FTSE 100

The FTSE 100 Index is a rules-based UK equity index containing 100 large eligible companies in the FTSE UK Index Series. Companies are ranked for this size segment by full market capitalization, while their index weights reflect investability adjustments such as free float. The index is commonly called the Footsie and identified by the headline ticker UKX.

Key Takeaways

• The FTSE 100 is a large-cap UK equity benchmark, not a list selected by an analyst’s opinion.
• “100 largest London-listed companies” is useful shorthand but incomplete: listing, nationality, free-float, liquidity, security-type, and other rules also matter.
• Full market capitalization is used for size ranking; float-adjusted capitalization influences index weight.
• Quarterly reviews use transparent rules and buffers to reduce unnecessary turnover near segment boundaries.
• A price index excludes ordinary dividend income; a total-return version incorporates reinvested distributions according to its methodology.
• The index is not a pure measure of UK domestic economic activity because constituents can earn substantial revenue abroad.
• Investors cannot buy the index directly. Funds and derivatives linked to it add costs and risks.

How the FTSE 100 Works

The FTSE 100 sits at the large-cap end of the FTSE UK Index Series. FTSE Russell defines an eligible UK universe, applies investability screens, ranks companies by full market capitalization, and assigns the top size segment to the FTSE 100 under the current ground rules.

The process distinguishes two quantities:

1. Full market capitalization for ranking: share price multiplied by the company’s eligible shares before free-float weighting.
2. Investable market capitalization for weighting: market value adjusted for the portion of shares treated as available to public investors and for other applicable restrictions.

This distinction prevents a large strategic or controlling holding from receiving the same index representation as freely traded shares.

Simplified Weight Formula

For constituent (i), an illustrative float-adjusted weight is:

where:

• (P_i) is price;
• (Q_i) is the share quantity used by the index;
• (F_i) is the investability or free-float factor; and
• (N) is the number of index constituents.

Actual index calculations also use a divisor and detailed rules for multiple security lines, corporate actions, currencies, timing, and rounding.

Worked Example: Ranking Is Not Weighting

Assume both companies are eligible and the figures below are simplified:

Alder ranks above Birch by full market capitalization. If these were the only two constituents, however, their illustrative weights would be:

The smaller company receives twice the weight because its free-float-adjusted market value is larger. This does not predict either company’s return; it only shows how the selected inputs affect index influence.

Reviews, Rebalancing, and Corporate Actions

FTSE Russell reviews the FTSE 100 quarterly as part of the FTSE UK Index Series. Scheduled reviews can move companies between the FTSE 100 and FTSE 250. Entry and exit buffers help prevent a company near the boundary from switching segments solely because of a small short-term rank change.

Not every constituent change waits for a quarterly review. Initial public offerings, mergers, takeovers, delistings, suspensions, share changes, and other corporate actions can be handled under separate maintenance rules. Review announcements normally identify the measurement date, implementation date, additions, and deletions.

Effective from the June 2026 review, the minimum free-float eligibility threshold for both UK- and non-UK-incorporated companies in the series is 10%, subject to all other conditions. This is an eligibility floor, not a target index weight. Always confirm the rule version applicable to the historical date being analyzed.

Price Return vs. Total Return

The familiar FTSE 100 headline level is a price index. It measures price changes after methodology adjustments but does not represent the full economic return received by a shareholder who also receives distributions.

Suppose the price index rises from 8,000 to 8,240 during a period:

If an official total-return series rises by 4.20% over the same dates, the 1.20 percentage-point difference reflects the series’ dividend-reinvestment methodology and timing. Do not add a separately quoted dividend yield to a price return unless the dates and cash-flow assumptions support that calculation.

FTSE 100 vs. Other UK Indexes

The FTSE 100 and FTSE 250 are complementary segments. A company promoted into the FTSE 100 generally leaves the FTSE 250 rather than appearing in both.

Why the FTSE 100 Matters

The index is used for:

• large-cap UK equity benchmarking;
• passive funds and exchange-traded products;
• futures, options, swaps, and structured products;
• performance attribution and risk reporting;
• market news and historical comparison; and
• asset-allocation policy where the selected benchmark matches the mandate.

The index’s visibility does not make it the right benchmark for every UK portfolio. A strategy holding mid- and small-cap shares may be better compared with the FTSE All-Share or another mandate-matched index.

Is the FTSE 100 a UK Economic Indicator?

The index can reflect financing conditions, investor expectations, commodity prices, exchange rates, and corporate outlooks. It should not be treated as a direct reading of UK gross domestic product, household income, or domestic business conditions.

Many constituents operate internationally. A fall in sterling can increase the translated GBP value of foreign-currency revenue while also signaling other economic stress. Sector concentration can cause the index to move differently from a domestically focused mid-cap index. Analysts should compare constituent revenue exposure, sectors, currency, interest rates, and the economic statistic actually being studied.

Risks and Limitations

• Concentration: a few large constituents or sectors can drive a significant share of performance.
• Market-cap bias: higher-valued companies receive more weight, which is not the same as equal economic importance or lower risk.
• Foreign-revenue exposure: company results may respond more to global demand and exchange rates than to UK domestic activity.
• Methodology risk: eligibility, free-float, review, or calculation rules can change.
• Turnover: additions and deletions can create trading costs for products that track the index.
• Return-series mismatch: price and total-return versions answer different performance questions.
• Product risk: index funds and derivatives introduce fees, liquidity, tracking, leverage, counterparty, and tax considerations.
• Historical comparability: constituent, sector, and methodology changes limit simple comparisons across decades.

How to Evaluate a FTSE 100 Claim

1. Identify the exact index variant, ticker, currency, and data vendor.
2. Record the start and end dates and whether levels are closing, intraday, or estimated.
3. Distinguish full-market-cap ranking from float-adjusted weighting.
4. Confirm price return, gross total return, or net total return.
5. Match constituent and weight data to the same effective date.
6. Separate index performance from a fund’s return after fees and tracking difference.
7. Check the current FTSE UK ground rules and latest review notice.

Common Mistakes

• Describing the index as exactly the 100 largest companies on the London Stock Exchange without eligibility qualifications.
• Assuming a constituent must be incorporated in the UK merely because it is in a UK index.
• Using market capitalization without saying whether it is full or float adjusted.
• Reading an index-point change as a percentage change.
• Comparing a price index with a total-return portfolio.
• Treating quarterly review dates as the only dates constituent data can change.
• Calling index inclusion a quality rating, investment recommendation, or guarantee of liquidity.
• Treating the FTSE 100 as a complete proxy for the UK economy or entire UK equity market.

Authoritative Sources

• LSEG: FTSE 100 explains full-market-cap ranking, free-float assessment, quarterly review, and daily corporate-action maintenance.
• LSEG: FTSE UK Index Series provides current factsheets, constituents, methodology, and return-series descriptions.
• FTSE UK Index Series Ground Rules is the controlling methodology source; verify its version and effective date.
• LSEG: FTSE 100 Turns Forty documents the 1984 launch and the transition from the older FT 30 benchmark.
• LSEG 2026 free-float criteria change documents the threshold aligned from the June 2026 review.

Related Terms

• FTSE: The provider brand and broader index-family concept.
• FTSE All-Share: A broader UK benchmark that includes the FTSE 100, FTSE 250, and FTSE SmallCap.
• Market Capitalization: Share price multiplied by the relevant shares outstanding.
• Float-Adjusted Market Capitalization: Market value adjusted for shares available to public investors.
• Total Return: Price change plus income over a defined period.
• Tracking Error: Variability of active return relative to a benchmark.

FAQs

This article is educational and does not recommend buying or selling an index-linked product or any constituent security.