Direct Answer
The Nikkei 300 is a rule-based measurement of Japanese broad equity benchmark, maintained by JPX/Nikkei. 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 Nikkei 300 covers Japanese broad equity benchmark, within the Japan market. JPX/Nikkei 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
Market capitalization weighted: current market value is the sum of share price times issued shares (government owned shares are excluded), divided by a base market value and multiplied by 100, with the index set at 100 on October 1, 1982. The formula uses issued shares, so it is not adjusted for free float. The base market value is corrected for constituent changes and share count changes to keep the series continuous.
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
Drawn from the Tokyo Stock Exchange Prime Market. The universe first drops issues with very low pricing ratios, trading volume or turnover ratios, abrupt shifts in turnover rank over two years, no dividends for a considerable period, consecutive pretax losses or excessive debt. Candidates are then judged on their share of market value within Nikkei's 36 industries to keep the market and its industrial sectors represented.
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
Issues that have not been listed on the Tokyo Stock Exchange for two years cannot, in principle, be candidates for addition at the periodic review.
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
Reviewed once a year in September, with changes usually effective at the beginning of October, and a review can produce no changes at all. There is no upper limit on the number of changes. Vacancies from delistings or market transfers are filled by extraordinary replacement, and an addition and its deletion take effect on the same day to hold 300 names.
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
The count of 300 is restored by an adjustment step: a shortage is filled with the largest non-members from the industry whose constituents cover the smallest share of that industry's market value, and an excess is trimmed from issues with the lowest market value share in their industry. Stock splits do not change the base market value.
References
- JPX/Nikkei: Nikkei Inc. Nikkei Stock Index 300 Index Guidebook (July 24, 2025 version), as published 2025. Every rule described on this page is taken from this document.
Current Nikkei 300 constituents
JPX/Nikkei publishes and licenses the Nikkei 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 Nikkei 300?
JPX/Nikkei maintains the Nikkei 300 and publishes the methodology document that defines its eligibility and weighting rules.
What does the Nikkei 300 measure?
It measures Japanese broad equity benchmark. 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 Nikkei 300 constituents?
JPX/Nikkei publishes the current constituent list. Swoopr does not republish it, because index providers license constituent data and Swoopr holds no such licence.