---
title: Index fund — what it means and why it matters | Allocate AI
description: A mutual fund or ETF that holds every stock in a market index like the S&P 500, so you own a slice of the whole market at very low cost.
canonical: https://allocateai.io/glossary/index-fund
site: Allocate AI — https://allocateai.io
---

# Index fund

A mutual fund or ETF that holds every stock in a market index like the S&P 500, so you own a slice of the whole market at very low cost.

An index fund is a fund that does not try to beat the market; it tries to be the market. Instead of a manager picking stocks, the fund buys every company in an index, such as the S&P 500 or a total US or world stock index, in proportion to their size. When the index goes up 8%, the fund goes up about 8% minus a tiny fee.

The appeal is cost and consistency. Because no one is paid to pick stocks, index fund fees are often 0.03% to 0.10% a year, compared with 0.5% to 1% for actively managed funds. Over decades that gap matters: on $500,000 growing at 7%, a 0.05% fee versus a 0.75% fee is a difference of more than $180,000 after 25 years. And most active managers do not beat their index after fees over long periods, so the low-cost option tends to win on both counts.

Index funds come as mutual funds (priced once a day, bought through the fund company or a 401(k)) and ETFs (traded like stocks on an exchange). They are functionally similar; ETFs are slightly more tax-efficient in taxable accounts.

A common portfolio is nothing more than a US total-market fund, an international fund, and a bond fund, held in proportions that fit your timeline.

The common mistake is owning several index funds that track nearly the same thing and believing that is diversification, or picking a fund in a 401(k) by its name without checking the expense ratio.

[Try the Compound Interest Calculator →](https://allocateai.io/calculators/compound-interest)

## Related terms

- [Expense ratio](https://allocateai.io/glossary/expense-ratio) — The yearly fee a fund charges, expressed as a percentage of your balance and taken out invisibly; 0.05% is cheap, 1% is expensive.
- [Diversification](https://allocateai.io/glossary/diversification) — Spreading money across many investments so that no single company, industry, or country can sink your whole plan.
- [Asset allocation](https://allocateai.io/glossary/asset-allocation) — How your investments are split between stocks, bonds, and cash; the mix drives most of your long-term return and how bumpy the ride is.
- [Dollar-cost averaging](https://allocateai.io/glossary/dollar-cost-averaging) — Investing a fixed amount on a regular schedule regardless of price, which spreads out timing risk and removes the need to guess when to buy.

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Allocate AI is a budgeting and financial-planning app: link your accounts, say what you want in plain words, and it writes the plan and stages every change for your approval. Read more at https://allocateai.io or see all pages in https://allocateai.io/llms.txt.