An index fund, also called a tracker, holds the shares or bonds in a market index so that its return follows that index, minus costs. Before buying one, check five things: the total cost, how closely it has tracked, how it copies the index, which unit class you are getting and which tax wrapper it will sit in.
Trackers are now a large slice of the UK fund market. They held £455 billion at the end of August 2026, 26.0% of industry funds under management, and took net retail inflows of £1.6 billion that month while active funds lost £707 million, according to the Investment Association. The shift was already under way: trackers attracted £12.8 billion in 2025 after a record £27.6 billion in 2024, and ended 2025 with a 25.2% share (Investment Association).
What an index fund does, and what it does not
An index is a rulebook. A provider such as MSCI or FTSE Russell decides which securities belong in it and how much weight each gets, usually by market value. A tracker follows those rules. It does not pick companies it likes or avoid ones it fears, so you get that market’s return, good years and bad, less the fund’s costs.
That makes the choice of index the biggest decision you make. A UK index tracker gives you UK-listed companies only. The MSCI World index holds 1,249 companies across 23 developed markets, with the United States at 72.94% of the index and the 10 largest holdings at 27.85% on 30 September 2026 (MSCI factsheet). Two funds with similar names can own very different things, so read the name of the index in the fund documents rather than the fund’s marketing name. Our guide to global tracker funds compares the main world indices side by side.
Cost: the ongoing charge is where you start, not where you finish
The ongoing charges figure (OCF) is the fund’s yearly running cost, expressed as a percentage and taken from the fund rather than billed to you. It is the number most often quoted, and it leaves out several things you also pay.
| Cost | Who takes it | Where to find it |
|---|---|---|
| Ongoing charges figure | Fund manager, deducted inside the fund | Factsheet and fund disclosure document |
| Transaction costs | Incurred inside the fund when it buys and sells holdings | Fund cost disclosure |
| Platform or account fee | The firm that holds your investments | Its charges page and your annual cost statement |
| Dealing charge | The firm that places your order | Contract note |
| Bid-offer spread | The market, on exchange-traded funds | Live buying and selling prices |
| Stamp duty reserve tax | HMRC | Not charged when you buy fund units or shares from the fund manager |
The stamp duty point comes from GOV.UK: buying shares in an open-ended investment company (OEIC) or units in a unit trust directly from the fund manager does not attract the tax. For the rest, the FCA requires investment firms to give you a reasonable estimate of all costs and charges before you invest and, where they provide an ongoing service, a personalised annual statement of the costs actually incurred, totalled as a cash amount and as a percentage (FCA Handbook, COBS 6.1ZA.14B). That annual figure is the most useful one to compare across providers.
Fund disclosure documents are themselves changing. The FCA’s regime for consumer composite investments began on 6 April 2026 with an optional transition, and its rules come into effect on 8 June 2027 (FCA PS25/20). Until then, expect some funds to use the old documents and some the new ones.
Tracking difference: what you actually received
Tracking difference is the gap between the fund’s return and the index’s return over a period, usually a calendar year. Because the fund pays costs and the index does not, the gap is normally negative and roughly the size of the fund’s costs. A fund that trails its index by much more than its ongoing charge is losing money somewhere: trading costs, cash held back from the market, tax withheld on overseas dividends or an imperfect sample of the index. A fund that trails by less may be earning income from lending its holdings to other market participants.
Tracking error is a different measure. It shows how much that gap varies from period to period. A low tracking error means the fund moves closely with the index day to day; it says nothing about whether the fund lags by a steady amount each year. Compare tracking difference over several years, against the exact index version the fund names (usually a net total return index, which assumes dividends are reinvested after withholding tax). Past performance is not a guide to future returns.
Replication: how the fund copies the index
- Full replication. The fund holds every security in the index at its index weight. This suits indices with a manageable number of liquid holdings.
- Sampling, or optimised replication. The fund holds a representative subset. It is used for very broad indices: the FTSE All-World had 4,204 constituents on 30 September 2026 (FTSE Russell factsheet). Sampling cuts trading costs but can widen tracking error.
- Synthetic replication. The fund holds a basket of assets and uses a swap with a bank to receive the index return. It is more common among exchange-traded funds and adds counterparty risk, which the fund’s documents should explain along with the collateral it holds.
Some trackers also lend out part of their holdings for a fee. Check what share of that income the fund keeps and what collateral it takes.
Accumulation or income units
Most index funds come in two versions. Income units pay dividends or interest to you in cash. Accumulation units keep the income inside the fund and reinvest it, so the unit price rises instead. The holdings and the ongoing charge are usually identical.
Outside a tax wrapper, the difference matters at tax time. HMRC says reinvested amounts “are taxed as income accruing to investors in the same way as if they had been distributed” (HMRC Investment Funds Manual, IFM03120). Those notional distributions are then treated as allowable expenditure for capital gains tax, which raises your base cost (HMRC Capital Gains Manual, CG57707). Keep the annual tax vouchers. Inside an ISA or a pension none of this affects your tax.
Fund or ETF?
The same index can be tracked by an open-ended fund (an OEIC or unit trust) or by an exchange-traded fund (ETF). Funds are bought from the manager and FCA rules say “all deals must be at a forward price” (FCA Handbook, COLL 6.3.9R), so you do not know the exact price when you place the order. ETFs trade on a stock exchange during market hours at a live price, with a spread between buying and selling. The practical differences are price certainty, dealing costs, stamp duty and tax status, set out in our comparison of funds and ETFs and in our explainer what is an ETF.
Where index funds sit: ISAs, pensions and general accounts
The wrapper changes the tax, not the fund. In the 2026 to 2027 tax year you can save up to £20,000 in ISAs (GOV.UK), and a stocks and shares ISA can hold “unit trusts and investment funds” (GOV.UK). You do not pay tax on dividends from shares in an ISA (GOV.UK) and you do not pay capital gains tax on investments held in one (GOV.UK).
Index funds are also widely held in self-invested personal pensions. Income from investments held for a registered pension scheme is free of income tax (Finance Act 2004, section 186) and gains on those investments are not chargeable gains (Taxation of Chargeable Gains Act 1992, section 271), but you pay tax when you take money out (GOV.UK). The pension annual allowance is £60,000 this tax year (GOV.UK).
In a general investment account for 2026 to 2027, dividends above the £500 dividend allowance are taxed at 10.75%, 35.75% or 39.35% depending on your income tax band (GOV.UK). Gains above the £3,000 annual exempt amount (GOV.UK) are taxed at 18% within the basic rate band and 24% above it (GOV.UK). If the fund is domiciled overseas, check that it has UK reporting fund status: without it, gains on disposal are taxed “as if those gains were income” (HMRC Investment Funds Manual, IFM13100). Our guide to tax-efficient investing in the UK covers how the wrappers fit together.
A checklist before you buy
- Name the index and look at what it holds: countries, number of companies and the weight of the largest positions.
- Add up the total cost: ongoing charge, transaction costs, platform fee and any dealing charge or spread.
- Compare tracking difference over several calendar years against the index version the fund names.
- Check the replication method, any securities lending and, for swap-based funds, the counterparty and collateral.
- Choose accumulation or income units, and know how each is taxed if held outside a wrapper.
- Check the domicile and, for an overseas fund held outside an ISA or pension, its reporting fund status.
- Decide the wrapper before the fund: ISA, pension or general account.
An index fund removes the risk that a manager trails the market. It does not remove market risk: when the index falls, the fund falls with it. For the latest data on what UK investors are buying, see our report on UK fund flows in August 2026.
