Index funds explained for UK beginners
How index funds work, why costs matter, the risks involved and which UK tax wrappers you can hold them in.
An index fund is one of the simplest ways to invest in the stock market. Instead of trying to pick winning companies, it buys the companies in a market index and holds them, usually at a low cost.
This guide explains how index funds work, what they cost, the risks, and how UK investors can hold them in tax-efficient accounts such as a Stocks and Shares ISA or a pension.
In 30 seconds
- An index fund tracks a market index rather than picking individual shares.
- Low costs matter because they reduce your return every year.
- Values go up and down; index funds suit money you can leave for five years or more.
- In the UK, ISAs and pensions can shelter investments from tax.
- Compare funds on total cost, what the index holds and how closely the fund tracks it.
What an index fund is
A market index is a list of companies chosen by a set rule, such as the largest companies listed in one country, or a broad mix of companies across the world. An index fund, sometimes called a tracker fund, aims to match the return of that list by holding the same companies in roughly the same proportions.
Because the fund follows a rule, it does not need a team of managers researching which shares to buy. This is called passive investing. Active funds, by contrast, have managers who try to beat the market. They usually charge more, and many do not beat their index after costs over long periods.
Index funds can be traditional funds (unit trusts and OEICs) or exchange-traded funds (ETFs). For a long-term investor, the form matters less than the cost and what the index holds.
Why costs matter so much
Every fund has an ongoing charge, shown as a percentage of your investment each year. On top of that, the platform you use may charge its own fee, either a percentage or a fixed amount. There can also be dealing charges and transaction costs inside the fund.
A difference of less than one percentage point looks small. Over twenty years it adds up, because the cost comes out every year and the money lost can no longer grow.
These are illustrations, not forecasts. Real returns vary and can be negative. With everything else equal, the cheaper option ends up with more money.
How to compare costs
- Ongoing charges figure (OCF): the fund’s own yearly charge, shown on its factsheet.
- Platform fee: what the platform charges to hold your investments. A percentage fee suits small balances; a fixed fee can be cheaper for large balances.
- Dealing charges: some platforms charge each time you buy or sell, especially for ETFs.
Diversification and risk
One index fund can hold hundreds or thousands of companies. If one company fails, the effect on the fund is small. This spread is called diversification, and it is one of the main reasons people use index funds.
Diversification does not remove market risk. When the whole market falls, as it has several times in recent decades, an index fund falls with it. Falls of a third or more have happened, and recovery has sometimes taken years.
| Index type | What it holds | Main risk to understand |
|---|---|---|
| Single-country shares | Large companies in one country | Tied to one economy and a few large sectors |
| Global shares | Companies across many developed countries, sometimes emerging markets too | Currency movements; can be weighted heavily to one large market |
| Bonds | Loans to governments or companies | Values fall when interest rates rise; lower expected growth |
| Multi-asset | A fixed mix of shares and bonds | Mix may not match your own attitude to risk |
Time horizon: when index funds make sense
Because prices move up and down, index funds suit money you will not need for at least five years, and ideally longer. Money you might need sooner, such as an emergency fund or a house deposit due next year, is usually better in cash.
Regular monthly investing spreads your purchases over time. You buy more units when prices are low and fewer when prices are high. It does not guarantee a better result, but it avoids putting everything in at one bad moment.
Example: if you invest £150 a month for 20 years and the investment grows at an assumed 5% a year after costs, you would pay in £36,000 and could end with about £61,655. This assumes a steady return. Real returns will vary, and the final amount could be higher or much lower.
UK tax wrappers for index funds
Where you hold a fund affects how much tax you pay. The UK has several accounts, often called wrappers, that shelter investments from tax.
- Stocks and Shares ISA: growth and income are free of UK Income Tax and Capital Gains Tax. There is a yearly allowance shared across all your ISAs.
- Lifetime ISA: for people within the eligible age range saving for a first home or for later life. The government adds a bonus, but there is a charge if you withdraw for other purposes. Check the current rules carefully.
- Workplace pension: many workplace schemes invest in index funds by default (see our guide to retirement saving). Contributions get tax relief and often an employer contribution, but the money is normally locked away until minimum pension age.
- Self-invested personal pension (SIPP): a personal pension where you choose the investments, with tax relief on contributions and the same access restrictions as other pensions.
- General investment account: no yearly limit, but dividends and gains can be taxable once they go above the relevant allowances.
Allowances and rules for these accounts are set by the government and change. Check the current figures on GOV.UK before you decide how much to put into each. Our UK tax basics guide explains how investment income is taxed.
How to get started
- Check your foundations
Clear expensive debt and build an emergency fund before investing. Investing money you might need next month forces you to sell at the wrong time.
- Decide on the wrapper
Most beginners start with their workplace pension and a Stocks and Shares ISA. Consider how soon you might need the money.
- Choose a platform
Compare total costs for your expected balance, the range of index funds offered, and whether the firm is authorised by the FCA. Check the FCA register.
- Read the fund documents
Look at the index the fund tracks, the ongoing charge, the tracking difference and the risk indicator in the Key Information Document.
- Set up a regular contribution
Choose an amount you can keep paying in bad months as well as good ones.
- Leave it and review yearly
Avoid checking daily. Review once a year to confirm the fund and costs still suit you.
Official sources
- Financial Conduct Authority (FCA): check that a platform is authorised, using the FCA register.
- GOV.UK: current ISA allowances and pension tax relief rules.
- Financial Services Compensation Scheme (FSCS): what investment protection covers and its limits.
Frequently asked questions
Can I lose money in an index fund?
Yes. An index fund follows the market it tracks, so when that market falls, the fund falls too. Losses can be large over short periods. This is why index funds are usually suited to money you will not need for at least five years.
What is the difference between an index fund and an ETF?
Both can track an index. A traditional index fund is bought and sold at one price per day through a platform. An exchange-traded fund (ETF) trades on a stock exchange during the day like a share. Costs, dealing charges and minimum amounts can differ, so compare the total cost on the platform you plan to use.
How much do I need to start?
Many UK platforms accept small regular monthly contributions, and some let you start with a modest lump sum. Check each platform’s minimums and whether it charges a fixed fee, which weighs more heavily on small balances. Build an emergency fund before you invest.
Do I pay tax on index funds held in a Stocks and Shares ISA?
Growth and income inside an ISA are free of UK Income Tax and Capital Gains Tax, and you do not need to report them on a tax return. There is a yearly limit on how much you can pay into ISAs. Check the current ISA allowance on GOV.UK.
Should I pick a global fund or a UK fund?
This page cannot recommend a specific fund. In general terms, a fund that tracks a single country depends on that country’s companies, while a global index spreads money across many countries and sectors. Read the fund factsheet to see what the index holds before you decide.
How do I know if a fund really tracks its index well?
Look at the tracking difference, which compares the fund’s return with the index over the same period. A small and consistent gap suggests the fund follows its index closely after costs. Factsheets and platform research pages usually show this information.
