If you want to grow your money over the long term without picking individual shares or paying high fees, index funds are one of the simplest ways to do it. They let you own a small slice of hundreds or even thousands of companies in a single investment, usually at a very low cost. That’s why they’ve become a popular choice for beginners, experienced investors and pension savers alike.
This guide explains index funds in plain English: what they are, how they work, the main types, costs, active vs passive investing, risks, how to choose and buy an index fund in the UK, how they’re taxed, how ISAs and pensions can help, and the common mistakes to avoid.
Table of Contents
What Is an Index Fund?
A Simple Definition
An index fund is a type of investment fund that aims to track the performance of a specific market index, such as the FTSE 100, the S&P 500 or a global stock market index. Rather than trying to beat the market, it aims to match it.
What Is a Market Index?
A market index is a list of companies or bonds that represents part of the market. Examples include:
| Index | What It Tracks |
|---|---|
| FTSE 100 | The 100 largest companies listed on the London Stock Exchange |
| FTSE 250 | The next 250 largest UK-listed companies |
| FTSE All-Share | Most companies listed in the UK |
| S&P 500 | 500 large US companies |
| MSCI World | Large and mid-sized companies in developed countries |
| FTSE Global All Cap | Large, mid and small companies worldwide |
| MSCI Emerging Markets | Companies in emerging economies |
| Global aggregate bond indices | Government and corporate bonds worldwide |
How an Index Fund Works
If you invest £1,000 in a global index fund, your money is spread across thousands of companies in proportion to their size in the index. When the index rises, your investment rises; when it falls, your investment falls.
Index Funds vs ETFs
Two Ways to Track an Index
Index tracking funds come in two main forms: traditional index funds (often unit trusts or OEICs) and exchange-traded funds (ETFs).
| Feature | Index Fund (OEIC/Unit Trust) | ETF |
|---|---|---|
| How it’s traded | Bought and sold once a day at a set price | Traded on a stock exchange throughout the day |
| Pricing | Single daily price | Live market price |
| Minimum investment | Often low, regular monthly investing available | Usually the price of one share (some platforms allow fractional shares) |
| Dealing costs | Often lower or none on many platforms | May have trading fees |
| Best for | Regular, automatic investing | Flexible trading and wide choice |
Both can be low-cost and suitable for long-term investors.
Active vs Passive Investing
What’s the Difference?
- Active funds: A fund manager picks investments aiming to beat the market
- Passive funds (index funds): Simply track a market index
How Active Funds Compare
Research consistently shows that many active funds fail to beat their benchmarks over the long run. S&P Dow Jones Indices’ SPIVA Europe Mid-Year 2026 report found that over the 10-year period, 98% of EUR-denominated Global Equity funds, 95% of GBP-denominated Global Equity funds underperformed their benchmarks. Its year-end 2025 report found that 81.8% of active equity funds lagged their benchmarks over the past year, rising to 97.0% over the 10-year period.
Morningstar-based analysis gives a similar picture. MoneyWeek reports that active managers’ success rate against passive alternatives stands at 20.3% over three years, 15.2% over five years and 11.9% over 10 years. For UK-focused funds, just 19% beat their passive peers in the first half of 2026.
Active management can still have a role, and there are periods when active funds do well. As one expert notes in the same article, there have been long windows over the past 15 years where active funds have performed well.
| Factor | Active Funds | Index Funds |
|---|---|---|
| Goal | Beat the market | Match the market |
| Fees | Usually higher | Usually lower |
| Long-term performance | Most underperform their benchmarks | Track the market, minus small costs |
| Transparency | Holdings change at manager’s discretion | Holdings follow the index |
| Manager risk | Depends on manager skill | Minimal |
| Turnover | Often higher | Usually lower |
Why Index Funds Are Popular
Key Benefits
- Low costs: Lower fees mean more of your returns stay with you
- Instant diversification: Spread risk across many companies
- Simplicity: No need to research individual shares
- Transparency: You know what the fund aims to track
- Long-term performance: Many beat most active funds over time
- Easy regular investing: Set up monthly contributions automatically
The Power of Low Fees
Fees compound over time, just like returns. Even small differences can make a big impact.
| Annual Fee | £10,000 Invested for 25 Years at 6% Return (Before Fees) |
|---|---|
| 0.1% | Approx £41,800 |
| 0.5% | Approx £38,100 |
| 1.0% | Approx £33,900 |
| 1.5% | Approx £30,100 |
Illustrative figures only. Returns are not guaranteed and values can fall.
Types of Index Funds
Equity Index Funds
Invest in shares. Examples include UK, US, European, global, emerging market and small-company trackers.
Bond Index Funds
Track government or corporate bond indices. Usually lower risk than equities, with lower expected long-term returns.
Global Index Funds
Invest across many countries in one fund. Popular for long-term investors wanting broad diversification.
Multi-Asset or Lifestyle Index Funds
Combine shares and bonds in set proportions, such as 60% shares and 40% bonds. Some automatically rebalance and reduce risk as you approach a target date.
Sector and Thematic Index Funds
Track specific sectors (technology, healthcare) or themes (clean energy, AI). These are more concentrated and often riskier.
ESG Index Funds
Track indices that screen companies on environmental, social and governance criteria.
Accumulation vs Income Units
| Unit Type | What Happens to Dividends |
|---|---|
| Accumulation (Acc) | Automatically reinvested into the fund |
| Income (Inc) | Paid out to you as cash |
Accumulation units are often used for long-term growth; income units suit people who want regular payouts.
Understanding Index Fund Costs
Fund Charges
- Ongoing charges figure (OCF): Annual cost of running the fund, shown as a percentage
- Transaction costs: Costs of buying and selling holdings within the fund
Platform Fees
Investment platforms may charge:
- A percentage of your investment each year
- A flat monthly or annual fee
- Trading fees for ETFs or shares
- Fees for certain accounts or transfers
Choose the Right Fee Structure
| Investment Size | Often Cheaper Option |
|---|---|
| Smaller portfolios | Percentage-based platform fees |
| Larger portfolios | Flat-fee platforms |
Compare total costs, including fund and platform fees, before choosing.
Risks of Index Funds
Market Risk
Index funds rise and fall with the markets. Your investment can fall in value, sometimes sharply, and you could get back less than you invest.
Concentration Risk
Some indices are dominated by a few large companies. For example, global and US indices are heavily weighted towards large US technology firms.
Currency Risk
If you invest in overseas indices, exchange rate movements can affect your returns in pounds.
Tracking Difference
A fund’s return may differ slightly from the index because of costs and how it tracks.
No Downside Protection
Index funds don’t try to avoid falling markets. They simply follow the index.
Investor Protection
The FSCS may protect you if an authorised investment firm fails, but it doesn’t protect against normal falls in investment value.
How to Choose an Index Fund
Step 1: Set Your Goals
- What are you investing for? (retirement, house deposit, children’s future)
- How long can you invest for? (ideally at least five years for shares)
- How much risk can you handle?
Step 2: Choose Your Asset Mix
| Risk Level | Example Mix |
|---|---|
| Cautious | 20–40% shares, 60–80% bonds |
| Balanced | 50–60% shares, 40–50% bonds |
| Growth | 80–100% shares |
Step 3: Choose the Market
A global equity index fund is a common core holding because it spreads money across many countries and sectors.
Step 4: Compare Funds
| Factor | What to Look For |
|---|---|
| Index tracked | Broad, well-diversified index |
| Ongoing charge | Low OCF |
| Fund size | Larger funds often have lower costs and more stability |
| Tracking record | Small tracking difference |
| Replication method | Physical (holds actual assets) or synthetic (uses derivatives) |
| Accumulation or income | Match to your needs |
| Provider | Reputable, well-established fund manager |
Step 5: Check the Key Information Document
Read the fund’s key information document or factsheet for details on objectives, costs, risks and holdings.
How to Buy Index Funds in the UK
Choose an Account
| Account | Best For |
|---|---|
| Stocks and shares ISA | Tax-free investing |
| Self-invested personal pension (SIPP) | Retirement saving with tax relief |
| Workplace pension | Employer contributions and tax relief |
| Lifetime ISA | First home or retirement (age limits apply) |
| Junior ISA | Investing for children |
| General investment account (GIA) | Investing after using tax-free allowances |
Choose a Platform
Compare platforms on fees, fund choice, ease of use and customer service.
Set Up Regular Investments
Investing a fixed amount each month (pound-cost averaging) can smooth out the effects of market ups and downs and build a strong habit.
Reinvest and Review
Reinvest dividends where appropriate and review your portfolio once or twice a year. Rebalance if your asset mix drifts away from your target.
Index Funds and Tax in the UK
Stocks and Shares ISAs
Investments held in an ISA are free from income tax and capital gains tax. The overall ISA allowance remains £20,000 a year. Note the changes ahead for savers: from 6 April 2027, the annual ISA cash limit will be set at £12,000, within the overall annual ISA limit of £20,000 for under-65s, with the rest available for stocks and shares ISAs and other investment ISAs.
Pensions
Pension contributions usually receive tax relief, and investments grow free of income tax and capital gains tax. You can usually take up to 25% tax-free when you access your pension, subject to limits.
Tax Outside ISAs and Pensions
| Tax | 2026/27 Position |
|---|---|
| Dividend Allowance | £500 |
| Dividend tax rates | 10.75% / 35.75% / 39.35% |
| Capital gains annual exempt amount | £3,000 |
| Capital gains tax rates on shares | 18% (basic rate) / 24% (higher and additional rate) |
| Interest from bond funds | Taxed as savings income above allowances |
Dividend tax rose in April 2026, when the basic and higher rates rose by two percentage points from 6 April 2026; the additional rate stayed at 39.35%. From April 2027, savings income tax rates (which apply to interest from bond funds) will also rise to 22%, 42% and 47%.
Accumulation Units Are Still Taxable Outside an ISA
Even if dividends are reinvested automatically, they’re still treated as income for tax purposes outside an ISA or pension.
Index Fund Investing Strategies
Buy and Hold
Invest regularly and hold for the long term, ignoring short-term market movements.
Core and Satellite
Use a low-cost global index fund as your “core” and add smaller “satellite” holdings in specific regions, sectors or themes.
Lifestyle Investing
Gradually move from shares to bonds as you approach your goal date to reduce risk.
Rebalancing
Bring your portfolio back to your target mix once or twice a year, selling what has grown and adding to what has lagged.
Common Index Fund Mistakes
Selling During Market Falls
Panic selling locks in losses. Long-term investors often benefit from staying invested.
Chasing Last Year’s Winners
Sector or thematic funds that performed well recently may not continue to do so.
Ignoring Fees
Even small fee differences add up significantly over decades.
Over-Diversifying
Owning several funds that track similar indices can create overlap without adding diversification.
Not Using ISAs and Pensions
Investing outside tax wrappers can mean paying unnecessary tax on dividends and gains, especially as tax rates rise.
Investing Money You’ll Need Soon
Shares are best for long-term goals. Keep short-term money in cash savings.
Forgetting Currency and Concentration Risk
Check how much of your fund is in one country, sector or a handful of companies.
Index Funds: Quick-Start Checklist
| Step | Action |
|---|---|
| 1 | Build an emergency fund and clear high-interest debt |
| 2 | Set your investment goals and timeframe |
| 3 | Decide your risk level and asset mix |
| 4 | Choose an account (ISA, SIPP, workplace pension) |
| 5 | Compare platforms and total costs |
| 6 | Choose a low-cost, broadly diversified index fund |
| 7 | Set up regular monthly contributions |
| 8 | Review and rebalance once or twice a year |
| 9 | Stay invested through market ups and downs |
Final Thoughts
Index funds offer a simple, low-cost way to invest in the stock and bond markets. By tracking an index rather than trying to beat it, they provide broad diversification, transparency and fees that are usually far lower than those of active funds. Long-term data, including the SPIVA Europe scorecards, shows that most active equity funds underperform their benchmarks over 10 years.
To get started, set clear goals, choose a sensible mix of shares and bonds, and pick a low-cost, well-diversified fund such as a global index tracker. Use tax-efficient accounts like stocks and shares ISAs and pensions, especially with dividend tax rising in April 2026 and savings tax rising in April 2027. Invest regularly, keep costs low and stay patient. Remember that the value of investments can fall as well as rise, and if you’re unsure, consider getting regulated financial advice.
Frequently Asked Questions
1. Are index funds good for beginners?
Yes. Index funds are low-cost, diversified and simple to understand, making them a popular choice for beginners investing for the long term.
2. Can you lose money in an index fund?
Yes. Index funds rise and fall with the markets, so the value of your investment can go down as well as up.
3. Are index funds better than active funds?
Over 10 years, most active equity funds underperform their benchmarks according to SPIVA data, which is why many investors prefer low-cost index funds.
