The Basics of Index Funds and Passive Investing

What an index fund actually does

An index fund is a fund that aims to mirror the performance of a market, rather than trying to pick the winners within it. If an index tracks the largest companies listed on a particular market, the fund buys those companies in roughly the same proportions and holds them. Its job is not to be clever. Its job is to be accurate and cheap.

That is the whole idea behind passive investing. Instead of paying a manager to try to beat the market, you accept the market's average return and let time do the heavy lifting. You are not aiming for spectacular. You are aiming for reliable, low-cost, long-term compounding — which, for most households, turns out to be a very respectable goal indeed.

Broad index funds typically hold hundreds or thousands of companies across different countries and sectors. That built-in spread means no single company failing can derail your savings, which is a comfort most people only appreciate when a favourite share price tumbles.

The fee gap matters more than you think

Costs are the clearest difference between passive and active investing, and they compound just like returns do — except against you. A typical index fund might charge an ongoing charge figure of around 0.1% to 0.25% a year. An actively managed fund often charges somewhere between 0.75% and 1.5%, before any platform fee is added on top.

On a £50,000 pot, a 1% difference in annual charges is £500 a year. Left alone for twenty years, with growth on top, that gap can easily run into tens of thousands of pounds of lost value. The manager has to outperform by that much simply to break even for you — and most do not manage it consistently.

• Fund charge: the ongoing charge figure, or OCF, shown on the fund factsheet.
• Platform charge: what your ISA or pension provider takes, often a percentage of your balance.
• Trading and FX costs: smaller, but worth checking on funds holding overseas assets.

A cheap fund on an expensive platform is not a cheap investment. Always look at the total.

Choosing between tracker options

Not all index funds are identical, and a few details are worth understanding before you commit.

• Accumulation versus income units. Accumulation units reinvest dividends automatically, which suits most long-term savers inside an ISA or pension. Income units pay dividends out to you.
• Physical versus synthetic. Physical funds buy the underlying shares. Synthetic funds use derivatives to replicate the index and can track slightly more tightly, but add counterparty risk.
• Tracking difference. This is the real measure of how closely a fund follows its index after all costs, and it is more useful than headline charges alone.
• Breadth. A fund tracking a narrow sector or a single country carries far more concentration risk than a broad global one.

For most households, a single low-cost global index fund held for the long term does the job perfectly well. Simplicity is a feature, not a compromise.

Where to hold them in the UK

The wrapper you choose can matter as much as the fund itself.

• Stocks and shares ISA. You can shelter up to £20,000 per tax year from income tax and capital gains tax. Growth and dividends stay tax-free.
• Workplace pension or SIPP. Contributions attract tax relief, and your money grows free of UK tax on dividends and gains. Access normally starts from age 57, rising to 58 from 2028.
• General investment account. Use this once tax-advantaged allowances are full, but be aware of dividend and capital gains rules.

If you hold index funds inside an ISA or pension, choose accumulation units so dividends are reinvested without you lifting a finger. And check whether your provider charges a percentage or a flat fee — flat fees suit larger pots, percentages suit smaller ones.

Building a plan you will actually keep

The best portfolio is the one you do not abandon in a bad month. A workable approach looks like this:

• Build a small emergency fund first — three to six months of essential spending.
• Clear expensive debt, particularly credit cards and overdrafts, before investing.
• Set up a monthly direct debit into your chosen fund, on a fixed date, and increase it when your pay rises.
• Check in once or twice a year. Not daily, and ideally not during a market fall.
• Keep the same fund for a long time. Switching in and out is how passive investors accidentally become bad active ones.

Regular monthly investing also smooths out the entry price, so you buy more units when markets are down and fewer when they are up. It is a quiet, unglamorous advantage.

Sticking with it when markets wobble

Index funds fall when markets fall. That is not a flaw — it is the deal you signed up to in exchange for lower costs and no manager risk. What matters is your time horizon. Money you need within five years belongs in cash, not equities.

If a 20% drop would push you to sell, hold a smaller proportion in shares. If you can leave it alone and keep contributing, history suggests patience is rewarded — though nothing is guaranteed, and past performance is never a promise.

Above all, keep it dull. Boring, cheap, automatic investing is one of the most reliable ways an ordinary UK household can build wealth over a working lifetime.