Most investing advice focuses on how to pick better stocks. An index fund takes the opposite approach: instead of trying to pick winners, you own a small slice of every stock in the market. No selection required.

An index fund is a type of investment fund designed to match the performance of a specific market index — like the Nifty 50 or the S&P 500 — by holding all (or a representative sample) of the securities in that index, in the same proportions.

The one-sentence answer

An index fund holds every stock in a market index automatically, so its performance tracks the market's performance — for a very low cost.

What is an "index"?

Before understanding index funds, you need to understand the index itself.

A market index is a list of companies, selected according to a rule, that represents some slice of the stock market. The Nifty 50, for example, contains India's 50 largest publicly traded companies by market capitalization. The S&P 500 contains the 500 largest US companies. The index is recalculated constantly as stock prices change, and the list of companies is updated periodically (when a company grows large enough to enter, or shrinks enough to exit).

An index is not an investment — it is a measuring stick. You cannot buy "the Nifty 50" directly. But you can buy a fund whose job is to replicate it.

How an index fund works

When you invest in a Nifty 50 index fund, the fund manager's job is simple: buy all 50 stocks in the index, in the same percentage as the index holds them. If Reliance Industries is 10% of the Nifty 50, the fund holds 10% in Reliance. If Infosys is 7%, the fund holds 7% in Infosys.

When the index is updated — say, a company is removed and replaced — the fund buys the new entry and sells the old one. This happens mechanically, not based on anyone's opinion.

Because the fund is not paying analysts to research companies or portfolio managers to make calls, the costs are very low. A typical Nifty 50 index fund in India charges an expense ratio (annual fee) of around 0.1%–0.2%. Actively managed funds charge 1%–2% or more.

Active management vs indexing

Traditional mutual funds use a team of analysts and a portfolio manager to actively pick stocks they believe will outperform the market. They charge higher fees for this expertise.

The problem is that outperforming the market consistently is genuinely difficult. Most stock prices already reflect publicly available information — analysts, algorithms, and professional investors have typically already acted on it. In any given year, roughly half of active managers beat the market. But the same managers rarely repeat that outperformance year after year.

Long-run data from SPIVA (S&P's active-vs-passive scorecard) consistently shows that over 10–15 year periods, more than 80–90% of active large-cap funds underperform their benchmark index after fees.

An index fund, by design, will never beat the market. But it will reliably match the market — and because most active managers fail to beat the market consistently, matching tends to outperform the average active investor over time.

A concrete example

Suppose Rahul and Priya each invest ₹10,000 per month for 20 years. Rahul picks an actively managed large-cap fund with a 1.5% expense ratio. Priya picks a Nifty 50 index fund with a 0.1% expense ratio. Both funds track approximately the same stocks and produce the same gross return — say, 12% per year before fees.

After 20 years at 12% gross return:

• Rahul's net return (after 1.5% fee): ~10.5% → final corpus: approximately ₹72 lakhs
• Priya's net return (after 0.1% fee): ~11.9% → final corpus: approximately ₹97 lakhs

The only difference was the fee. Over two decades, the 1.4% annual gap compounded into a ₹25 lakh difference — not because Priya invested more cleverly, but because she paid less for the same exposure.

Types of index funds

Mutual fund index funds: structured like traditional mutual funds, bought and redeemed at end-of-day net asset value (NAV). Suitable for systematic investment plans (SIPs) where you invest a fixed amount monthly.

Exchange-traded funds (ETFs): trade on the stock exchange like shares, with prices updating throughout the day. Typically have slightly lower expense ratios than mutual fund index funds, but require a demat account and a broker to buy.

Both track the same underlying index. The choice between them is mostly about how you prefer to invest (SIP vs lump-sum, ease of use vs slightly lower cost).

Why the expense ratio matters so much

The fee is deducted from your returns every year, and compounding amplifies even small differences over time. A 1% fee sounds small. Over 30 years at a 12% gross return, it reduces your final wealth by roughly 20%. The fee does not buy you better performance — the evidence says it reduces it.

Try this

Look up one fund you currently hold (or are considering) and find its expense ratio — it appears on the fund's factsheet or the AMC's website. Then look up the equivalent index fund for the same category (e.g., large-cap active fund → Nifty 50 index fund). Compare the expense ratios and ask: what would the active fund need to outperform by each year, after fees, to justify the difference?

If you keep investment notes in JustJot.ai, jot down the expense ratios of everything you hold alongside your expected returns. The friction of writing it down often clarifies whether you are paying for performance or just paying.