What Is an Index Fund — Explained Simply

The investment Warren Buffett recommends for most people — and why

6 min read Beginner

"An index fund holds a tiny piece of every company in an index — the S&P 500, the FTSE 100, the whole market. It costs almost nothing to run. It beats most professional fund managers over 20 years. This chapter explains why."

For educational purposes only. Nothing in this chapter is financial advice. All figures are illustrative examples. Tax rules, account types, contribution limits, and regulations differ by country and change over time. Always verify current rules with official government sources or a qualified financial adviser before making any investment decisions.

What an Index Fund Is

An index fund is a fund that tracks a market index — a pre-defined list of stocks or other assets — rather than attempting to select individual investments. The fund holds the same securities as the index, in the same proportions, and simply replicates its performance. No analyst picks stocks. No manager makes decisions. The fund just follows the index.

An index is a standardised list of securities that represents a market or market segment. The S&P 500 is an index of the 500 largest US publicly listed companies by market capitalisation. The FTSE 100 is an index of the 100 largest companies listed on the London Stock Exchange. The FTSE All-World covers over 3,500 companies across 49 countries. When "the market" goes up 10%, that is a reference to an index moving 10%.

This passive approach contrasts with active fund management, where a team of analysts and fund managers research, select, and trade securities in an attempt to beat the index. Active funds charge more for this service. The evidence on whether they deliver is clear.

The Expense Ratio — Why It Matters More Than You Think

The expense ratio (also called the Ongoing Charges Figure or OCF in the UK) is the annual percentage of your investment that the fund charges for management. For a passive index fund, this is typically 0.03% to 0.20% per year. For an active fund, it is typically 1% to 2% per year — sometimes higher with performance fees.

The difference sounds small. Over time it is enormous. On £10,000 invested for 30 years at 8% gross annual return: a 0.07% fee fund leaves you with approximately £98,900. A 1.5% fee fund leaves you with approximately £57,400. The fee difference consumed £41,500 — more than four times your original investment.

This is because every pound taken in fees is a pound that no longer compounds. The fee erosion compounds against you in exactly the same way that investment returns compound in your favour. The compounding works both ways.

The 20-Year Performance Evidence

S&P Global's SPIVA (S&P Indices Versus Active) scorecard publishes annual data on how many active fund managers beat their benchmark index. The findings are consistent: over 15 years, approximately 88% of large-cap active US equity fund managers underperform the S&P 500. In the UK, around 80% of active UK equity managers underperform the FTSE All-Share over 10 years.

This is not because active managers are incompetent. It is because fees create a structural headwind that is very difficult to overcome consistently. To beat a 0.07% index fund, an active fund must first overcome its own 1.5% fee disadvantage — before generating any alpha. Over long periods, very few do.

UK Examples

Vanguard FTSE All-World UCITS ETF (VWRL): Covers approximately 3,500 companies across 49 countries. Expense ratio: 0.22%. Available on most UK platforms in a Stocks and Shares ISA. One fund, entire world.

HSBC FTSE All-World Index Fund: Similar global coverage to VWRL but in mutual fund (unit trust) format. Expense ratio: 0.13%. Suitable for regular monthly investments through platforms that offer it.

Vanguard LifeStrategy funds: Pre-mixed allocations of global equity and bond index funds in various ratios (20/80 to 100/0). A single fund that handles rebalancing automatically. Expense ratio: 0.22%.

FAQs

Is an index fund the same as an ETF?

Not exactly — an index fund describes the investment strategy (tracking an index passively). An ETF describes the structure (traded on an exchange like a stock). Most ETFs are index funds, and most index funds in the UK are available as ETFs. The terms overlap significantly but are not identical.

What happens if the index drops 40%?

Your fund drops 40% too. This is the tradeoff for low fees and broad diversification — you capture the full market return, up and down. The relevant question is your time horizon: historically, globally diversified indices recover from major drops within 2–10 years.

Should I pick a UK index or a global one?

For most investors, a global index fund is better than a UK-only one. The UK represents only about 4% of global stock market value. Restricting to the UK concentrates your risk in one economy and one currency. A global fund gives exposure to the US, Europe, Asia, and emerging markets.

Can I lose all my money in an index fund?

For a global index fund to go to zero, every publicly listed company in every major economy would have to simultaneously become worthless. This is effectively impossible — by that point, the currency, the banking system, and society itself would have collapsed. An index fund can lose a large percentage of its value temporarily; it cannot go to zero.

Key takeaways

An index fund tracks a pre-defined market index passively — no stock picking, no active management, very low costs.
Expense ratios for index funds are typically 0.03–0.20% vs 1–2% for active funds — a difference that compounds enormously over decades.
Over 15 years, ~88% of active US equity managers underperform the S&P 500 index after fees.
UK examples: Vanguard FTSE All-World ETF (VWRL, 0.22%) and HSBC FTSE All-World Index Fund (0.13%) give global exposure in one product.
A global index fund is generally preferable to a UK-only one — the UK is only ~4% of global market value.