S&P/ASX 300 Index

The S&P/ASX 300 Index is an Australian equity-market benchmark designed to measure the performance of up to 300 of the largest eligible securities listed on the Australian Securities Exchange (ASX). It uses float-adjusted market capitalisation and is intended to represent large-, mid- and small-cap segments of the Australian listed market. ^[1]

Overview

The S&P/ASX 300 is part of the S&P/ASX index series, which is produced through a partnership between S&P Dow Jones Indices and the ASX. The index’s stated purpose is to provide broader exposure to Australian equities than narrower benchmarks, while applying size and liquidity considerations to the securities it includes. S&P describes it as covering large-cap, mid-cap and small-cap components of its Australian index family. ^[1]

The index is commonly referred to as the ASX 300. Its name indicates the approximate number of constituent securities, not a fixed promise that it will always contain exactly 300. S&P describes it as measuring “up to 300” securities; eligibility, additions, removals and corporate events can affect the actual composition over time. ^[1]

The S&P/ASX 300 is broader than the S&P/ASX 200 Index, which measures a smaller group of large, liquid Australian-listed stocks. The additional constituents extend coverage into smaller companies, but the index remains focused on securities meeting index-provider eligibility and liquidity requirements. It is not a census of every company listed on the ASX. ^[1][2]

Construction and eligibility

Securities must be listed on the ASX to be eligible for inclusion in the S&P/ASX series. Index rules set out how the eligible population is assessed, how additions and removals are determined, and how the index is maintained. ASX describes the S&P/ASX methodology as the public rule set governing index entry and removal. ^[2]

The index is float-adjusted, meaning its market-capitalisation measure accounts for the shares considered available to investors in the public market rather than treating all shares outstanding as freely tradable. S&P’s description identifies float-adjusted market capitalisation as the basis for ranking securities in the index. ^[1] The investable weight factor, or IWF, is the ratio of available free-float shares to total shares outstanding. ^[2]

In September 2025, modifications to the wider S&P/ASX index series changed aspects of the inclusion process for the S&P/ASX 200 and S&P/ASX 300. ASX reported that the calculation period for average float-adjusted market capitalisation and daily liquidity was reduced from six months to three months. It also described a two-way turnover approach, under which a security that rises above an index entry buffer may be added without waiting for an existing constituent to fall below an exit buffer. ^[2]

The September 2025 changes also reduced the minimum IWF for index inclusion from 0.30 to 0.15. ASX explained that the changes were intended to help the indices reflect changing market conditions more quickly and align the series with S&P DJI index conventions. These are general series-level changes described by ASX; the index provider’s applicable methodology remains the authoritative reference for detailed rules and specific eligibility decisions. ^[2]

Weighting and index calculation

The S&P/ASX 300 is weighted by float-adjusted market capitalisation. In general terms, a constituent with a larger eligible market value has a greater weight in the index than one with a smaller eligible market value. Because the calculation adjusts for free float, shares not considered available to public investors do not contribute in the same way as freely traded shares. ^[1][2]

An index level is a calculated measure of the combined price movements of its constituents, not the price of a single security or the value of a portfolio that an investor directly owns. A change in the index reflects movements in its constituents and may also reflect index maintenance and corporate-action adjustments under the applicable methodology. The index does not itself issue shares, hold assets or provide a return to an investor.

S&P identifies franking-credit versions of the index in addition to its standard index offering. Those versions are designed to account for Australian dividend imputation credits under their specific calculation rules; they should not be confused with the ordinary price-return index or with a particular fund’s realised return. ^[1] The published index methodology and product documentation determine the precise return series and treatment of dividends and credits.

Maintenance and review

Index providers periodically reassess securities against the series’ rules. A review can result in companies entering or leaving an index and can change constituent weights. ASX describes rebalancing as the regular review and adjustment of an index to reflect the eligible company population and applicable inclusion criteria. ^[2]

The rules are important because index membership is not simply a ranking of companies by their headline market capitalisation on a particular day. The process also uses eligibility criteria, float adjustment and liquidity measures, and it may incorporate entry and exit buffers to manage turnover. The 2025 changes described by ASX altered the calculation period and the way entry and exit can interact. ^[2]

Index changes can matter to institutions that use the benchmark for portfolio comparison or index-tracking strategies. When constituents or weights change, managers seeking to track an index may need to adjust holdings. The extent and timing of any trading depend on the relevant product or mandate; the index itself does not require investors to trade.

Uses

The index’s breadth makes it a reference point for measuring performance across a wider span of listed Australian companies than the S&P/ASX 200. S&P says the S&P/ASX 300 is intended to address investment managers’ need for a broad opportunity set with sufficient size and liquidity. ^[1] Fund managers and analysts may use an index as a benchmark for evaluating an investment strategy, describing a market segment or comparing returns over a stated period.

Index-linked products may seek to track or otherwise reference the S&P/ASX 300, but the existence of an index-linked product does not mean that the index itself is an investment vehicle. For example, S&P’s index page lists the Vanguard Australian Shares ETF among products linked to the index. ^[1] A product’s holdings, fees, tracking approach and performance are determined by its own documentation and can differ from the index’s calculated results.

The index can also serve as a market indicator. Its level and returns summarise the performance of its rule-defined constituent group, but should not be read as a complete representation of every Australian company, every ASX-listed security or the Australian economy. As a market-capitalisation-weighted index, larger constituents have greater influence on its aggregate movement than smaller constituents. ^[1]

Relationship to other Australian indices

The S&P/ASX index family includes benchmarks with different constituent counts and market segments. The S&P/ASX 200 focuses on 200 of the largest eligible stocks, while the S&P/ASX 300 adds broader large-, mid- and small-cap coverage. The S&P/ASX Small Ordinaries, in the methodology document surfaced by S&P, measures companies in the S&P/ASX 300 that are not in the S&P/ASX 100. ^[3]

The All Ordinaries Index is another established Australian equity-market measure, but it is a distinct index with its own rules and coverage. The S&P/ASX 300 should likewise be distinguished from the S&P/ASX 200 VIX Index, which concerns expected volatility derived from options on the S&P/ASX 200 rather than representing a broad portfolio of Australian shares.

Limitations and interpretation

Index membership does not guarantee that a security will remain eligible, nor does inclusion indicate an assessment of the company’s quality or future results. The index applies defined market and investability criteria; it does not determine whether a particular security is suitable for any investor. Its composition changes as companies’ relative size, liquidity and eligibility change, and as corporate events occur. ^[2]

Because the index uses float-adjusted market-capitalisation weights, its return can be influenced disproportionately by its largest constituents and by the industries in which those companies operate. A broad constituent count therefore does not imply equal weighting or equal influence across companies. Index users should also distinguish price returns from total-return or franking-credit-adjusted series, since these are different measures with distinct calculation treatments. ^[1]

The S&P/ASX 300 is consequently best understood as a rules-based benchmark for a selected, liquid and float-adjusted segment of ASX-listed equities. Its purpose is to provide broad market representation and a consistent comparison measure; it is not a guarantee of diversification across all economic risks, a forecast of future performance or a directly investable asset. ^[1][2]