View source for FTSE 100 Index

The '''FTSE 100 Index''' (pronounced '''"Footsie 100"''') is a market-capitalization-weighted stock market index comprising the 100 companies with the highest free-float market capitalization listed on the [[London Stock Exchange]] (LSE). Launched on 3 January 1984 with a base value of 1,000 points, the index is maintained by [[FTSE Russell]], a subsidiary of the [[London Stock Exchange Group]] (LSEG). It is widely regarded as the primary benchmark of the United Kingdom equity market and is among the most closely followed stock indices in the world.

The index employs a '''free-float market capitalization''' weighting methodology, whereby constituent companies are weighted according to the market value of shares available for public trading. Eligibility is determined based on market capitalization, liquidity, and free-float requirements, with the index composition reviewed quarterly to reflect changes in the market. The FTSE 100 includes multinational corporations spanning sectors such as financial services, energy, pharmaceuticals, consumer goods, mining, industrials, telecommunications, and healthcare. Because a significant proportion of constituent companies derive substantial revenues from international operations, the index is often considered a measure of globally diversified businesses rather than solely the domestic British economy.

The FTSE 100 serves as the underlying benchmark for a wide range of investment products, including index mutual funds, exchange-traded funds (ETFs), futures, options, and structured financial instruments. It is extensively used by institutional and retail investors to evaluate portfolio performance, measure market sentiment, and implement passive investment strategies. Alongside indices such as the [[S&P 500]], [[Dow Jones Industrial Average]], [[NASDAQ-100]], [[DAX]], [[CAC 40]], and [[Nikkei 225]], the FTSE 100 is regarded as one of the world's principal indicators of large-cap equity market performance.