FTSE 100 Explained: UK Index Weighting, Sectors and Sterling
The FTSE 100 is the UK’s best-known equity benchmark, comprising 100 major eligible companies listed on the London Stock Exchange and calculated under FTSE Russell’s index methodology. The index is reviewed quarterly and uses free-float adjustments when weighting constituents, while membership selection considers full market capitalisation and eligibility criteria.
Many FTSE 100 constituents operate internationally, so an index labelled “UK” does not exclusively reflect domestic UK consumer demand. Sterling exchange rates, commodity prices, global interest rates and the sector mix can all affect company earnings and valuations. Understanding the index requires separating those economic exposures from its London listing and sterling quotation.
What is the FTSE 100?
The FTSE 100 is a rules-based measure of large eligible London-listed companies within the FTSE UK Index Series. It is widely followed in British financial news and often used as a proxy for the UK blue-chip equity market. It is not a direct measurement of every UK business or the living standards of British households.
Its constituent securities and weights are updated under FTSE Russell’s review and corporate-action arrangements. Index levels are expressed in points, while actual fund returns depend on whether the investor tracks the price or total-return version, fees, currency treatment and investment structure.
How are FTSE 100 constituents selected?
FTSE Russell describes the FTSE 100 as comprising the largest 100 eligible UK companies by full market capitalisation before investability weight adjustments, subject to rules and review procedures. Listing, nationality, security eligibility and other factors matter. The exact selection process includes buffers and maintenance provisions intended to avoid unnecessary turnover.
A company becoming one of the hundred most valuable by a momentary market quote does not necessarily lead to instant membership. The official quarterly review and scheduled implementation determine the usual timing. Corporate actions or exceptional events can prompt other adjustments.
Full market cap versus free-float weighting
The distinction is fundamental. Membership can be based on full eligible market size, while the influence of each constituent in the index reflects its free-float-adjusted market capitalisation under applicable methodology. Shares held by strategic owners or otherwise classified as unavailable for public trading may receive reduced weighting recognition.
Suppose fictional UK Company A has full market value £30 billion with 50% eligible float and Company B has full market value £25 billion with 90% eligible float. Their simple adjusted values would be £15 billion and £22.5 billion. A could rank above B by full size but receive a lower index weight.
What happens during the quarterly review?
FTSE Russell conducts scheduled FTSE UK index reviews, normally on a quarterly timetable. Eligibility, ranking and free float are assessed, and announced additions or deletions are implemented according to the provider’s published schedule. Index trackers may rebalance holdings, creating trading demand around implementation dates.
Review-related flows are not a free or guaranteed opportunity. Markets often anticipate likely changes, and the exact execution impact depends on available liquidity, fund positions and unrelated news. Corporate events outside the usual reviews can also alter constituent membership or share counts.
Why is the FTSE 100 internationally exposed?
Many large London-listed companies sell products, earn profits or own assets outside the United Kingdom. Their revenue exposure may include US dollars, euros, emerging-market currencies, global commodity prices or multinational supply chains. Thus a weaker UK economy does not automatically cause every FTSE 100 constituent’s earnings to decline at the same rate.
Conversely, a stronger global economy or commodity rally can support some London-listed firms while domestic UK sectors struggle. Analysts should examine each sector’s geographic revenue and cost structure rather than label the entire FTSE 100 as a pure UK GDP trade.
Sterling strength and overseas earnings
Currency translation is one way GBP moves can affect reported corporate earnings. If a company earns dollars and reports in sterling, a weaker pound can increase the sterling value of unchanged dollar earnings, all else equal. But firms also have foreign expenses, hedges, debt and competitive responses that complicate the effect.
Suppose a fictional business earns $1 billion with no offsetting costs for a simplified demonstration. At $1.25 per £1, that equals £800 million; at $1.20 per £1 it equals about £833 million. The arithmetic illustrates translation, not the real profit sensitivity of a multinational business.
FTSE 100 versus FTSE 250
The FTSE 250 covers the next segment of eligible companies below the FTSE 100 under FTSE UK’s construction, generally focusing more on medium-sized listed businesses. Some FTSE 250 constituents have meaningful UK domestic economic exposure, but neither index is purely domestic or globally exposed by definition.
Comparing their relative performance can provide information about sector rotation, company size and expectations for UK economic conditions. However, the sector composition, valuation and liquidity of the two indices differ, so relative returns should not be attributed solely to sterling or domestic growth.
Sector weights and dividend characteristics
The FTSE 100 has historically featured significant exposure to global financial institutions, healthcare, energy, consumer staples and mining businesses, although actual weights change over time. A sector’s contribution depends on its current index weight and component returns. Some industries are highly sensitive to global commodity prices or borrowing conditions.
Dividend income can be an important part of long-term returns for many UK-listed large companies. Comparing a price-return FTSE 100 chart against another market’s total-return chart can therefore misstate historical performance. Use like-for-like index variants and time horizons.
What moves the FTSE 100?
Potential drivers include global equity sentiment, commodity markets, bank earnings, inflation, real yields, overseas growth, sterling exchange rates and the outlook for large constituents. A Bank of England interest-rate decision may matter, but some multinationals respond more strongly to US or global factors.
When a headline index moves sharply, examine which companies and sectors account for the largest weighted contributions. A weak mining sector can overwhelm positive performance elsewhere when metals prices decline or investors revise China demand expectations.
Commodity-price sensitivity
Energy and mining companies derive different earnings exposures from oil, gas and industrial metals. A crude oil price rise can support some upstream producers while increasing costs for airlines, industrial firms and consumers. Company hedging, costs, taxes and production volumes change the relationship between commodity prices and equity valuations.
As a result, the FTSE 100 does not move one-for-one with any single commodity. Research should identify which sector and constituent are economically sensitive to the commodity, their current weight and whether the market already expected the price change.
UK monetary policy and gilt yields
Bank Rate influences lending conditions and the valuation of future cash flows, while UK gilt yields and the wider global rates environment affect equity discount rates. Some financial businesses may benefit from certain rate structures but suffer from weaker credit demand or impairments. The net effect is company- and cycle-specific.
Higher gilt yields do not universally imply weaker FTSE 100 performance: they may accompany stronger nominal growth or inflation expectations. Assess real yields, earnings revisions and relative valuation instead of relying on one directional slogan.
Index level versus an investment return
A FTSE 100 level reflects the provider’s chosen return variant and calculation currency. An investor in a UK-listed FTSE 100 ETF pays brokerage costs and ongoing expenses and may receive cash dividends or accumulate them within the fund. The fund’s traded share price can differ from its NAV at a given moment.
For a long-term comparison, account for dividends and tracking difference. A flat FTSE 100 price index over several years need not imply that a dividend-reinvesting investor earned exactly zero, while fund fees and taxes could lower net outcomes.
FTSE 100 futures and UK derivatives risk
Futures contracts can provide leveraged exposure to an equity index, with variation margin and expiry rules. Their price can differ from the cash index because of financing costs, expected dividends and time remaining. UK CFD and spread-betting products have additional counterparty and regulatory characteristics.
For UK retail clients, FCA restrictions apply to CFD leverage, margin close-out and risk disclosures. A cash index quote is not itself a leveraged contract. See Stock Index Futures Explained.
FTSE 100 compared with S&P 500
The FTSE 100 and S&P 500 track different company universes, currencies, industry exposures and index methodologies. A period of rising US technology valuations may favour the S&P 500, while stronger dividends or commodity-related earnings may help certain UK large caps. Relative performance is contingent rather than guaranteed.
Comparisons should use matching return variants and a common reporting currency if the investor’s objective is to measure economic purchasing-power outcomes. A headline sterling FTSE level and a dollar S&P level do not provide that common basis.
How to analyse the FTSE 100
Start with official FTSE Russell methodology and current weights. Review sector and geographic exposure, global earnings, sterling changes, commodity and financial conditions, and dividend assumptions. Compare FTSE 100 with the FTSE 250 and other markets to identify whether a move reflects global risk or a UK-specific factor.
Consider the actual index product, its fees, tracking and available execution rather than treating benchmark points as a portfolio P&L calculation. Record scenarios for earnings, currency and interest-rate changes, along with alternative explanations and risks.
Worked reference table
| Hypothetical issuer | Full market capitalisation | Eligible float factor | Illustrative adjusted value |
|---|---|---|---|
| Company A | £30bn | 50% | £15bn |
| Company B | £25bn | 90% | £22.5bn |
Illustrative membership-size versus weight-size distinction; actual provider rules include additional adjustments.
Why the FTSE 100 is not a pure UK economy proxy
Many large London-listed companies earn revenue from customers around the world. Their profits may depend on global commodity prices, US dollar sales, international healthcare demand or emerging-market financial conditions. A London listing does not imply that domestic UK consumption determines every constituent’s performance.
To assess a UK-specific growth narrative, compare domestic revenue exposure, relevant FTSE 250 constituents and UK economic data. The composition and sector mix of each benchmark matter more than the country label alone.
Price return versus total return in the FTSE
The headline FTSE 100 price index normally excludes the reinvestment of ordinary dividends, while total-return variants incorporate them according to their methodology. This can produce materially different long-term return series. A flat price index does not imply that an investor receiving dividends made exactly zero gross total return.
For instance, a hypothetical price index remaining unchanged with reinvested dividends contributing 3% under simplified assumptions could have around 3% total return before costs. Real fund outcomes depend on tax, expenses and payment timing.
Sterling translation with foreign earnings
A company earning $500 million overseas would report an illustrative £400 million at $1.25 per pound and £416.7 million at $1.20 per pound. That currency movement increases the sterling translation by about 4.2% before considering costs or hedges. It does not prove that the firm’s actual profits or share value must rise by 4.2%.
Overseas expenses, foreign debt, pricing strategy and currency hedges can substantially change the exposure. Some UK-listed companies benefit from a weaker pound while others face higher input costs.
Sector attribution during commodity moves
Consider a fictional FTSE-like index with 15% in energy, 10% in mining and 75% in other sectors. If energy gains 8%, mining falls 5% and the rest rises 1%, the weighted outcome is 0.15×8% +0.10×−5% +0.75×1% = 1.45%. Energy drives much of the gain even though another commodity-related sector loses value.
Use updated sector weights and the actual companies’ operating exposures in a real analysis. Crude oil, natural gas and industrial metals have different market drivers, and individual company profits respond to costs, production and hedging.
UK gilt yields versus global financing conditions
Bank Rate and gilt yields influence UK financing and the valuation of future earnings, but multinational FTSE 100 constituents may also react strongly to dollar interest rates and global credit markets. A central-bank announcement that moves UK rates need not have a uniform effect across banks, oil producers, pharmaceuticals and consumer businesses.
Distinguish changes in the discount rate from changes in expected cash flow. Higher yields may accompany stronger growth or unexpected inflation, creating mixed effects on sectors and the aggregate index.
Quarterly reviews and tracker implementation
Scheduled FTSE UK index reviews can lead to additions, removals and weight changes. Index-tracking funds may trade near implementation dates, but prospective changes are often anticipated. The resulting price impact can be small, large or reversed depending on liquidity and market conditions.
Check FTSE Russell’s official review notices and methodology rather than predict membership from a temporary rank alone. Eligibility, ranking buffers and corporate-event procedures influence the actual outcome.
Comparing UK and US indices fairly
Imagine the S&P 500 gains 10% in dollars and the FTSE 100 rises 6% in sterling. This does not yet establish which performed better for a sterling-based investor because dollar currency moves and dividend treatment can change the comparison. A UK holder of unhedged US shares experiences both equity and foreign-exchange returns.
Convert both to a common currency and consistent dividend-reinvestment basis. Then compare sector mix and valuation rather than attribute the entire difference to economic performance in one country.
Building a FTSE 100 three-scenario assessment
An upside case could assume stronger global earnings, stable real yields and healthy commodity demand. A base case might involve modest revenue growth with unchanged valuations. A stress case could combine weaker industrial demand, widening credit spreads and falling profits among heavily weighted constituents.
Specify which sectors contribute to each scenario and their index weights at the research date. Such scenarios are conditional illustrations, not fixed price targets or guaranteed outcomes. They help reveal which assumptions must hold for a market thesis to remain plausible.
Different index versions and international investors
The FTSE 100 can be viewed through price-return or total-return calculations, and investment products can express it through different currencies and fund share classes. A UK-based investor using sterling may experience an outcome different from an overseas investor after converting returns into their home currency.
Before interpreting comparative performance, align the observation period, dividends and reporting currency. A fund can also differ from its benchmark because of expenses, tracking implementation and taxes, even when it holds the intended UK large-company constituents.
Frequently asked questions
Is the FTSE 100 only influenced by the UK economy?
No. Many constituents earn substantial overseas revenue and respond to global currencies, commodities and markets.
Does the FTSE 100 track the largest 100 by free float?
Official FTSE Russell describes selection by full market capitalisation, with free-float factors applied to weights under relevant rules.
When is the FTSE 100 reviewed?
The ordinary FTSE UK index review schedule is quarterly, with other corporate-action adjustments as required.
Does weaker sterling always make the FTSE 100 rise?
No. Translation may help some firms, but foreign costs, hedging, sector moves and expectations also matter.
Are dividends included in FTSE 100 performance?
The price and total-return variants differ. Comparisons should use the appropriate version.
Related Stock Indices Education
Compare how indices work, index weighting, market drivers, and the six-pillar analysis framework.
Sources and further reading
- LSEG — FTSE 100 Official Overview
- FCA — Contracts for Differences
- S&P Dow Jones — Index Methodology Literacy
Important notice: General financial education only, not advice, a trading recommendation or a forecast. Numerical examples are hypothetical. Equity indices, funds and derivatives can experience significant losses, with additional risks from leverage, currency and execution.