S&P Quarterly Index Rebalance

What an index rebalance is

An index such as the S&P 500 is a list of companies with a weight assigned to each. Over time, the list and the weights need updating: companies issue or buy back shares, the portion of shares available to public investors changes, and companies are acquired, spun off or shrink below the index’s standards.

S&P Dow Jones Indices, the company that owns and calculates the S&P 500, S&P MidCap 400, S&P SmallCap 600 and many other benchmarks, handles routine updates in a quarterly rebalance. Because a very large amount of money in index funds and benchmarked portfolios tracks these indexes, the rebalance can trigger large trades in the affected stocks.

When it happens

The quarterly rebalance takes effect after the close of trading on the third Friday of March, June, September and December, so the updated index is in place when markets open the following Monday. That Friday is the same day as quad witching, which adds to the heavy trading usually seen at the close.

Dates on this page follow that rule and are labelled computed until S&P publishes its announcement. Details of each rebalance are released by S&P Dow Jones Indices on its index announcements site.

What changes at the rebalance

Share counts and float

At each quarterly rebalance, S&P updates each company’s shares outstanding to the latest public filings, and may update its investable weight factor (IWF), the share of a company’s stock that is available for public trading after excluding large strategic holdings. These updates change each company’s weight in the index even if its share price is unchanged.

Additions and deletions

S&P often announces a batch of companies joining or leaving the S&P 500, MidCap 400 and SmallCap 600 to coincide with the quarterly rebalance. These announcements typically come out after the close on a Friday about two weeks before the effective date, and name the companies and the effective date.

Membership changes are not limited to the quarterly dates, however. When a company is acquired or otherwise has to leave an index, S&P adds a replacement on whatever schedule the event requires. Many constituent changes happen outside the quarterly cycle.

How companies are chosen

The S&P 500 is not a mechanical list of the 500 largest U.S. companies. An index committee selects members using published eligibility rules covering factors such as company size, liquidity, the share of stock available to the public, and financial viability. Meeting the rules makes a company eligible; the committee decides when and whether to add it.

Why the rebalance can move stocks

Index funds aim to match their benchmark as closely as possible, so they generally trade at or near the closing price on the effective date. That creates a large, predictable demand to buy stocks being added and to sell stocks being removed, along with smaller trades in stocks whose weights change.

Several effects are commonly described:

• The announcement effect. Shares of a company announced for addition to the S&P 500 often move on the news, since it implies buying by index funds.
• Closing-auction volume. Rebalance days frequently see very large trading volumes in the closing auction.
• Front-running and reversal. Some traders try to anticipate the index trades, and prices can reverse after the effective date once the forced buying or selling is done.

These are tendencies described in market commentary. The size and even the direction of any price effect varies, and many additions see little lasting impact.

Common misreadings

• Thinking all S&P 500 changes happen quarterly. Many happen in between, triggered by mergers and other corporate events.
• Assuming joining the index makes a company more valuable. Inclusion changes who owns the shares, not the company’s business.
• Confusing S&P with Russell. FTSE Russell runs its own, separate reconstitution schedule; see Russell reconstitution.

Frequently asked questions

When is the S&P 500 rebalanced?

Quarterly, effective after the close on the third Friday of March, June, September and December. Changes are in place for the open on the following Monday.

When are S&P 500 additions announced?

Quarterly-rebalance changes are typically announced after the market close on a Friday about two weeks before the effective date. Changes driven by corporate events can be announced at any time.

Why do stocks jump when they are added to the S&P 500?

Index funds tracking the S&P 500 must buy the stock, and traders often position ahead of that demand. The effect is common but not guaranteed, and it can reverse.

Is the S&P 500 just the 500 biggest companies?

No. A committee selects members using eligibility criteria that include size, liquidity, public float and financial viability.

Explainer updated November 13, 2026.