Thailand's SET Stock Indices Explained: SET to SET100

Home - Finance - Thailand’s SET Stock Indices Explained: SET to SET100 Last Updated on September 17, 2026 by Jeff Tomas For local and international investors, the Stock Exchange of Thailand (SET) offers a clear way to track Thailand’s listed companies and broader economic direction. Yet the SET isn’t represented by one number alone. Its index family includes measures for the broad market, large companies, smaller companies, and specific industry groups. When the SET Index rises, it usually means the market’s overall value has increased, even though some individual stocks may have fallen. A declining index can show broad weakness without telling you that every company is losing value. Understanding that distinction is essential, especially when reviewing challenges facing Thailand’s SET Index. Next, we’ll explain how the major indices work, how SET calculates their values, how constituents are reviewed, where to find reliable data, and which risks investors should consider. Thailand’s index family answers different market questions. The SET Index is the broad benchmark, while SET50 and SET100 concentrate on larger, more actively traded companies. Smaller-company and sector indices add useful detail, but none provides a complete picture alone. The SET Index asks, “How is the wider Thai stock market moving?” It covers common stocks on the SET Main Board, with official exclusions such as stocks suspended for more than three months. The definition also includes unit trusts of property funds. SET50 tracks 50 large-cap, highly liquid stocks. SET100 includes those companies plus the next 50 qualifying stocks, giving investors a wider large and mid-cap view. Both use market capitalization and liquidity screens, with a maximum 10% weight for each constituent. For example, a bank-heavy SET50 may rise more than the wider SET Index when financial stocks lead the market. Differences also come from company size, sector exposure, and constituent weights. These benchmarks describe market movement; they don’t promise your investment will earn the same return. News reports may quote the headline SET Index, such as this coverage of Thailand’s leading stock market index, even when individual investors hold very different portfolios. The sSET Index focuses on smaller listed companies outside the SET100. It can reveal activity that large-cap benchmarks miss, but limited liquidity may produce wider spreads, fewer trades, and sharper reactions when buying interest changes. Meanwhile, SET Industry Group and SET Sector indices show whether banks, energy companies, consumer businesses, technology firms, or another group is driving the market. Free-float versions, including SET50FF and SET100FF, adjust weights based on shares available for public trading. Use them for narrower questions, not as a full measure of Thailand’s economy. The SET, SET50, and SET100 are market-capitalization-weighted price indices. Their levels change mainly when constituent prices move, but SET also adjusts the calculation when share counts or other corporate details change. The basic formula is: Index = (Current Market Value / Base Market Value) x Base Value Current market value is the combined value of eligible shares at their latest prices. For example, if a company has 1 billion shares priced at THB 20, its market value is THB 20 billion. Base market value is the comparable starting value used when the index begins or resets. The divisor and base value keep the index comparable across time. Market-cap weighting gives larger companies more influence because their total equity value is greater. A THB 500 billion company usually affects the index more than a THB 5 billion company after a similar percentage price change. Full market capitalization and investable market capitalization aren’t always the same. Free float is the portion of a company’s shares realistically available for public trading. Shares held by founders, governments, controlling families, or strategic investors may count toward full market value but remain largely unavailable to everyday investors. As a result, SET50FF and SET100FF apply free-float-adjusted market capitalization. These versions can offer a more investability-focused view because companies receive weight based partly on the shares investors can actually buy. Under the current ground rules, each SET50 and SET100 constituent has a maximum 10% weight. The limit reduces the influence of the largest companies, but it doesn’t remove concentration. Several large constituents can still dominate the index, and SET reviews the cap within its regular index maintenance process. SET adjusts the divisor or calculation base when a corporate action changes a company’s share count. Otherwise, a stock could appear to add value to the market simply because more shares exist. For example, suppose a company issues new shares through a public offering. Its market capitalization may rise because the share count increases, even if its price doesn’t move. SET adjusts the index calculation so that mechanical dilution or expansion doesn’t look like a genuine market gain. Similar adjustments can apply to exercised warrants or the conversion of preferred shares into common shares. These changes preserve consistency over time. They don’t protect investors from real losses caused by falling prices, poor business results, or dilution. They only prevent the index formula from confusing a structural change with market performance. Finally, a price index mainly tracks price movements. A total return index also includes distributions, such as dividends, assumed to be reinvested. Therefore, the index level isn’t a stock price or a direct return for every investor. Fees, taxes, portfolio weightings, dividends, and currency movements can all produce different results. SET reviews its tradable indices using measurable screens rather than choosing companies by reputation alone. Market capitalization, trading activity, turnover, free float, suspensions, and surveillance status all help determine whether a stock can enter or remain in the SET50 or SET100. The headline SET Index has a broader universe. By contrast, SET50 and SET100 apply tighter eligibility rules to create benchmarks that investors can trade more easily. Review timing and criteria can differ across SET-native indices and FTSE SET products, so check the latest official ground rules before acting on a review announcement. A company added to the SET50 or SET100 often attracts more attention because index funds, exchange-traded products, and institutional portfolios may need exposure to it. That demand can increase trading activity around the effective date. A removal may create selling pressure when tracking funds reduce or eliminate their holdings. Weight changes can have a similar effect. If a company’s market capitalization or free-float estimate rises, its index weight may increase, giving its price movements more influence over the ben Information source: Chiang Rai Times