What Is an Index Fund?
- Filed
- Length
- 4 min
In this page (8)
- Start with the index itself
- How an index fund differs from an actively managed fund
- Mutual fund or ETF?
- What diversification does and does not do
- Costs to look for
- Questions to ask before choosing one
- Where index funds sit among other assets
- The short version
Ask people how they started investing and many will mention an index fund. The idea sounds almost too plain to be useful: rather than picking the companies that will do best, you buy a small piece of a whole group and accept the group's result.
A quick note before we go further: this article is general background, not financial advice. We do not know your income, goals or tolerance for loss, and a qualified adviser who does can tell you whether any of this suits you.
Start with the index itself
An index is simply a list with rules. A stock market index names a set of companies, such as the largest firms listed on a particular exchange, and calculates one number that rises and falls with their combined share prices. Bond indices do the same for loans issued by governments or companies. On its own, an index is a measuring stick. You cannot buy it directly; it only describes how a slice of the market has moved.
An index fund is a product built to copy that measuring stick. The fund manager buys the securities on the list, in roughly the proportions the index uses, and adjusts the holdings when the list changes. The goal is not to beat the index. The goal is to match it as closely as possible, minus the running costs.
How an index fund differs from an actively managed fund
In an actively managed fund, a manager or team chooses which securities to buy and sell, hoping to do better than the market. That research takes time and people, and the cost is usually passed on to investors through higher fees. Some active managers do outperform for a period; many do not, and it is hard to know in advance which will.
An index fund takes the opposite approach. Because the rules decide what is held, there is less research to pay for and less trading. Lower costs are the main structural advantage, and over long periods even a small difference in yearly fees can add up.
| Question | Index fund | Actively managed fund |
|---|---|---|
| What decides the holdings? | The rules of the index | The manager's judgement |
| Aim | Match the index | Beat a chosen benchmark |
| Typical cost level | Generally lower | Generally higher |
| Trading activity | Mostly when the index changes | Whenever the manager decides |
Mutual fund or ETF?
Index funds come in two common wrappers. A traditional mutual fund is bought from the fund company or through a platform, and orders are filled once a day at that day's price. An exchange-traded fund, or ETF, trades on a stock exchange like a share, so its price moves through the trading day. Both can track the same index; the differences lie in how you buy them, minimums, trading costs and, in some countries, tax treatment. Read the documents for the specific product in front of you.
What diversification does and does not do
Holding many companies at once spreads out the damage if one of them fails. A single business can collapse; it is far less likely that every firm in a broad index does so at the same time. That is the protective side of diversification.
It is important to be clear about its limits, though. When an entire market falls, a fund that tracks it falls too. Diversification reduces the risk tied to individual companies; it does not remove market risk. An index fund can lose value, sometimes sharply, and there is no guarantee about when it recovers.
Costs to look for
Before comparing funds, it helps to know where money leaks out. The main items are:
- Ongoing charge or expense ratio — a yearly percentage taken from the fund's assets to run it.
- Platform or account fees — what the broker or app charges you to hold investments.
- Trading costs — commissions or spreads when you buy or sell, which matter more if you trade often.
- Tracking difference — the gap between the fund's return and the index's return over time, which reflects costs and how well the fund copies its index.
Questions to ask before choosing one
- Which index does it follow, and what does that index actually contain? A fund tracking one country's largest firms is very different from one tracking global bonds.
- How long can you leave the money alone? Broad equity funds suit long horizons better than money you might need next year.
- What are the total yearly costs once fund and platform charges are added together?
- How would you feel if the value dropped noticeably for a year or two? If the honest answer is "I would sell", the mix may be too aggressive.
- Is there an emergency cushion in cash first, so you are not forced to sell at a bad moment?
That last point connects investing to everyday money management. Our older piece on getting personal finances in order covers the budgeting groundwork that usually comes before any fund at all.
Where index funds sit among other assets
Index funds are one tool, not the whole toolbox. Some people also hold cash savings, property or physical assets. If you are curious how a tangible holding compares, our article on why some investors keep gold bars looks at a very different kind of asset with its own trade-offs around storage and price swings.
The short version
An index fund copies a market index instead of trying to outguess it. Its appeal is broad diversification at generally lower cost; its main risk is that it moves with the market, downturns included. Know the index, add up every fee and get personal advice where the stakes are high.