Every morning, before the first trade is placed, financial desks across London, Frankfurt, Tokyo and New York glance at the same numbers. Not individual stock prices — there are simply too many of those to process at a glance — but indices. A single figure, rising or falling, that distils the collective movement of dozens or hundreds of companies into one instantly readable measure. That figure is what most people mean when they say “the market is up” or “markets fell sharply today.” The index has become so embedded in how we talk about financial markets that it is easy to take for granted just how elegant and useful a construction it really is.

For a retail trader in the UK, understanding indices is not merely academic background. The FTSE 100 is the market in the eyes of the financial press, the government, and the public. It is the primary benchmark against which professional fund managers are measured, the underlying reference for billions of pounds worth of derivative contracts, and the starting point for a great deal of trading and investment activity. Before you can trade an index, hedge against it, or use it to assess your own portfolio performance, you need to understand what it actually is — and what it is not.

What an Index Actually Measures

The concept behind a stock market index is straightforward: select a basket of representative companies, track their prices, and express the combined movement as a single number. That number tells you, at any given moment, whether the companies in the basket are worth more or less in aggregate than they were at some defined starting point. It is a barometer — a compressed, real-time summary of market sentiment across a broad swathe of the economy.

Consider the challenge without one. The London Stock Exchange lists well over two thousand companies. Checking each one individually to form a view on the direction of the market would be both impractical and misleading — a handful of very large companies moving sharply would be drowned out by noise from hundreds of smaller ones. The solution is to pre-select a curated group of the most significant companies, weight their contributions appropriately, and track them continuously. The result is an index: a manageable, meaningful, and constantly updated picture of where the market stands.

It is worth being precise about what an index measures and what it does not. An index reflects market prices, not underlying economic reality. When the FTSE 100 rises, it means that investors, in aggregate, have bid up the prices of the companies within it — usually because they expect earnings to improve, conditions to become more favourable, or risk to recede. When it falls, the reverse is true. The index does not measure the economy directly; it measures the expectations of market participants about the economy’s future. That is a subtle but critical distinction, and one that matters enormously when interpreting index movements in the context of real-world events.

There is a further subtlety specific to the UK’s flagship index. The FTSE 100 is dominated by large multinational corporations — oil majors, global banks, mining groups, and pharmaceutical companies — many of which earn the vast majority of their revenue outside the United Kingdom. This means the FTSE 100 can, and often does, move in ways that appear disconnected from the domestic UK economy. When sterling falls sharply, for example, the FTSE 100 often rises: the overseas earnings of its constituent companies are worth more in pound terms. This international character is one of the index’s most distinctive features and something every UK trader should keep firmly in mind.

From the FT 30 to the Footsie: Four Decades of History

The modern era of UK index tracking begins on 3 January 1984, when the FTSE 100 was launched at a base level of 1,000 points. But the practice of summarising the market in a single figure goes back further. Before the FTSE 100 existed, the primary gauge of the London market was the Financial Times Ordinary Share Index — commonly known as the FT 30 — which tracked just thirty leading industrial companies. The FT 30 was maintained by editors at the Financial Times rather than by a formal index methodology, which gave it a somewhat subjective character and limited its usefulness as a trading benchmark.

By the early 1980s, the growing sophistication of institutional trading and the emergence of financial derivatives created a pressing need for something more rigorous: a broader, more representative index whose value could be calculated in real time and used as the basis for futures and options contracts. The London Stock Exchange worked with the Financial Times to create a new benchmark covering one hundred companies. The name FTSE — Financial Times Stock Exchange — acknowledged the joint venture, though in a small piece of early institutional politics, the Stock Exchange initially referred to the product simply as the “SE 100,” without the FT prefix. The Financial Times’ involvement in the official launch was ultimately recognised as commercially valuable, and the familiar FTSE branding prevailed.

The index found its footing quickly. By 1986, Margaret Thatcher’s programme of financial deregulation — the so-called Big Bang — had transformed the City of London, and a wave of popular privatisations, including British Telecom, British Gas, and British Airways, brought millions of new retail investors into the market for the first time. The FTSE 100 became a household name almost overnight, appearing on the evening news and in daily newspaper headlines as the common measure of whether Britain’s economic fortunes were rising or falling.

The index’s first great test came in October 1987. On the morning of Monday the 19th — Black Monday, as it became known — global stock markets collapsed with extraordinary speed. London markets had been closed the previous Friday due to the Great Storm, and when they reopened, the selling was ferocious. The FTSE 100 fell 10.8% on the day and a further 12.2% the following session, wiping out more than a fifth of the market’s value in just two trading days — the largest two-day decline since the Wall Street Crash of 1929. The immediate cause was a combination of overvalued stocks, rising US interest rates, and the then-novel phenomenon of computerised programme trading, which automatically triggered selling when certain loss thresholds were reached, accelerating the downward spiral. Recovery, when it came, was relatively swift: by 1989 the index had reclaimed its pre-crash highs, and those who had held their positions through the worst of the panic were eventually rewarded.

The 1990s brought sustained growth, interrupted by Black Wednesday in September 1992 when sterling’s forced exit from the European Exchange Rate Mechanism briefly unsettled markets, though it ultimately proved beneficial for the FTSE 100’s internationally exposed companies. By the end of 1999, the index had reached 6,930 — nearly seven times its starting level — driven by the technology boom and a broad bull market that seemed at the time as though it might continue indefinitely. It did not. The dot-com crash of 2000 to 2003 pushed the index below 4,000, and it would not reclaim its millennial high for another fifteen years.

The 2008 financial crisis delivered the sharpest single-year loss in the index’s history — a fall of 31% as the collapse of Lehman Brothers triggered a global credit crisis. The index recovered 22.1% the following year as central banks intervened aggressively and confidence slowly returned. More recent shocks — the Brexit referendum of 2016, the Covid-19 pandemic in 2020, and the inflationary surge that followed — each left their mark on the index, though the underlying pattern across four decades has been one of recovery and, for patient investors, growth. Accounting for dividends reinvested, the annualised return on the FTSE 100 since its 1984 launch has been approximately 8.6% per year — well above the long-run rate of inflation.

How the Index Is Built and Maintained

Understanding index construction is not merely academic — it determines exactly what you are buying exposure to when you trade an index, and helps explain why two indices tracking ostensibly the same market can behave quite differently from one another. The FTSE 100 is maintained by FTSE Russell, a subsidiary of the London Stock Exchange Group, and follows a set of published ground rules that govern every aspect of how the index is compiled, weighted, and revised.

Membership of the FTSE 100 is open to the 100 largest companies listed on the London Stock Exchange by full market capitalisation, subject to meeting certain eligibility requirements including minimum liquidity thresholds, a minimum free float of 10%, and a UK nationality assignment under FTSE Russell’s classification rules. Crucially, companies need not be British in any operational sense — the requirement is to be listed on the London Stock Exchange. HSBC, Shell, Rio Tinto, Antofagasta, and many other FTSE 100 constituents earn the majority of their revenues far beyond the UK’s borders. The index is, in that sense, a reflection of what London attracts as a global financial centre as much as a measure of British economic activity.

The weighting of each constituent is determined using a free-float market capitalisation methodology. Free-float market capitalisation refers not to the company’s total market value, but specifically to the value of shares that are genuinely available for trading by public investors — excluding shares held by company insiders, strategic investors, governments, or other parties whose holdings are effectively locked in and not available to the market. A company might have a substantial total market capitalisation but a relatively modest free float if, for example, the founding family retains a large controlling stake. FTSE Russell rounds the free float factor for each constituent to the nearest multiple of 5%, and this adjusted figure is used in the index calculation formula.

The practical consequence of free-float weighting is that the largest companies exert a disproportionate influence on the index’s daily movement. As of early 2026, HSBC sits at the top of the FTSE 100 by market capitalisation, typically accounting for somewhere between 5% and 8% of the index’s total weight. AstraZeneca, Shell, Unilever, and a handful of other mega-caps follow closely behind. A 2% move in any one of these heavyweights can shift the index by as much as several points in isolation, while a 5% move in a small-cap constituent near the bottom of the index will barely register in the headline number. This is not a flaw in the design — it reflects where the real money is concentrated — but it is something that traders must keep in mind when interpreting index movements.

Membership of the index is reviewed every quarter, in March, June, September, and December. The market capitalisation cut-off date for each review falls on the Tuesday before the first Friday of the review month, giving FTSE Russell a snapshot of company rankings. The results of each review are announced approximately a week before any changes take effect, which are then implemented after the close of business on the third Friday of the review month. This publication of pending changes in advance is significant for traders: the announcement of a company’s promotion to the FTSE 100 typically triggers buying pressure as tracker funds and index-linked products must purchase the new constituent, while a demotion triggers the reverse. These events can create short-term trading opportunities around the announcement and implementation dates.

Companies removed from the index between quarterly reviews — due to a takeover, delisting, or other structural event — are replaced immediately by the highest-ranking company in the FTSE All-Share that is not already a constituent of the FTSE 100, rather than waiting for the next scheduled review. This keeps the index continuously at or near one hundred members.

The Family of UK Indices

The FTSE 100 is the most watched of a broader family of UK indices, all maintained by FTSE Russell under the same general framework of free-float market capitalisation weighting and quarterly review. Understanding the relationships between these indices — and what each one actually measures — is important for traders who want to use them as genuine analytical tools rather than simply as background noise.

The FTSE 250 is the index of the 101st to 350th largest companies on the London Stock Exchange by market capitalisation, launched in October 1992. It is often described as a more domestically oriented index than the FTSE 100, and with good reason: while the FTSE 100 is dominated by global corporations that happen to be listed in London, the FTSE 250 contains a much higher proportion of companies whose revenues are predominantly UK-based. As a result, the FTSE 250 tends to respond more directly to UK-specific economic and political developments. The divergence between the two indices in the years following the Brexit referendum is a particularly clear illustration of this: the FTSE 100 initially rose as sterling fell (boosting the overseas earnings of its multinational members), while the FTSE 250 fell sharply as investors priced in the economic uncertainty facing UK-focused businesses. For traders who want genuine exposure to domestic British economic conditions, the FTSE 250 is often the more informative benchmark.

The FTSE 350 is simply the combination of the FTSE 100 and FTSE 250 — the top 350 companies on the London Stock Exchange by market capitalisation — and it forms the backbone of the FTSE All-Share Index. The All-Share extends further down the size spectrum to include the FTSE SmallCap Index, which covers companies ranked approximately 351st to around 600th by market cap, and together these indices aim to represent at least 98% of the full capital value of all UK-eligible companies on the London Stock Exchange. As of early 2026, the FTSE All-Share comprises around 532 companies with a combined market capitalisation of approximately £2.7 trillion.

Beyond these mainstream indices sits the Alternative Investment Market, or AIM, which is a sub-market of the London Stock Exchange designed for smaller, growth-oriented companies that do not yet meet the requirements for a full main-market listing. The FTSE AIM All-Share tracks the companies listed on AIM, and the FTSE AIM 100 covers the largest one hundred of them. AIM companies are typically earlier-stage businesses with higher growth potential but commensurately higher risk, and the index’s behaviour reflects this — it tends to be more volatile than the main FTSE indices and less liquid, making it a very different trading environment.

FTSE Russell also maintains a wide range of sector-specific indices within the UK market, organised according to the Industry Classification Benchmark (ICB) — a global classification system jointly developed by FTSE and Dow Jones. These sectoral indices track the performance of companies within individual industries: financials, energy, healthcare, basic materials, consumer staples, industrials, and so on. For a trader who has formed a view on a particular sector rather than the broad market — say, that the energy sector will outperform following a supply disruption — sectoral indices provide a way to express that view with more precision than a broad market position would allow.

Putting Indices to Work in Your Trading

An index serves four distinct purposes for the active retail trader, and each is worth understanding separately because each calls for a different application of the index as a tool.

The first and most fundamental use is information. An index tells you, instantly and without ambiguity, whether the broad market is advancing or retreating. This matters because stock prices rarely move in isolation from the wider market environment. If the FTSE 100 is down 1.5% on the day and a stock you are watching has fallen 1%, the stock is actually holding up well relative to the market — an observation that would be invisible without the index as a reference point. Conversely, a stock rising 0.5% while the index rises 2% may be subtly underperforming. The index provides the essential background against which individual stock behaviour becomes legible.

The second use is benchmarking. If your portfolio has returned 8% over the past twelve months and the FTSE All-Share has returned 12%, you have underperformed the broad market by four percentage points, meaning you would have been better served simply holding a tracker fund. This is not a comfortable conclusion to reach, but it is a valuable one. Without an index benchmark, there is no way to know whether your stock selection is adding value or destroying it relative to a passive alternative. Professional fund managers are measured against their benchmark index as a matter of contractual obligation; retail traders should apply the same discipline to their own performance assessment.

The third use is hedging. An investor holding a diversified portfolio of FTSE 100 stocks who fears a near-term market correction has a problem: selling individual stocks to reduce risk is cumbersome, potentially triggers tax events, and may leave the investor out of position when the market recovers. A more elegant solution is to take a short position on the FTSE 100 index through futures or options, or through a spread bet or CFD. If the market falls, gains on the index short position offset losses in the underlying portfolio. The mechanics of these instruments are explored in detail in the derivatives section of this series, but the principle is straightforward: the index provides a single liquid instrument through which broad market risk can be managed efficiently.

The fourth use — and for many active traders the primary one — is direct trading. The FTSE 100 futures contract, traded on ICE Futures Europe, is one of the most liquid derivative instruments in the UK market. Spread betting and CFD providers offer the same exposure to retail traders with much smaller capital requirements, allowing traders to take long or short positions on the index across any timeframe from intraday to several weeks. Trading the index rather than individual stocks carries distinct advantages: the index is far more difficult to manipulate than any single stock, its behaviour is shaped by broad macroeconomic forces rather than company-specific news that can be unpredictable, and its liquidity means that positions can typically be entered and exited cleanly without significant slippage.

One of the most practically useful aspects of the FTSE 100 for retail traders is the behaviour it exhibits around large, scheduled macro events — budget announcements, Bank of England interest rate decisions, major economic data releases from the Office for National Statistics. Because the FTSE 100 reflects aggregate expectations, it often moves sharply and directionally in response to information that surprises the market. A trader who has formed a well-grounded view on the likely content of a policy announcement can, in principle, position in the index ahead of the event and profit from the subsequent move. The risk, of course, is that the market’s reaction to a given piece of news is not always the reaction that seems logically obvious — sometimes good news is already priced in, and the index falls on the announcement itself, even when the data confirms what most expected. Learning to distinguish between what the news says and what the market has already priced is one of the most valuable skills a trader can develop, and the index is the purest instrument through which that skill can be practised.

For longer-term investors, the index serves a different but equally important purpose: it defines the passive return that active selection must beat to justify its additional effort and cost. Tracker funds and exchange-traded funds that replicate the performance of the FTSE 100 or FTSE All-Share can be held within an ISA or SIPP entirely free of UK capital gains tax and income tax respectively, making them highly tax-efficient vehicles for long-term wealth accumulation. The total return of the FTSE 100 — price appreciation plus dividends reinvested — has consistently exceeded inflation over most ten-year periods since 1984, offering investors who can tolerate short-term volatility a reasonable probability of real wealth growth over time.

The index, in the end, is both a mirror and a measuring stick. It reflects what the market collectively believes about the future, and it provides the standard against which every investor’s performance, every analyst’s forecast, and every trader’s read of the market can ultimately be judged. Mastering the indices that matter in your market is not a preliminary step before the real work begins — it is the real work, and everything else builds from it.