Most investors spend their time trying to pick the funds that will beat the market. The uncomfortable truth is that after costs, most of them don't — and an index fund that simply matches the market will outperform the majority of active managers over a 10-year horizon.

This guide explains exactly how index funds work, why the math favors them, how to pick one, and the mistakes that trip up investors who are new to passive investing.

TL;DR

• An index fund holds every stock in a specified index (like the Nifty 50) in the same proportions as the index itself.
• Because it trades rarely and requires no research team, its annual cost (expense ratio) is far lower than active funds — typically 0.1–0.2% vs 1–2%.
• That cost difference compounds dramatically over decades: a 1.5% annual drag on a ₹10 lakh investment over 20 years costs you roughly ₹17 lakh in lost growth.
• The right index fund to pick depends on your time horizon and what slice of the market you want to own.
• The biggest mistakes are choosing a fund with a high tracking error, holding too many overlapping index funds, and abandoning the strategy during a market correction.

Start with what you already know: a mutual fund

You are probably familiar with a mutual fund. You put money in, a fund manager picks stocks, and you own a share of the resulting portfolio. The manager's job is to pick better stocks than the average — to "beat the market."

An index fund is a mutual fund with one important difference: the manager does not pick stocks. Instead, the fund automatically holds every stock in a specified index, in the exact proportions the index uses.

Think of an index like a recipe. The Nifty 50 index is a recipe that says: "hold these 50 large-cap Indian companies, weighted by their market capitalisation." An index fund follows that recipe exactly. When the index changes its ingredients — because one company grows and another shrinks — the fund updates its holdings to match. No judgment calls, no research, no manager.

The mechanics: NAV, units, and expense ratios

When you invest ₹10,000 in an index fund, here is what happens:

1. The fund calculates its NAV (Net Asset Value) — the total value of all its holdings divided by the number of units outstanding. If the NAV is ₹100 today, you receive 100 units.
2. The fund uses your money to buy stocks in the proportions the index dictates.
3. Each day, as the underlying stocks move, the NAV changes. If the Nifty 50 rises 1%, your fund's NAV rises approximately 1%.
4. The fund charges an annual expense ratio — deducted automatically from the NAV, not as a separate bill. A 0.2% expense ratio means the fund skims ₹2 per year from every ₹1,000 you hold.

The expense ratio is the only number you fully control before you invest. Everything else — market returns, index composition — is beyond your influence.

Why costs compound against you

This is the key insight, and it is best understood through a worked example.

Suppose the Nifty 50 returns 12% per year on average (roughly its long-run real return). You invest ₹10 lakh.

The ₹17 lakh difference between the index fund and the average active fund is not from better stock picking — it is simply the arithmetic of costs. The active fund must outperform the index by 1.6% every single year just to break even with the index fund after costs. Over a 20-year period, very few funds achieve this consistently.

This is not a theoretical argument. The SPIVA India Scorecard (S&P Dow Jones Indices) tracks this: over rolling 10-year windows, roughly 70–80% of large-cap active funds in India underperform their benchmark after costs.

Framework: the cost rule Before investing in any fund, ask: "What is the expense ratio of the cheapest index fund that tracks the same market segment?" If the active fund costs more than 1% above that, it needs to reliably outperform by more than 1% per year to justify itself. The evidence says most don't.

The index fund landscape in India

Not all index funds are the same. They track different indices, which represent different slices of the market.

A practical starting point: for most investors building a core portfolio, a Nifty 50 index fund (or Nifty 500 for broader exposure) covers the majority of Indian equity growth without complexity.

How to pick an index fund

Two numbers determine whether one index fund is better than another tracking the same index:

1. Expense ratio — lower is always better. For Nifty 50 funds, direct plans are available from 0.10% to 0.20%. Avoid regular plans (sold through distributors) — they carry an embedded commission of 0.5–1% that benefits the distributor, not you.

2. Tracking error — this measures how closely the fund's actual returns match the index returns. A tracking error of 0.10% means the fund's daily NAV moves deviate from the index by about 0.10% on average. Lower tracking error = the fund is doing its job well.

The pick rule: among all index funds tracking the same index, choose the one with the lowest expense ratio and lowest tracking error. If these conflict, weight expense ratio more heavily for long holding periods (it compounds harder).

What index funds are not good at

Index funds are not the answer to every investing question. Be clear about what they do not do:

• They cannot protect against a broad market crash. If the Nifty 50 falls 40%, your Nifty 50 index fund falls approximately 40%. There is no active manager to shift to cash.
• They include every company in the index, including poor ones. By definition, you also own the worst-performing stocks in the index. This is actually fine — the good ones more than compensate — but investors who find it psychologically uncomfortable should know this upfront.
• They require patience. Index funds are designed for 7–10+ year horizons. Over shorter periods, an active manager with a concentrated bet can easily outperform.

Common mistakes

1. Buying a "regular" plan instead of a "direct" plan. Every mutual fund in India offers two versions: regular (where a broker/distributor receives a commission from your expense ratio) and direct (where you buy directly from the AMC and avoid that commission). The direct plan of the same fund is always cheaper. Always buy direct.

2. Owning too many index funds. A Nifty 50 fund and a Sensex fund overlap almost entirely — you are not diversifying, you are duplicating. A Nifty 50 fund and a Nifty 500 fund also overlap heavily (the 500 includes the 50). Keep it simple: one or two index funds with genuinely different exposures.

3. Stopping SIPs during a market correction. The temptation during a 20% market drop is to pause your monthly SIP. This is the exact wrong move — you are buying units at a 20% discount. The SIP is working harder for you when markets are down, not less. Stay the course.

4. Chasing "thematic" or "sectoral" index funds. A Banking Index fund or IT Index fund is technically an index fund but concentrates all your risk in one sector. These behave more like active bets than diversified passive holdings. Treat them separately from your core index fund allocation.

5. Conflating tracking error with underperformance. A fund with a 0.15% tracking error is not "losing" to the index — it is replicating it within normal bounds. Expect a small, persistent gap between fund return and index return roughly equal to the expense ratio. That gap is not a problem; it is the cost of the service.

Summary and next step

Index funds work because they remove the two biggest costs in active management — stock-picking errors and management fees — and replace them with a simple rule: own the market.

The evidence is consistent: over long horizons, owning the market at low cost outperforms most attempts to beat it. That is not because markets are perfectly efficient, but because the bar for consistent outperformance, net of costs, is higher than most active managers clear.

Try this now: find your existing mutual fund investments. Look up each fund's expense ratio (direct plan TER) and note how it compares to a comparable index fund. If you hold a large-cap active fund charging 1.5–2%, run the 20-year cost comparison above with your actual invested amount. The number is often surprising.

Related reading on this site: [How to Think About Asset Allocation](how-to-think-about-asset-allocation.md) explains how index funds fit into a broader portfolio structure. [The Mechanics of Compounding](the-mechanics-of-compounding.md) shows why starting early with a low-cost fund has an outsized effect on the final number.