FTSE 100 Explained: What It Is, How It Works, and Why It Matters to UK Investors

If you follow UK business news even casually, you’ve heard the phrase “the FTSE 100 closed higher today” more times than you can count. But what is the FTSE 100 actually tracking, why does it move the way it does, and should it factor into your own investing decisions? This guide breaks down everything worth knowing about the UK’s most-watched stock market index, in plain language.

What Is the FTSE 100?

The FTSE 100 (pronounced “footsie”) is an index of the 100 largest companies listed on the London Stock Exchange, ranked by market capitalization. Its full name is the Financial Times Stock Exchange 100 Index, a reflection of its joint origin between the Financial Times and the London Stock Exchange Group, which still manages the index today through FTSE Russell.

Launched on 3 January 1984 with a starting value of 1,000 points, the FTSE 100 has become shorthand for “how is the UK stock market doing” in the same way the Dow Jones or S&P 500 serves that role in the United States. As of early September 2026, the index trades in the region of 10,700 points, having climbed from a 52-week low near 9,107 to an all-time high above 10,989.

A few defining facts about the index:

• It covers roughly 80% of the entire market capitalization of the London Stock Exchange’s main market.
• It’s a free-float index, meaning only shares available for public trading count toward a company’s weighting shares locked up by founders, governments, or controlling shareholders are excluded.
• Constituents are reviewed quarterly, in March, June, September, and December, so the 100 companies in the index today won’t necessarily be the same 100 in a year’s time.

How Is the FTSE 100 Calculated?

The index uses a market-capitalization weighted methodology, adjusted for free float. In practical terms, that means:

1. Each company’s market cap is calculated (share price × number of shares in public circulation).
2. Larger companies have a proportionally bigger influence on the index’s overall movement.
3. The combined value of all 100 companies is converted into the index points figure you see quoted on financial news sites.

This is why a single large company having a bad day can drag the whole index down, even if dozens of smaller constituents are rising. A 2% fall in a heavyweight like HSBC or Shell moves the index far more than a 2% fall in a smaller constituent near the bottom of the list.

Free Float Adjustment

The free-float rule matters more than most casual readers realize. A company might have a huge headline market cap, but if a large chunk of its shares are held by a founding family or a sovereign wealth fund and never trade, only the publicly available portion counts toward its FTSE 100 weighting. This keeps the index a better reflection of what investors can actually buy and sell.

Who Decides Which Companies Are in the FTSE 100?

FTSE Russell, the index provider, runs a quarterly reconstitution process. Companies are ranked by full market capitalization on set review dates, and the rules broadly work like this:

• A company ranked 90th or above by market cap will be promoted into the FTSE 100 if it isn’t already a member.
• A company ranked 111th or below will typically be relegated to the FTSE 250.
• This buffer zone between 91st and 110th prevents companies from bouncing in and out of the index every quarter due to minor price fluctuations.

Promotions and relegations get real financial news coverage because index-tracking funds which hold trillions of pounds globally are forced to buy or sell shares to match the new index composition. A company being added to the FTSE 100 often sees a short-term bump in its share price purely from this “passive flow” effect, independent of its underlying business performance.

Which Companies Currently Sit at the Top of the FTSE 100?

The index is dominated by a handful of sectors: banking, energy, pharmaceuticals, mining, and consumer goods. Based on current market capitalization, the largest constituents include:

• HSBC Holdings: The index’s biggest bank and, at present, its largest company overall by market value.
• AstraZeneca: The pharmaceutical giant behind a broad portfolio of cancer, respiratory, and vaccine treatments.
• Shell: The energy major, whose share price tends to track oil and gas prices closely.
• Rolls-Royce Holdings: After a well-documented turnaround, now one of the index’s standout performers of the past few years.
• Unilever: The consumer goods company behind everyday household brands.
• British American Tobacco: A consistent high dividend payer.
• BP: The second of the two UK oil majors.
• GSK (GlaxoSmithKline): Another pharmaceutical heavyweight.
• Glencore: A mining and commodities trading business.
• Barclays: One of several UK banks represented in the index alongside Lloyds, NatWest, and Standard Chartered.

Because index weighting is capitalisation-based, these top names collectively account for a disproportionate share of the FTSE 100’s total value, even though the index technically contains 100 companies.

Why the FTSE 100 Moves the Way It Does

A common misconception is that the FTSE 100 is a pure barometer of the UK domestic economy. It isn’t and understanding why helps explain some of its quirks.

It’s More International Than It Looks

A large share of FTSE 100 constituents earn most of their revenue outside the UK. Energy majors, miners, pharmaceutical companies, and global banks generate profits in dollars, euros, and a range of other currencies from operations spanning dozens of countries. This means the index often responds more to global commodity prices, US interest rate decisions, and international demand than to UK-specific economic data.

Currency Effects Cut Both Ways

Because so many constituents report earnings in foreign currencies, a weaker pound can actually push the FTSE 100 higher. When sterling falls, overseas profits are worth more once converted back to pounds, which flatters earnings and share prices for the index’s most internationally exposed companies. Conversely, a stronger pound can act as a modest drag on the index even during periods of domestic economic strength.

Sector Concentration

Energy, financials, materials, and healthcare make up a large proportion of the index’s weighting, while technology a sector that has driven much of the growth in US indices over the past decade is comparatively underrepresented on the London market. This is one reason the FTSE 100’s long-term growth profile has often looked different from the S&P 500’s: London simply doesn’t have the equivalent of the world’s largest technology companies listed on its main board.

How Can You Invest in the FTSE 100?

You can’t buy “the index” directly it’s a calculated number, not a tradable asset but there are several practical ways to gain exposure to it.

• Index tracker funds and ETFs. These funds are built to replicate the FTSE 100’s performance as closely as possible, holding the same 100 companies in roughly the same proportions. They’re widely used because of their low fees relative to actively managed funds.
• Individual shares. You can buy shares in specific FTSE 100 constituents directly through a stockbroker or investment platform, giving you control over exactly which companies you hold.
• CFDs and spread betting. These derivative products let traders speculate on the index’s short-term price movements without owning the underlying shares. They carry higher risk and are generally used by more experienced traders rather than long-term investors.
• Pension and ISA wrappers. Many UK workplace pensions and Stocks & Shares ISAs include FTSE 100 tracker funds as a default or recommended option, often without savers realising it.

Things Worth Knowing Before You Invest

• Dividends matter more here than in many other markets. FTSE 100 companies, particularly in banking, energy, and tobacco, have historically paid relatively high dividend yields compared to US large caps. Many long-term investors in UK trackers are drawn as much by the income as by capital growth.
• Past performance doesn’t guarantee future returns. The index has had long stretches of relatively flat performance, notably through much of the 2010s, before accelerating more recently. It shouldn’t be assumed to rise in a straight line.
• Diversification still matters. Because the FTSE 100 is concentrated in a handful of sectors and dominated by its largest names, holding only a FTSE 100 tracker still leaves you with concentrated exposure to energy prices, interest rates, and global banking conditions.

What Moves the FTSE 100 Day to Day?

If you’re trying to understand why the index rose or fell on a given day, these are the usual suspects:

• Oil and gas prices: Because Shell and BP carry significant index weight, a jump in Brent crude often lifts the whole index, even if other sectors are flat or falling.
• Bank of England interest rate decisions: Rate changes affect bank profitability, borrowing costs for listed companies, and the relative attractiveness of dividend-paying shares versus bonds.
• Sterling’s exchange rate: As covered above, a weaker pound tends to support the index due to the earnings translation effect.
• Global economic data: US inflation figures, Chinese manufacturing data, and eurozone growth numbers all influence sentiment, given how internationally exposed FTSE 100 companies are.
• Company-specific news: Mergers, takeover bids, profit warnings, and earnings releases from the largest constituents can move the index meaningfully on their own, as seen when major takeover approaches or multibillion-pound acquisition deals hit the newswires.
• Inflation and consumer data: UK retail price data, wage growth figures, and housing market statistics shape expectations for interest rates and consumer spending, indirectly affecting sentiment toward the index.

Common Misunderstandings About the FTSE 100

“A rising FTSE 100 means the UK economy is doing well.” Not necessarily. Because so much of the index’s earnings come from overseas and from commodity-linked businesses, the FTSE 100 can rise while UK domestic growth is sluggish, and vice versa. It’s a better measure of large-cap corporate profitability and global market sentiment than of Main Street economic conditions.

“The FTSE 100 and the London Stock Exchange are the same thing.” The London Stock Exchange is the marketplace where shares are traded; the FTSE 100 is simply one index calculated from a subset of companies listed there. Thousands of other companies trade on the exchange without being part of the index.

“Higher index points always mean better returns for investors.” The index level is a price measure, not a total return measure. Total return indices, which reinvest dividends, tell a more complete story of investor returns over time, since dividend income makes up a meaningful portion of what FTSE 100 investors actually earn.

FAQs About the FTSE 100

What does FTSE stand for? FTSE stands for Financial Times Stock Exchange, reflecting the index’s joint development by the Financial Times and the London Stock Exchange.

How often does the FTSE 100 constituent list change? FTSE Russell reviews the index quarterly in March, June, September, and December promoting and relegating companies based on market capitalisation rankings.

What is the FTSE 100’s all-time high? The index has set fresh record highs during 2025 and 2026, moving well past its earlier record levels set in the years following its 1984 launch, as strong corporate earnings and a weaker pound have supported valuations.

Is the FTSE 100 a good investment for beginners? A low-cost FTSE 100 tracker fund is often cited by financial educators as an accessible way for beginners to gain diversified exposure to large UK and multinational companies in a single purchase, though as with any investment, capital is at risk and value can fall as well as rise.

Why is the FTSE 100 sometimes called the “Footsie”? It’s simply a phonetic nickname derived from the acronym FTSE, and it has stuck as informal shorthand used across UK financial media.

Does the FTSE 100 include dividends in its headline figure? No. The standard FTSE 100 index quoted in the news is a price index and does not include reinvested dividends. FTSE Russell also publishes a separate Total Return index that does account for dividend income.

How is the FTSE 100 different from the FTSE All-Share? The FTSE All-Share is a much broader index covering the vast majority of companies listed on the London Stock Exchange’s main market over 600 constituents rather than just the top 100 by market capitalisation.

Final Thoughts

The FTSE 100 is less a snapshot of the UK high street and more a window into how the world’s largest London-listed companies spanning energy, banking, mining, and pharmaceuticals are performing against a backdrop of global economic conditions, commodity prices, and currency movements. Understanding what drives it, and what it doesn’t measure, makes the daily headlines about the index a lot easier to interpret, whether you’re an active investor or simply trying to make sense of the business news.

As with any stock market benchmark, the FTSE 100 should be read as one data point among many rather than a complete verdict on the UK economy. Anyone considering investing based on the index, directly or through a tracker fund, should weigh their own financial goals, time horizon, and risk tolerance, and consider speaking with a qualified financial adviser before making investment decisions.