At a glance
- Provider
- FTSE Russell; benchmark administrator FTSE[1]
- Launched
- 3 January 1984, starting value 1,000[2]
- Constituents
- 100 — a constant number is maintained[1]
- Weighting
- Market capitalisation adjusted for free float and foreign ownership limits[1]
- Review schedule
- Quarterly in March, June, September and December; changes after the close on the third Friday[1]
- Region / currency
- UK companies on the LSE Main Market; base currency GBP[1]
Index levels and returns change every trading day and are not shown here. Check the provider's own page for current figures.
Quick answer
The FTSE 100 holds the 100 largest UK companies by full market capitalisation listed on the London Stock Exchange's Main Market. It is free float-adjusted, calculated in pounds sterling and reviewed every quarter by FTSE Russell [1].
Key points
- The FTSE 100 always has 100 companies; a constant number is maintained at reviews and between them.
- Ranking uses full market capitalisation, while index weights are adjusted for free float.
- Reviews happen four times a year, using data from the Tuesday before the first Friday of the review month.
- A company joins if it rises to 90th or above and leaves if it falls to 111th or below, which limits churn.
- "UK company" is a defined term in the rules, based on incorporation, listing and other factors.
#What is the FTSE 100?
FTSE Russell's ground rules say the FTSE 100 "will consist of the largest 100 UK companies by full market capitalisation" [1]. Market capitalisation (the British spelling) is share price times the number of shares; see market capitalization. The index was launched on 3 January 1984 with a starting value of 1,000 points [2].
It sits inside a family. The rules maintain a constant number of constituents for the FTSE 100, the FTSE 250 and the FTSE 350, and replacements are drawn from the wider FTSE All-Share [1]. For what an index is and why it exists, read what a stock market index is.
#Which companies are eligible?
A company first has to be treated as a UK company. The rules say that if a company is UK incorporated and has a sole listing in the UK, FTSE will allocate it UK nationality; for others, FTSE looks at factors such as country of incorporation, headquarters location and shareholder base [1]. Its shares must also pass trading and float screens.
| Test | Requirement |
|---|---|
| Market | Admitted to trading on the Main Market of the London Stock Exchange (Equity Shares, commercial companies category) |
| Trading price | Sterling, euro or U.S. dollar denominated trading price on SETS, the exchange's order book |
| Free float | Minimum 10% |
| Liquidity | Must pass a median liquidity test set out in a separate FTSE guide |
| Nationality | Allocated UK nationality under the ground rules |
#How is the FTSE 100 weighted?
Constituents of the FTSE UK Index Series "are adjusted for free float and foreign ownership limits" [1]. Free float is the share of a company's stock available to ordinary investors. So although companies are ranked by full market capitalisation to decide who is in, their weight once inside depends on the float-adjusted value. A company with a large block held by a founder ranks by its full size but counts for less in the index.
#How does the quarterly review work?
The FTSE 100 is reviewed quarterly in March, June, September and December, using data from the FTSE UK Monitored List at the end of the day on the Tuesday before the first Friday of the review month [1]. Changes take effect after the close of business on the third Friday of the review month [1].
The entry and exit rules include a cushion. A company is inserted if it rises to 90th position or above, and a constituent is deleted if it falls to 111th or below [1]. If more companies qualify to come in than to go out, the lowest-ranking constituents are deleted so the count stays at 100, and vice versa [1].
Worked example
Worked example: applying the 90th / 111th rule
At a hypothetical review, six companies sit near the cut-off. Ranks are by full market capitalisation.
- Alpha, not in the index, ranked 88th
- Inserted (90th or above)
- Bravo and Charlie, not in the index, ranked 95th and 99th
- Not inserted (below 90th)
- Delta and Echo, in the index, ranked 104th and 109th
- Stay (above 111th)
- Foxtrot, in the index, ranked 113th
- Deleted (111th or below)
- Insertions vs deletions
- 1 vs 1 — count stays at 100
Bravo and Charlie are larger than Delta and Echo yet stay out. The cushion means the index is not always exactly the 100 largest at every moment, in exchange for less churn.
Hypothetical companies and ranks; the rule check was run in Python.
Joining vs leaving the FTSE 100
Non-member
- Ranked 90th or higher → inserted
- Ranked 91st–100th → waits
- Can be added early if a member leaves between reviews
Member
- Ranked 110th or higher → stays
- Ranked 111th or lower → deleted
- May also be deleted to make room if more companies qualify to join
#What happens if a company leaves between reviews?
Because the count is fixed, a gap is filled straight away. The rules say companies removed from the FTSE 100 intra-quarter will be replaced by the highest-ranking FTSE All-Share company that is not currently a constituent [1]. A takeover, for example, can therefore bring a new member in without waiting for the next review.
The index is calculated in pounds: the ground rules give GBP as its base currency [1]. Where a company is listed says nothing about where it earns its money, so the index is a measure of UK-listed companies rather than of the UK economy. You cannot buy the index itself; funds that track an index can trail it because of fees and costs [3]. See index funds explained.
Common beginner mistakes
Treating the FTSE 100 as the UK economy
It measures 100 large companies listed in London. Their businesses, and their revenues, can be spread around the world.
Assuming it is always the exact top 100
The 90th/111th cushion lets some smaller members stay and keeps some larger non-members waiting until they clear the 90th position.
Confusing ranking with weighting
Full market capitalisation decides membership; free float-adjusted value decides weight. A company can rank high but carry a smaller weight.
Ignoring the currency
The index is in pounds. An investor whose home currency is not sterling will see an extra gain or loss from exchange rates. Compare with the dollar-based MSCI World.
What's the bottom line?
The FTSE 100 is a fixed-size, free float-adjusted index of the largest UK companies on the London Stock Exchange, reviewed quarterly with a 90th/111th cushion and calculated in pounds. Those few rules explain why membership changes when it does. To compare with a broad U.S. benchmark, see the S&P 500.
Frequently asked questions
Does the FTSE 100 always have exactly 100 companies?
The ground rules maintain a constant number of constituents. If a member leaves between reviews, it is replaced by the highest-ranking eligible FTSE All-Share company that is not already in the index.
When does the FTSE 100 change?
At quarterly reviews in March, June, September and December, with changes taking effect after the close on the third Friday of the review month, and between reviews when a member has to be replaced.
What does "90th or above" mean for joining?
A company outside the index is added at a review if it ranks 90th or higher by full market capitalisation. A member is deleted if it falls to 111th or lower.
Are foreign companies in the FTSE 100?
Only companies FTSE allocates UK nationality. A non-UK incorporated company can qualify after FTSE reviews factors such as its headquarters, shareholder base and where its shares trade most.
Sources
Grade A = primary source (regulator, government agency, official rulebook or the index provider's own documents). Numbers in brackets in the text point here.
- FTSE Russell (LSEG). Ground Rules: FTSE UK Index Series v17.3 (September 2026) (2026). Accessed 2026-10-03.A
- FTSE Russell (LSEG). FTSE 100 Index turns forty (Index Insights) (2024). Accessed 2026-10-03.A
- U.S. SEC — Investor.gov. Index Funds (2026). Accessed 2026-10-03.A
This page is general education, not personal financial, tax or legal advice. Figures in worked examples are hypothetical and calculated before taxes and fees unless stated. Rules and limits change; check the linked primary sources for the current version. How we check every page.
