money alone and keep buying, you benefit from the long-term growth of the market. That discipline is worth more than picking the perfect fund.

Frequently Asked Questions

Can I buy an index fund with a small amount of money?

Yes. Most brokerages have no minimum deposit, and ETFs let you buy fractional shares starting at $1. Mutual funds sometimes require a $1,000 to $3,000 minimum initial investment, but that minimum is often waived if you set up automatic monthly contributions of $50 or $100. Start with whatever you can afford.

Do I need to pick different index funds or just buy one?

One broad market index fund is enough for a complete portfolio. A fund tracking the total U.S. stock market (like VTI or VTSAX) gives you exposure to thousands of companies. If you want international stocks or bonds, you can add a second fund, but many investors stick with a single total market fund for simplicity.

What happens if the company running the index fund goes out of business?

Your shares don't disappear. The fund would be sold to another company or merged into a similar fund. Your investment is protected by law; the fund company is a custodian, not the owner of your money. Your shares and their value remain yours.

Can I lose money investing in an index fund?

Yes, in the short term. If the market drops 20%, your index fund drops 20%. But historically, the stock market has recovered from every downturn and reached new highs. If you're investing for 10 years or longer, the odds of a loss are very low. If you need the money in the next few years, index funds are not the right place for it.

Should I buy the same index fund at multiple brokerages?

No. Pick one brokerage and buy there. Spreading money across multiple brokerages makes tracking and rebalancing harder without any real benefit. The only reason to use multiple brokerages is if one goes out of business, which is extremely rare.