Wealth Services

What is an Index Fund?

By Tata Capital
13 October 2026
11 mins read
2266

Summary: An index fund is a mutual fund that simply copies a market index, such as the Nifty 50, instead of trying to beat it. This blog explains what an index fund actually is, how it works day to day, and the different types you may come across. It also lays out the honest case for index investing, low costs and a strong track record against active funds, alongside the real risks and the fact that not every “index fund” is as diversified as it sounds.

An index fund is a mutual fund that passively tracks a market index, such as the Nifty 50 or the Sensex, by holding the same stocks in the same proportions as that index. Rather than trying to beat the market, the goal is simply to match its return, and it does this at a very low cost compared to a fund where a manager actively picks stocks.

This blog covers how index funds actually work, the different types available, the honest case for using them (low cost, and the fact that most actively managed funds trail their benchmark over long periods), the real risks involved, how they are taxed and how to go about choosing one. Note that index funds are low cost, but low cost does not mean low risk.

Index Fund Meaning

An index fund is a passively managed mutual fund that replicates a market index by holding its constituent stocks in the same weights. Instead of a manager picking stocks, it simply mirrors the index, so when the index rises or falls, the fund does too.

That is what an index fund is in its most compact form, and it also answers what an index fund is meant to do differently from a typical mutual fund. There is no attempt to outguess the market but only to replicate it as closely as possible. With that, the next immediate question is how a fund manager keeps a portfolio in line with an index that itself keeps changing.

Also Read – Types of mutual funds in India

How Does an Index Fund Work?

An index fund buys all or a representative sample of the stocks that make up its target index, in the same proportions the index uses. When the index composition changes, a stock gets added or removed, or the weights are rebalanced, the fund adjusts its own holdings to match. There is no discretionary stock-picking involved, which is exactly why this approach is called passive.

In practical terms, if the Nifty 50 rises by 1% on a given day, the fund’s NAV rises by roughly 1% too, minus a small amount for tracking error and running costs. A fund manager is still involved here, but their job is replication and execution, thereby keeping the portfolio aligned with the index instead of deciding which stocks to buy or sell based on their own judgement.

Here, two factors explain why two funds tracking the exact same index can still post slightly different returns:

Tracking error, i.e., the gap between the fund’s return and the index’s return.
Expense ratio, the annual cost charged to run the fund.

Both of these become central once you start comparing what different index funds actually offer.

Types of Index Funds

Index funds are not a single uniform product. The type of index a fund tracks changes how diversified or concentrated your exposure actually is:

Broad-Market Index Funds: Track a wide index such as the Nifty 50, Sensex or Nifty 500, thereby giving you exposure across large sections of the market. This is the classic diversified version of an index fund mutual fund most people have in mind.
Large-cap, Mid-cap, and Small-cap Index Funds: Track indices built around companies of a specific size, such as the Nifty Midcap 150, narrowing exposure to one segment of the market.
Sectoral or Thematic Index Funds: Track a specific sector, such as banking or IT. These are concentrated bets and not diversified ones, despite carrying the same “index fund” label.
Factor or Smart-beta Index Funds: Track an index built around a specific factor such as value, momentum or low volatility, rather than simple market-capitalisation weighting.
International Index Funds: Track a foreign index, such as the S&P 500, giving you exposure to overseas markets through a domestic fund structure.
Exchange-Traded Funds (ETFs): A close family of the index fund, tracking the same kinds of indices but trading on the stock exchange like a share. This also means you need a demat account to hold one.

Note that a broad-market index fund tracking the Nifty 50 is a fundamentally different proposition from a narrow sectoral index fund. The two should not be treated as equally diversified or equally “safe” simply because both carry the index fund label.

Also Read – What Is Debt Fund Investment?

Why Index Funds Have Become So Popular: The Honest Case

The case for index funds rests on two things, i.e., cost and evidence. Here, both hold up under scrutiny. On cost, index funds in India typically charge an expense ratio of around 0.1% to 0.3% for direct plans, compared with roughly 1% to 2% for actively managed equity funds. That gap looks small on paper, but it compounds meaningfully over long periods.

As an illustration, on a monthly SIP of ₹10,000 over 20 years (assuming an identical gross market return for both and only the cost difference varying), a fund with a 1.5-percentage-point lower annual cost could end up worth roughly ₹15 to ₹20 lakh more than the higher-cost equivalent by the end of the period.

This is an illustrative calculation only. Actual outcomes depend entirely on real market returns and fund performance.

On evidence, the SPIVA India Year-End 2025 scorecard, published by S&P Dow Jones Indices, found that roughly 76% of actively managed Indian large-cap equity funds underperformed their benchmark over the 10 years ending December 2025, and the figure was higher still, around 84%, over five years. Paying more for active management, in other words, has often bought less return in this segment.

This shift in preference shows up in the numbers too. Passive assets in India (index funds, ETFs, and related fund-of-funds) had grown to somewhere between ₹14 and ₹15 lakh crore by early 2026. This makes it close to a fifth of total mutual fund industry assets, according to AMFI-linked data, up from a small fraction of that just a few years earlier.

To keep this honest, the case is strongest specifically for large-cap equity and for long investment horizons. In less efficient corners of the market (such as mid-cap and small-cap stocks), skilled active management has at times still added value over the same period. That balance is exactly what makes the case for index funds credible rather than absolute. Additionally, it is also why the risks below deserve equal attention.

Risks and Limitations (Low Cost Is Not Low Risk)

Being inexpensive does not make an index fund safe. Here is how:

Full Market Risk: An index fund falls exactly as much as its underlying index does in a downturn, since there is no manager available to turn defensive or move to cash. Equity index funds carry real volatility and suit investors with a genuinely long horizon.
No Chance of Outperformance: By design, you get the index’s return minus costs, nothing more. Choosing an index fund means deliberately giving up any possibility of beating the market in exchange for certainty of tracking it.
Tracking Error: The small ongoing gap between a fund’s actual return and its index’s return is a real cost. It is also one of the key factors worth checking before choosing between funds tracking the same index.
Concentration in Narrow Indices: A sectoral or thematic index fund is not a diversified whole-market bet, regardless of what its name suggests. Treating it as equivalent to a Nifty 50 fund is a common and costly mistake.

Mutual fund investments, including index funds, are subject to market risk, and this applies in full here.

Read More – What is Net Asset Value (NAV)?

Index Funds vs Active Funds vs ETFs

The table below lays out the practical differences across index funds, active funds and ETFs:

Factor	Index Fund	Active Fund	ETF
Management style	Passive, replicates an index	Active, manager selects stocks	Passive, replicates an index
Goal	Match the index return	Beat the index (often does not, over the long run)	Match the index return
Typical cost	Low (roughly 0.1%-0.3%)	Higher (roughly 1%-2%)	Often the lowest of the three
How you buy/sell	End-of-day NAV, like any mutual fund	End-of-day NAV, like any mutual fund	Traded on the exchange during market hours
Account needed	None beyond a regular mutual fund account	None beyond a regular mutual fund account	Demat account required
Best suited for	Hands-off, SIP-based investors	Investors seeking a manager’s active judgement	Investors wanting intra-day flexibility

Although index funds versus ETFs cover a deeper analysis of the same, both of these largely aim for the same outcome. However, the mechanics of buying and holding them differ meaningfully.

An ETF trades like a share with intra-day pricing and a bid-ask spread, while an index fund is bought and sold at the day’s closing NAV and works naturally with a SIP.

In short, index funds suit investors who want a hands-off SIP-friendly approach, while ETFs suit those who already have a demat account and want the flexibility of intra-day trading.

Read More – Mutual Fund Portfolio Analysis

How Are Index Funds Taxed?

An index fund is taxed based on what it actually holds and not on the fact that it is an index fund. An equity index fund, one tracking the Nifty 50 or a similar equity index, is taxed the same way as any other equity-oriented scheme:

Long-term capital gains (units held over 12 months) attract 12.5% tax above an annual exemption of ₹1.25 lakh.
Short-term capital gains (units held for 12 months or less) are taxed at 20%.

A debt or international index fund, by contrast, does not qualify for this equity treatment and is generally taxed at your applicable income tax slab rate regardless of holding period, following the rules introduced for non-equity mutual funds since April 2023.

Additionally, equity fund gains have not carried any indexation benefit since July 2024, so do not confuse an “index fund,” which tracks a market index, with “indexation,” which is a separate tax concept used to adjust an asset’s purchase cost for inflation.

Tax rules change from time to time, so it is worth verifying the current rates with a qualified chartered accountant before filing.

How to Invest in and Choose an Index Fund

Getting started with any index fund mutual fund follows the same basic process:

Complete Your KYC: This is a one-time requirement using your PAN and Aadhaar and it is largely paperless today.
Decide Between a Lump sum and a SIP: Either route works for an index fund. An SIP suits investors who prefer to invest gradually over time.
Invest Through an AMFI-registered Platform or the AMC Directly: This ensures your investment is routed through a properly regulated channel.

Choosing between two funds tracking the identical index is a narrower decision than it might first appear, since both funds hold the same stocks in the same weights, and their past “returns” will be nearly identical for that reason. What actually differs and what is worth comparing, comes down to three factors:

Lowest Expense Ratio: Since the underlying portfolio is fixed by the index, a lower cost translates almost directly into a higher net return over time.
Lowest Tracking Error: A smaller and consistent gap between the fund and its index suggests more disciplined replication.
Adequate Fund Size (AUM): A reasonably sized fund tends to offer more stability and liquidity than a very small one.

Direct plans are generally worth preferring over regular plans for this reason, since they exclude distributor commission and therefore carry a lower expense ratio. It is also worth matching the specific index to your own goal and time horizon, rather than choosing one because it appears on a “best index fund” list. This is since two funds tracking the same index are close substitutes by design.

If you want to explore index funds available through a single platform, you can do so on Tata Capital Moneyfy. Note that Tata Capital, as an AMFI-registered distributor, cannot recommend a specific scheme to you. For personalised guidance, speaking with a qualified financial adviser remains the appropriate route.

Conclusion

In a nutshell, index funds can be described as a mutual fund that passively tracks a market index at very low cost, thereby handing you the index’s return minus a small amount for expenses and tracking error.

At the same time, index funds carry full market risk. Low cost is not the same as low risk. When comparing funds tracking the same index, expense ratio and tracking error are the two factors that actually matter. You can explore index funds on Tata Capital Moneyfy as part of your own research.

Disclaimer: Tata Capital (Moneyfy) is an AMFI-registered mutual fund distributor. Investments are subject to market risk. Please read all scheme related documents carefully, and consult a qualified chartered accountant or financial adviser regarding your specific tax position, before investing.

FAQs

What is an index fund in simple terms?

An index fund is a mutual fund that copies a market index, such as the Nifty 50, by holding the same stocks in the same proportions. Instead of a manager trying to pick winning stocks, the fund simply mirrors the index, rising and falling along with it.

How does an index fund work?

Index funds buy the constituent stocks of their target index in matching weights and adjust holdings whenever the index itself changes composition. There is no active stock selection involved and hence, the fund's return closely tracks the index's return, minus a small amount for costs and tracking error.

Are index funds safe?

Not in the sense of being low risk. Index funds carry the full market risk of the index they track. In a market downturn, an equity index fund will fall by roughly the same amount as the index itself, since there is no manager positioned to move to safer assets. Low cost should not be mistaken for low risk here.

Do index funds give good returns?

Index funds are designed to match their benchmark's return, not beat it. So their returns reflect exactly how that index performs over your holding period. Over long horizons, particularly in large-cap equity, this has often meant returns comparable to, or better than, many actively managed alternatives, once costs are accounted for.

What is the difference between an index fund and an ETF?

Both are types of index fund mutual fund products that track the same kinds of indices, but an ETF trades on the stock exchange like a share, with intra-day pricing and a bid-ask spread and requires a demat account. An index fund, by contrast, is bought and sold at the day's closing NAV and works naturally with a SIP, with no demat account needed.

Which is the best index fund?

There is no single "best" index fund in absolute terms, since funds tracking the same index hold nearly identical portfolios. The more useful comparison for index funds tracking your chosen index, is the lowest expense ratio, the lowest tracking error and an adequately large fund size, rather than past returns which will be nearly the same across funds tracking that index.

How are index funds taxed?

Equity index funds follow standard equity taxation. 12.5% long-term capital gains tax above ₹1.25 lakh annually for units held over 12 months and 20% short-term capital gains tax for units held 12 months or less. Debt or international index funds are taxed at your income slab rate instead, following the rules for non-equity mutual funds.

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