What Is an Index Fund and How Does It Work?

When you buy one share of an index fund, you are not buying a manager's opinion. You are buying a slice of a list.

That list already exists, published by an index provider, and it says which companies belong in it and how much of each one to hold. The fund's entire job is to own that list as closely as possible. If Apple is 6 percent of the index, the fund holds roughly 6 percent Apple. When a company drops out of the list, the fund sells it. No meeting, no forecast, no conviction call.

That single design decision, replacing human judgement with a published rulebook, is what makes index funds cheap, and cheap is what makes them work.

What an index actually is, and who decides what goes in it

An index is a measuring stick. The S&P 500 measures 500 large US companies. The FTSE 100 measures the 100 largest companies listed in London. The S&P/TSX 60 measures 60 large Canadian companies. A global index like the MSCI World stitches together developed markets across dozens of countries.

Two things surprise people here.

First, indexes are usually capitalisation weighted. You do not own equal slices of each company. You own each company in proportion to its market value, so the largest handful can make up a very large share of a "diversified" fund. That is not a flaw, but it is worth knowing before you assume 500 companies means 500 equal bets.

Second, the list has rules and the rules get applied. Companies join and leave on a schedule, and the fund follows. Nobody at the fund gets a vote, which is precisely the point.

Index fund, ETF, or actively managed fund

These three get muddled constantly. They are answering different questions: what is the strategy, and what is the container.

	Index mutual fund	Index ETF	Actively managed fund
Who picks holdings	A published index rulebook	A published index rulebook	A fund manager and research team
How you buy it	Direct from the fund, priced once a day at net asset value	On a stock exchange, live price all day	Direct from the fund, priced once a day
Typical ongoing cost	Very low	Very low	Substantially higher
Trading inside the fund	Minimal	Minimal	Frequent
Realistic goal	Match the index minus fees	Match the index minus fees	Beat the index after fees

The SEC's investor guide is blunt about why the passive versions cost less: less trading of the portfolio means fewer transaction costs, more favourable tax consequences from lower realised capital gains, and lower fees than actively managed funds. Three separate savings, all flowing from the same lack of activity.

The fee number is the part you actually control

You cannot control what markets return. You can control almost exactly what you pay, and over a long holding period that is not a rounding error.

Here is the arithmetic, using a single 10,000 pound (or dollar) lump sum, no further contributions, and an assumed 6 percent annual return before charges. Fund A charges 0.06 percent a year. Fund B charges 0.85 percent. Both track the same index, so before costs they do the same thing.

Years held	Fund A at 0.06 percent	Fund B at 0.85 percent	Gap
10	17,808	16,524	1,284
20	31,712	27,304	4,408
25	42,315	35,094	7,221

By year 25 the fee gap has cost you 7,221, which is more than 70 percent of everything you originally put in, on a difference of less than one percentage point a year. Nothing else in the comparison changed. Same index, same market, same 25 years.

The reason the gap accelerates is the same reason saving works at all: the money the fee takes out this year is money that cannot compound for the remaining 24. If that mechanism is not yet second nature, our guide to compound interest covers it properly, and the SEC publishes a free compound interest calculator you can run your own numbers through.

Two practical warnings on fees. The ongoing charge is not the only cost: your platform, brokerage or workplace provider may add an account fee, and buying an ETF may cost a trading commission and a bid-offer spread. And a very cheap fund that tracks its index badly is not a bargain, so check the fund's tracking record against the index it claims to follow, not just its price tag.

What an index fund will not do for you

Honest limits, because the enthusiasm around index investing tends to skip these.

It will not cushion a fall. An index fund tracking a falling index falls with it. There is no manager to step aside, and stepping aside is explicitly not the product.

It will not diversify you as much as the holdings count suggests. A fund holding 500 companies that are all large US firms is concentrated in one country and, lately, in a small number of very large firms. Holding 500 names is not the same as holding 500 risks.

It will not beat the index. By construction it returns the index minus costs. If you want the possibility of beating it, you are accepting the far likelier possibility of trailing it after fees.

It will not fix your behaviour. The cheapest fund in the world still loses to panic selling in a bad quarter.

The wrapper matters more than the fund

The same index fund produces different outcomes in different countries, because the account you hold it in decides what the tax authority takes. This is where most beginner guides go quiet, and it is where the real money is.

United States. Funds and ETFs are regulated by the SEC, and FINRA regulates the brokers selling them. Most people meet index funds inside a 401(k), where the menu is chosen by the employer and a broad index or target date fund is very often the cheapest thing on it. Outside that, traditional and Roth IRAs shelter growth, with the Roth taking tax up front and the traditional deferring it. In an ordinary taxable brokerage account the structure starts to matter: mutual funds can distribute capital gains to holders even in a year you sold nothing, while ETFs generally distribute far less, which is why the ETF version of an identical index is often the better taxable-account choice. Every fund's expense ratio is in its prospectus, and FINRA's fund analyser compares them.

United Kingdom. Funds are regulated by the Financial Conduct Authority, and the cost figure to compare is the ongoing charges figure, or OCF. Held inside a Stocks and Shares ISA, growth and income are free of UK capital gains tax and dividend tax, which is why the ISA is the default home for a tracker for most people. A self invested personal pension adds tax relief on contributions instead, with access deferred to pension age. Outside a wrapper you are into the dividend allowance and the capital gains annual exempt amount, both of which have been cut materially in recent years, so check the current thresholds on GOV.UK rather than relying on a figure you half remember. One more UK quirk: most trackers come in accumulation units, which reinvest income automatically, and income units, which pay it out. Pick deliberately.

Canada. Provincial regulators coordinate through the Canadian Securities Administrators, and the Ontario Securities Commission's investor education site is the plain-English reference. The cost figure here is the management expense ratio, or MER, and the spread is unusually wide: index mutual funds sold through bank branches have historically carried MERs many times those of equivalent ETFs tracking the same index, so the same strategy can cost wildly different amounts depending on where you bought it. Recent total cost reporting rules improved what appears on your annual statement, which makes it easier to see what you are actually paying. Both TFSAs and RRSPs shelter growth, but they are not interchangeable for US holdings: under the Canada-US tax treaty, US dividend withholding tax is generally not applied to US shares held in an RRSP, while in a TFSA that withholding is deducted and cannot be reclaimed. If you hold a US-focused fund, that detail is worth real money.

Four things to check before you buy

Keep it to a page.

1. What does it actually track? Read the index name, not the fund's marketing name. "Global" and "world" mean different things in different fund families.
2. What is the all-in cost? Ongoing charge plus platform fee plus, for ETFs, trading cost. Compare against a mainstream equivalent tracking the same index.
3. Has it tracked the index closely? Compare the fund's return to the index return over several years. Persistent underperformance beyond the fee is a red flag.
4. What account will it sit in? Fill your tax-sheltered room first. It is the highest-certainty return available to you.

The bottom line

An index fund is the least clever product in investing, and that is its entire advantage. It buys a published list, holds it, and charges almost nothing to do so, which leaves the two variables that actually decide your outcome squarely in your hands: what you pay, and what account you hold it in. Get the ongoing charge as low as a mainstream provider will sell it to you, put it inside a 401(k), IRA, ISA, SIPP, TFSA or RRSP before you put it anywhere else, and then leave it alone. Most of the money in index investing is made by not doing anything else.