Direct Answer
The S&P/ASX 300 is a rule-based measurement of top ~300 companies listed on the Australian Securities Exchange by float-adjusted market cap, maintained by S&P Dow Jones Indices/ASX. Like every index it owns nothing: it is a number produced by applying a published weighting formula to a selected list of companies, revised on a schedule the provider sets.
What this index covers
The S&P/ASX 300 covers top ~300 companies listed on the Australian Securities Exchange by float-adjusted market cap, within the Australia market. S&P Dow Jones Indices/ASX defines which securities are eligible, how they are selected from that pool, and how much each one contributes to the published number.
The name states a target of 300 companies. A target is not a guarantee of the exact count on any given day: corporate actions, dual share classes and pending additions all move the real number around it.
How constituents are weighted
Constituents are weighted by float-adjusted market capitalization (FMC) using the standard S&P Dow Jones divisor method. Share counts are reviewed quarterly and rounded to the nearest thousand.
The weighting method is the single biggest driver of how an index behaves. Two indexes covering the same market can diverge for years purely because they weight differently, which is why comparing index levels without knowing the method tells you very little.
What qualifies for inclusion
Measures 300 of the largest, highly liquid ASX securities by FMC, spanning the LargeCap, MidCap and SmallCap components. It needs a minimum Relative Liquidity of 30%, and a stock whose liquidity falls below half of that threshold becomes ineligible and is removed at the next rebalancing.
Eligibility rules are the part of an index most readers never look at and the part that decides what it can possibly measure. A screen that excludes a whole sector, or admits only one listing venue, shapes the result far more than any day of trading does.
How long a listing must exist first
Market capitalization is measured on a six-month daily average. An IPO or direct listing is added only when a vacancy appears or at a rebalance, and only after proven liquidity for at least eight weeks, unless an extraordinary large offering justifies an exception.
A seasoning rule is why a large new listing does not appear in an index the week it starts trading. It is a deliberate lag, and it means an index can under-represent a fast-growing part of the market for months at a time.
When the membership is revised
Rebalanced semi-annually, effective after the close on the third Friday of March and September. A non-member is considered at rank 274 or higher, and a member is removed at rank 326 or lower. Data are taken on the second to last Friday of the prior month.
Reconstitution is the one date an index changes shape by design rather than by price. Funds tracking it have to trade toward the new membership, which is why the schedule matters to anyone holding the tracker rather than only to the provider.
The rule that surprises people
Unlike the S&P/ASX 20, 50, 100 and 200, it is not a fixed count index. Between rebalances a stock is added only when a non-member is added to the S&P/ASX 200 or a higher index, which pulls it into the 300 as well.
References
- S&P Dow Jones Indices/ASX: S&P Dow Jones Indices, S&P/ASX Australian Indices Methodology, as published 2025. Every rule described on this page is taken from this document.
Current S&P/ASX 300 constituents
S&P Dow Jones Indices/ASX publishes and licenses the S&P/ASX 300 constituent list. Swoopr does not republish it here. The provider's own page is the authoritative source for current membership and weights.
Related reading
- All stock market indexes: how indexes are built, weighted and revised.
- Asia-Pacific indexes: others covering the same region.
- Index concentration: measuring how much of an index sits in its largest holdings.
- Index rebalancing and inclusion effects: what happens to a stock when index membership changes.
Frequently Asked Questions
Who maintains the S&P/ASX 300?
S&P Dow Jones Indices/ASX maintains the S&P/ASX 300 and publishes the methodology document that defines its eligibility and weighting rules.
What does the S&P/ASX 300 measure?
It measures top ~300 companies listed on the Australian Securities Exchange by float-adjusted market cap. The index is a calculated number applying a weighting formula to a selected list of companies, and it holds nothing itself.
Where can I see the current S&P/ASX 300 constituents?
S&P Dow Jones Indices/ASX publishes the current constituent list. Swoopr does not republish it, because index providers license constituent data and Swoopr holds no such licence.