Index rebalancing and reconstitution explained: S&P 500, Nasdaq-100, FTSE and more

Index rebalancing is the scheduled process by which an index provider updates an index: adding and removing members (reconstitution) and recalculating the weights of the members that remain (rebalancing). Because trillions of dollars in index funds and ETFs must hold exactly what the index holds, these changes force large, predictable trades, mostly concentrated in the closing auction on the day the change takes effect. For traders, review days mean unusual volume, occasional sharp moves in individual shares, and a calendar worth knowing.

The stock index guide introduces the idea briefly. This guide goes further: how the major indices actually run their reviews, what happens to prices, and what matters if you trade index CFDs or share CFDs.

Reconstitution, rebalancing and reweighting

The terms are used loosely, but they describe different things.

• Reconstitution changes the membership list. A company that meets the rules is added, and one that no longer does is removed.
• Rebalancing updates the inputs for existing members: share counts, free-float factors (the proportion of shares available to the public), and any weight caps. Even with no change in membership, a rebalance moves weights.
• Reweighting or capping is a specific kind of rebalance that limits the weight of the largest members, either at scheduled dates or, in some indices, whenever thresholds are breached.

There are also unscheduled changes. When a member is taken over, delisted, spun off or goes bankrupt, the provider replaces it, often with a few days' notice.

Why rebalancing forces trading

An index fund's job is to match the index as closely as possible. If the index adds Company X at a 0.5% weight on a given date, every fund tracking it needs to own Company X at a 0.5% weight from that date. Buying earlier means holding something the index does not, and buying later means missing the index's return. The cleanest way to match is to trade at the official closing price on the day before the change takes effect, because the index itself will use that price.

That is why index changes produce some of the largest closing auctions of the year. The stock market mechanics guide explains how closing auctions set the official close.

Worked example: how big the forced trade can be

All numbers are illustrative.

• Assets tracking an index: 1,000 billion dollars (1 trillion).
• New member's weight in the index: 0.5%.
• Index funds must therefore buy: 1,000 × 0.005 = 5 billion dollars of the stock.
• The stock's average daily trading value: 1 billion dollars.

The required purchase is five times a normal day's trading. Some of it is pre-positioned by active managers and banks who buy after the announcement and sell to index funds at the close, but the point stands: index changes can require trades that are large relative to a company's normal liquidity. For the company being removed, the same logic works in reverse.

The effect is much smaller for a large company moving between two indices that are both heavily tracked, and much larger when a company joins an index such as the S&P 500 for the first time from outside any major benchmark.

How the main indices run their reviews

Rules differ between providers and change over time. What follows is the framework; always check the provider's current methodology and announcements.

S&P 500

The S&P 500 is run by S&P Dow Jones Indices, and membership decisions are made by an index committee rather than a purely mechanical formula. To be eligible, a company must meet a set of criteria that includes US domicile, a market capitalisation above a threshold that is revised periodically, adequate liquidity and free float, and positive earnings: under generally accepted accounting principles, the sum of the last four quarters' earnings and the most recent quarter's earnings must both be positive.

Meeting the criteria is not enough on its own. The committee chooses among eligible companies, partly to keep the index's sector balance representative. Scheduled quarterly rebalances take effect after the close on the third Friday of March, June, September and December, and additions are often announced in the evening, typically one to two weeks before. Off-cycle changes happen whenever a member is acquired or otherwise leaves.

The best-known recent example is Tesla, which joined the S&P 500 on 21 December 2020. Because of its size, it was the largest company ever added to the index, and index funds had to buy tens of billions of dollars' worth of its shares at the close on the preceding Friday.

Nasdaq-100

The Nasdaq-100 holds the largest non-financial companies listed on the Nasdaq exchange, weighted by a modified market capitalisation. Its annual reconstitution is announced in early December and takes effect after the close on the third Friday of December. Weights are rebalanced quarterly in March, June, September and December.

The index also has a rule for special rebalances. If the largest member's weight exceeds 24%, or if the combined weight of members each above 4.5% exceeds 48%, Nasdaq can rebalance outside the normal schedule to bring the large members back down. That happened in July 2023: the combined weight of the biggest technology companies breached the 48% limit, Nasdaq announced a special rebalance on 7 July, and new weights applied from 24 July, reducing the aggregate weight of the largest members to 40%. It was only the third special rebalance in the index's history, after December 1998 and May 2011. The Nasdaq-100 trading guide covers the index's concentration in more detail.

FTSE 100

FTSE Russell reviews the FTSE 100 quarterly, in March, June, September and December. The rules are mechanical, with a buffer to stop companies bouncing in and out:

• A non-member is added if its market capitalisation ranks 90th or higher among eligible companies.
• A member is removed if it falls to 111th or lower.

Companies ranked between those levels generally keep their current status, unless removals are needed to make room for mandatory additions. Ranking uses market values at a set date shortly before the review, and the results are published a few days before they take effect. Promotions and relegations between the FTSE 100 and FTSE 250 are routine news in the UK financial press. The FTSE 100 trading guide covers the index itself.

Nikkei 225, Euro Stoxx 50 and others

• Nikkei 225: periodic reviews twice a year, in April and October, with a limit on the number of regular replacements per review. Nikkei can also lower a member's price adjustment factor to cap its influence. See the Nikkei 225 trading guide.
• Euro Stoxx 50: an annual composition review in September, with quarterly weight updates and a 10% cap per member. STOXX consulted in 2026 on moving to twice-yearly reviews.
• DAX: Deutsche Börse reviews membership on a regular published schedule, with rules on size, liquidity and profitability.

The quarterly coincidence: expiry and rebalancing

Many rebalances take effect on or around the third Friday of March, June, September and December. Those are also the standard expiry dates for quarterly equity index futures and options in the US and Europe. In the US, the day is often called triple witching, because index futures, index options and single-stock options expire together.

Put the two together and you get some of the highest-volume closing auctions of the year. For index CFD traders this matters in two ways:

• Volume and volatility near the close. Large imbalances in the closing auction can move individual shares sharply in the final minutes, and sometimes the index itself.
• Futures-based CFDs expire or roll. If your provider's index CFD is priced from futures, its reference contract changes around these dates. The index futures vs CFDs guide covers what that means for pricing and costs.

What happens to prices

The index inclusion effect

The classic observation is that a stock tends to rise when its addition to a major index is announced, and that stocks being removed tend to fall. The logic is simple: there is a known, price-insensitive buyer (or seller) for a large amount of stock on a known date.

Two qualifications matter.

1. It is widely known. The effect has been studied since at least the 1980s, and banks, hedge funds and event-driven managers position for index changes as soon as they are announced, and sometimes before, by predicting which companies will qualify.
2. It appears to have shrunk. Research published by S&P Dow Jones Indices and others has reported that the price impact of S&P 500 additions and deletions has become much smaller than it was in the 1990s. One plausible reason is that more traders now compete to supply the shares that index funds need.

Some of the gain on announcement can also reverse after the effective date, once the forced buying is complete. None of this is a reliable trading edge for a retail trader.

Moves after the effective date

Once the change takes effect, the added company's share price depends on its business again, not on index flows. What does change is the composition of the index you are trading. After the July 2023 Nasdaq-100 special rebalance, for instance, the largest members carried less weight than before, which in principle makes the index slightly less sensitive to any one of them. Over long periods, reconstitution also means an index's character drifts: the sectors that grow get more weight, and the ones that shrink lose it.

Index changes and share CFD traders

If you trade CFDs on individual shares, review days can produce:

• Large closing prints. The official close on the effective date may be some distance from the last continuous trade, because of the auction.
• Gaps on the next open. A stock that moved in a large closing auction may open the next session at a different level once normal trading resumes. The gap and weekend risk guide explains how stops behave when price gaps.
• Wider spreads near the close. Providers often widen CFD spreads around auctions.

If a company you trade is about to join or leave a major index, check the effective date and the time of the closing auction, and consider whether you want to hold a position through it.

Worked example: a weight cap and what it does to the index

All numbers are illustrative.

Imagine a 10-stock index with a 20% cap. One member has grown to a 30% weight. At the next rebalance the provider caps it at 20% and spreads the excess 10% across the other nine members in proportion to their weights.

• Before: if that stock rose 10%, the index would gain about 3% from it alone (30% × 10%).
• After: the same 10% move contributes about 2% (20% × 10%).

The cap does not change the stock, but it changes how much the index reacts to it. If you trade an index because of one dominant member, a rebalance can quietly reduce your exposure to that member. To see what a weight change means for your own position size, the instruments position size calculator converts index point moves into account currency.

Common mistakes

• Treating an announcement as a free trade. Index additions are public, studied and heavily traded by professionals. The obvious move is usually priced in quickly.
• Forgetting the effective-date close. The forced trading happens in the closing auction, which can move a stock sharply in minutes.
• Confusing the announcement date with the effective date. They are typically one to two weeks apart for scheduled changes.
• Ignoring the third-Friday coincidence. Rebalancing, futures expiry and options expiry together can make the final hour of those sessions unusual.
• Assuming the index is static. Reconstitution means the index you trade this year is not quite the index you traded five years ago. Long-term comparisons of index charts mix different baskets of companies.
• Assuming rules are fixed. Providers consult on and change their methodologies, including review frequencies and caps. Check the current rulebook before relying on a schedule.

Frequently asked questions

How often is the S&P 500 rebalanced?

Scheduled rebalances happen quarterly, taking effect after the close on the third Friday of March, June, September and December. Membership changes can also happen at any time when a member is acquired, delisted or otherwise leaves, and the index committee decides on additions rather than a fixed formula.

When is the Nasdaq-100 reconstituted?

Once a year. The annual reconstitution is announced in early December and takes effect after the close on the third Friday of December. Weights are rebalanced quarterly, and a special rebalance can be triggered if the largest members' combined weight breaches set thresholds, as happened in July 2023.

What is the 90/111 rule for the FTSE 100?

At each quarterly review, a company outside the FTSE 100 is added if it ranks 90th or higher by market capitalisation, and a member is removed if it ranks 111th or lower. The gap between the two acts as a buffer that reduces turnover from small changes in ranking.

Does a stock always rise when it joins an index?

No. Additions have historically been associated with gains around the announcement, but the effect is widely anticipated, appears to have become smaller over time, and can partly reverse after the effective date. It is not a dependable pattern to trade.

Does rebalancing affect index CFDs?

Indirectly. An index CFD follows the index level, which is unaffected by the change itself because providers adjust the divisor to keep the series continuous. What you may notice is unusual volume and volatility near the close on review days, especially when they coincide with quarterly futures expiry, and, over time, a change in which stocks drive the index.

Where can I find upcoming index changes?

The index providers publish announcements and methodology documents on their websites: S&P Dow Jones Indices, Nasdaq, FTSE Russell, STOXX, Nikkei and Deutsche Börse. Financial news services also report the main changes, usually on the evening they are announced.