How to pick the right index fund 

Choosing an index fund is like standing in that narrow stretch of the cereal aisle reserved for rabbit-food bran flakes.

Any broad index fund is good for your financial health, much like any bran flake beats fluorescent sugar hoops. But, there are grades of good.

The boxes promise fibre. The backs are crammed with percentages and acronyms. That's where the real differences live.

This guide helps you read the back properly, so when you do something sensible for your future self, you don't accidentally buy the bran flakes that somehow contain more sugar than the technicolour E-number festival next door.

What is an index fund?

An index fund is a type of investment fund that simply tracks a market index, like the FTSE 100 or the S&P 500.

Instead of trying to pick the best-performing companies, it buys all (or most) of the companies in that index. The goal isn't to beat the market, but to match it. 

That list might contain, for example, the 500 largest companies in America, or the investable stock market of the entire planet, and the fund simply follows it, owning whatever the rules include and adjusting itself whenever the list changes.

The fund's job is dull in the most comforting way imaginable: keep close to the list, keep costs tight, avoid unnecessary fuss.

Your job, unfortunately, is to decide which list deserves your money in the first place.

How do I pick an index?

You pick an index by asking yourself which slice of capitalism you're willing to sit with, year after year, through its enthusiasms and its sulks, without reaching for the "sell" button the moment it behaves badly.

Because it will behave badly from time to time, and it rather enjoys reminding investors that confidence is not the same thing as control.

The American obsession: S&P 500 funds

(S&P 500)

Begin, if you like, with the S&P 500, which is a club of 500 of the largest listed companies in the United States. Together, those companies represent a huge chunk of the US stock market. When people say "the US market was up 1% today", they're often talking about this index.

What you acquire here is a thick slab of American corporate power, priced in dollars and, in recent years, dominated by tech, financials and healthcare.

It's performed very strongly since the year 2000, but this hasn't always been the case: anyone who has been around long enough can recall periods where it did little more than mark time and test the faith of its admirers.

The usual suspects: developed world funds

(MSCI World, FTSE Developed Index)

Move on to something like MSCI World or FTSE Developed Index, and you still find yourself holding a great deal of America – around 70%, in fact.

That's because company weightings are determined by their value, also known as their market capitalisation. And, like it or not, most of the world's biggest companies in the developed world are American.

However, alongside the unavoidably large slice of Uncle Sam, with a developed world index fund, you'll also acquire Europe, Japan, Australia and other developed markets with established financial systems and relatively stable governments.

It's a way to spread your risk across multiple advanced economies. Growth may be steadier rather than spectacular, but these markets have a history of weathering downturns and political change.

The full spread: global funds

(FTSE All-World, MSCI ACWI)

This is the "own the planet" option.

A full global index fund doesn’t just buy the US, Europe and Japan. It also includes emerging markets – countries like China, India, Brazil and Taiwan.

In simple terms, an "emerging market" is an economy that isn't too rich, isn't too poor, and is on the way to becoming what we in the West would class as "developed".

Adding them in through funds like FTSE All-World or MSCI ACWI increases diversification and gives you exposure to higher-growth regions of the world.

But it also increases volatility, as emerging markets tend to be more sensitive to political change, currency swings and economic shocks.

In practice, global index funds typically allocate around 10–15% to emerging markets, with the rest in developed economies.

Why so many different global funds? Well, that's because different index providers use slightly different rulebooks.

All aim to represent the global stock market, covering developed and emerging economies, but they don’t define "global" in exactly the same way. They may classify certain countries differently, include a different number of companies, or apply slightly different size thresholds and free-float rules.

The everything-included version: all-cap funds

(FTSE Global All Cap)

Companies are usually grouped into large, mid and small cap, depending on the total value of a company on the stock market. An all-cap fund focuses on all three, across developed and emerging markets.

Choose FTSE Global All Cap, for example, and you move beyond the corporate aristocracy into the industrious middle classes of capitalism.

You'll not only be investing in the titans whose logos glow on sky scrapers, but the mid-sized machine tool manufacturers in Bavaria that make components for factories you will never visit, the regional Canadian banks that finance housing developments in provinces you can't spell, the South Korean suppliers of specialised semiconductors and industrial bearings whose quarterly reports are read mainly by accountants and their mothers.

It's closer to owning the investable equity market in something approaching its full ecological diversity, which means the ride will occasionally feel untidy. Smaller firms tend to rise faster in good times and fall harder in bad ones.

The filtered variants: ESG funds

(FTSE4Good, MSCI ESG Screened)

Then there are the screened versions, such as FTSE4Good or MSCI ESG Screened, which apply rules about environmental or governance criteria and alter sector weights in the process. 

The sort of investor drawn to these is often the same conscientious soul who pauses mid-stride to relocate a snail from a pavement before the commuter rush, who reads the small print on recycling bins, who believes that capital allocation is a moral act as well as a financial one.

There is nothing unserious about that; it simply means you're accepting a portfolio that excludes certain industries, and therefore will not behave identically to its unscreened sibling.

Just be aware, your idea of which companies make the world a better place may not be the same as the fund manager's.

For example, a sustainable fund might exclude oil producers, but happily include a company like Nvidia, whose chips power everything from AI tools to military systems. Another ESG fund might exclude defence companies entirely. A third might allow both, but screen out tobacco and gambling.

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How to compare two funds tracking the same index

Once you've chosen the index – the "what" – the next job is to pick the fund – the "how".

This is because two funds can track the same index but still deliver slightly different results, mainly due to fees and how closely they track it.

They should both behave almost identically. If the S&P 500 rises 10%, both funds should rise by roughly the same amount. But in practice, small differences creep in.

Think of it like buying the same product from two different shops. The contents are the same. The price and packaging might not be.

With index funds, those differences usually come down to fees, how closely the fund tracks the index, and how it’s structured.

Over one year, the gap may look tiny. Over decades, those details can start to matter.

OCF

The ongoing charges figure is the annual percentage taken from the fund’s assets, a small number that applies itself with metronomic consistency. This is sometimes also called a TER – a Total Expense Ratio.

Given time, modest percentages become immodest sums.

• Take £10,000 compounding at 6% for 30 years.
• At 0.2% a year, you reach roughly £54,000.
• At 1% a year, you arrive nearer £43,000.

The difference is the cumulative effect of allowing a slightly larger slice to be removed every year.

For broad equity index funds, an OCF around 0.05% to 0.25% is standard.

Check it as a matter of routine. Paying 1% for exposure that can be obtained for a fraction of that should prompt a raised eyebrow and, in most cases, a straightforward switch.

Here's the effect that different fees can have on your overall compounded value, assuming a £10,000 annual investment for 30 years, with a 7% average annual return:

Tracking difference vs tracking error

When comparing two funds tracking the same index, look first at tracking difference. This is the gap between the fund's return and the index's return over a given period.

If the index rose 10% and the fund delivered 9.8%, the shortfall is 0.2%. That 0.2% is what you actually paid in slippage.

For a mainstream equity index fund with an OCF of 0.10% to 0.25%, a tracking difference that broadly matches the fee over three or five years is entirely normal. 

If a fund charging 0.15% is lagging by 0.40% or 0.50% year after year, that is a red flag. Something in the implementation is costing more than advertised.

Tracking error is different, measuring how much that gap moves around from month to month. For long-term investors building a core holding, variability matters less than the size of the persistent gap.

Open the performance table on the factsheet. Compare "fund net of fees" with "benchmark" over one, three and five years. If the difference looks close to the OCF, you're in the realm of normal; on the other hand, if it consistently exceeds it by a wide margin, walk away.

Still not sure where to start? If you're more of a visual learner, check out our index funds for beginners video course. Hosted by Damien from Damien Talks Money, it's totally free, and takes less than two hours.

ETF or OEIC?

An ETF trades on the stock exchange. You buy it like a share, meaning there is a bid price, an offer price, and a spread between the two.

An OEIC by contrast, is a type of mutual fund that deals once a day at a single price based on the value of its holdings. You place the order and receive that end-of-day price.

• ETFs suit platforms with low dealing fees and investors who don't mind exchange pricing 
• OEICs suit regular monthly investing and platforms that allow free fund trades.

Inside an ISA or pension, over decades, the structure itself matters far less than the fee and the tracking record.

If you're unsure on the differences between mutual funds and ETFs or which one's right for you, this guide breaks it all down.

Spread and dealing friction

On a large, mainstream index ETF, the spread should usually sit around 0.05% to 0.20% in normal conditions. Again, that means the difference between the price you can buy at, and the price you can sell at.

If you're seeing 0.50% or more on a broad global or S&P 500 tracker, something is off. Either liquidity is thin or the market is unsettled.

That percentage is what you effectively give up the moment you trade.

Next, check your platform's dealing fee. £5 on a £500 trade is 1%; on a £5,000 trade it's 0.1%. Some platforms don't charge a dealing fee at all.  

Foreign exchange (FX) charges can also bite. If your ETF trades in dollars and your platform clips 0.75% for conversion, that can dwarf the spread.

Practical rule:

• Use large, liquid ETFs for core holdings
• Avoid trading during obvious market stress
• Keep dealing costs sensible relative to the amount invested – or try to avoid them altogether.

Unlike the OCF and the tracking difference that stay with you for years, the spread is a one-off friction.

The stuff you can check without obsessing over

Think of these variables as being like page five of your dog's blood test results – interesting, slightly technical, worth glancing at, but rarely the thing that decides whether everything is fine.

Physical vs synthetic

A physical fund actually owns the shares in the index.

A synthetic fund does not necessarily hold all the shares. Instead, it uses a derivative – usually a swap – which is a contract with a bank. In that contract, the bank agrees to pay the fund the return of the index, and the fund gives the bank something else in exchange.

In simple terms, it is a legally binding deal that says, “You deliver the index return, we'll settle up with you daily.”

Why it matters

Physical replication is simple. The fund owns the assets.

Synthetic replication relies on that swap contract working as promised. It can track very tightly, especially in harder-to-access markets, but it introduces an extra layer between you and the underlying companies.

Where to check

Open the fund factsheet. Look for a line that says:

• "Replication method"
• "Methodology"
• "Physical" or "Synthetic"

It's usually near the top of the first or second page.

What you're looking for

If it says physical, that is straightforward. If it says synthetic, check that it mentions collateral and swap exposure.

How much to worry

Low to medium. For mainstream developed-market equity trackers, physical is common and easy to understand. Synthetic isn't inherently dangerous – it's regulated, usually collateralised and is standard for some types of funds – but if you prefer fewer moving parts, physical is the simpler option.

We've explained this in loads more detail here.

Securities lending

This is when the fund temporarily lends some of the shares it owns to other investors – usually large financial institutions – in exchange for collateral (something of equal or greater value held as security).

In return, the fund earns a small fee.

That extra income can help offset costs and slightly improve returns. But it also adds a small layer of counterparty risk – if the borrower failed, the collateral is there to protect investors.

Most large index funds do this to some extent, and the risk is generally considered low, but it's worth knowing it happens.

Where to check

In the factsheet or annual report, search for:

• "Securities lending"
• "Stock lending"
• "Lending revenue"

Many providers also have a dedicated "Securities lending policy" page on their website.

What you're looking for

• Does the fund lend?
• What percentage of lending revenue goes back to investors?

If the fund keeps most of the revenue for investors, that's normal.

How much to worry

Low. This is common practice among large providers. 

Fund size and liquidity

Fund size is how much money is invested in it. Liquidity is how easily you can trade it.

Where to check

On the factsheet, look for:

• "Fund size"
• "Assets under management" or "AUM"

For ETFs, your platform may also show:

• Average daily volume
• Bid-offer spread.

What you're looking for 

As a rough guide:

• Billions in assets = established, mainstream
• Tens of millions = small, possibly niche.

If it tracks a major index and has been around for years with substantial assets, liquidity is unlikely to be a problem.

How much to worry

Medium for tiny, niche ETFs. Low for large, mainstream trackers.

Bottom line: checklist before buying an index fund

Before you make a decision, try asking yourself these questions and running through the checkpoints on the list.

Question	What to check	What "normal" looks like	What to watch out for
Have I chosen the right index?	Exact benchmark name on the factsheet	Matches the exposure you actually want (e.g. S&P 500 = US only; MSCI World = developed markets; FTSE All-World = developed + emerging)	Assuming "global" means everything, when it may exclude emerging markets or smaller companies
Is the fee sensible?	OCF/TER	~0.05%–0.25% for a broad equity tracker	Anything approaching 1% for plain index exposure
Does it actually track properly?	Compare fund return vs benchmark over 1, 3 and 5 years	Tracking difference roughly matches the fee	Persistent gap much wider than the OCF
Are dealing costs under control?	Platform fee, FX charge	Spread ~0.05%–0.20% on major ETFs; reasonable platform fees (or none at all)	FX charges of 0.75%+, dealing fees that exceed £5
Is the structure straightforward?	ETF or OEIC; replication method	Physical replication; clear documentation; held inside ISA or pension if possible	Synthetic with unclear collateral or complexity you don’t understand
Is it established and liquid?	Assets under management (AUM); trading volume	Billions in assets for mainstream trackers	The AUM seems unusually small for a major index, or trading volume looks thin

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