Is the IHSG Just 10 Stocks? The Concentration Risk Inside Indonesia's Own Index
7 min read
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7 min read
If you own an IHSG-tracking fund, you probably think you own a slice of Indonesia's whole economy, spread across hundreds of companies. On paper, the IHSG (Indeks Harga Saham Gabungan, Indonesia's main stock index) does list more than 900 companies. In practice, a small handful of them carry most of the weight, and one sector alone accounts for close to a quarter of the entire exchange's value.
That gap between how diversified an index looks and how concentrated it actually is, is called index concentration risk. It matters because a fund that tracks the IHSG is not really betting on 900 companies equally. It is betting heavily on whichever few happen to be biggest, whether you realise it or not.
Start with raw size. As of 13 July 2026, Bank Central Asia (BBCA) alone was the single largest company on the Indonesia Stock Exchange (BEI), worth Rp760 trillion, equal to 7.23% of the entire exchange's market value that day (Kontan). One company, more than seven cents of every rupiah on the whole exchange.
The same report placed the next four largest companies at:
| Rank | Company | Market cap (13 Jul 2026) |
|---|---|---|
| 1 | Bank Central Asia (BBCA) | Rp760 trillion |
| 2 | DCI Indonesia (DCII) | Rp475 trillion |
| 3 | Barito Renewables Energy (BREN) | Rp432 trillion |
| 4 | Bank Rakyat Indonesia (BBRI) | Rp431 trillion |
| 5 | Bank Mandiri (BMRI) | Rp393 trillion |
Source: Kontan, "Klasemen Market Cap BEI Bergeser," 13 July 2026.
Add those five together and you get roughly Rp2,491 trillion. BBCA's own reported 7.23% share implies the whole exchange was worth about Rp10,512 trillion that day (Rp760 trillion ÷ 7.23%, self-derived from the two sourced figures above), which puts the top five names alone at close to 24% of the entire market, out of more than 900 listed companies. The total value of the exchange has since grown further, to about Rp10,923 trillion by the week ending 1 August 2026 (Infobanknews), but the shape of the problem, a handful of names carrying an outsized share, has not changed.
Zoom out from individual stocks to sectors and the concentration looks even starker. As of the end of April 2026, the financial sector, mostly the big banks, made up Rp2,862 trillion, or about 23.12% of the IHSG's total market capitalisation of Rp12,382 trillion at that time (Cetro Trading Insight). Almost a quarter of the entire Indonesian stock market's value sits in one sector.
Foreign investors clearly treat these bank stocks as the index's real engine. In mid-July 2026, a wave of foreign buying concentrated in the big banks was reported as a direct driver of the IHSG's gains that week (CNBC Indonesia, 17 July 2026).
Here is the twist: the raw market cap ranking above (BBCA, DCII, BREN, BBRI, BMRI) is not actually how the IHSG weighs its constituents. Since 1 October 2021, phased in fully by April 2022, the IHSG has calculated its weights using free-float adjusted market capitalisation, meaning only the shares that genuinely trade in the open market count toward a stock's index weight. Shares held by founders, controlling families, or other insiders who are not realistically sellers are stripped out of the calculation.
This matters because some of Indonesia's largest companies by raw market cap have very little of their stock actually available to the public. Barito Renewables Energy (BREN), the third-largest company on the exchange by market cap, had a free float of just 12.3% as of 31 March 2026 (SWA.co.id; Investor.id). So while BREN's headline market cap makes it look like Indonesia's third-biggest company, less than an eighth of its shares actually move in the market, which pulls its real weight inside the free-float-adjusted IHSG well below what its raw size suggests.
Free float being low is not just a technical footnote. On 2 April 2026, the Indonesia Stock Exchange (BEI) and its central depository, KSEI, published their first ever list of High Shareholding Concentration (HSC) stocks: nine companies where a small group of related shareholders together control the overwhelming majority of the company. BREN was one of them, flagged at 97.31% ownership concentration among linked shareholders, alongside Dian Swastatika Sentosa (DSSA) and seven smaller names (ROCK, IFSH, SOTS, AGII, MGLV, LUCY, RLCO) (CNBC Indonesia, 21 April 2026).
The consequence was not just a warning label. BREN and DSSA were subsequently removed as constituents of the LQ45 and IDX80 indices, two of the exchange's other benchmark indices, specifically because of this concentration (IDNFinancials). A stock can be one of the largest on the exchange by market cap and still get excluded from a diversified benchmark for being too concentrated in the hands of too few owners.
The IHSG itself carries no cap on how large a single stock's weight can grow. But IDX30, the exchange's own curated blue-chip index, does: it enforces a hard 15% maximum weight per stock, specifically to stop the index from being dominated by one or two names.
That cap was just enforced again. In the rebalance effective 3 August 2026, BBCA's weight in IDX30 was trimmed to 15.00% (down from 16.62%) and BBRI's to 15.00% (down from 15.76%), purely to bring them back under the ceiling. Bank Mandiri's weight rose to 12.87% and Telkom Indonesia's (TLKM) fell to 8.66% in the same rebalance (Kontan, "Resmi! 2 Saham Ditendang Dari Indeks IDX30," 3 August 2026). Even inside a 30-stock, already-curated index, two banks kept bumping up against the ceiling built to stop exactly this kind of dominance.
None of this means the IHSG is a bad benchmark, or that Indonesian banks are bad businesses. It means that "I hold an IHSG-tracking fund" is not the same claim as "I am spread across the Indonesian economy." In practice, it is closer to "I hold a large position in Indonesian banking, plus exposure to whichever conglomerate happens to be biggest that quarter."
That is worth knowing before a bad week in bank earnings, or a fresh concentration warning on a name like BREN or DSSA, moves the whole index more than the average headline number suggests it should.
Open the Market page in NetWort to see current sector and stock-level moves, rather than assuming an "IHSG is up" headline means the average Indonesian company had a good day. If you want to see how one of the index's biggest constituents is doing on its own, Bank Central Asia's page breaks out its price history separately from the index it dominates. It pairs well with the same lesson applied to a single portfolio: how much a 40%-weighted position can cost you when it has a bad day, the same math, just at the scale of an entire national index instead of one account.