The IHSG Isn't the Average Stock Price. Here's the Formula Behind the Number
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6 min read
On 31 August 2026, the IHSG (Indeks Harga Saham Gabungan, Indonesia's main stock index) closed up 0.11% at 6,525.478 (Liputan6, "IHSG Hari Ini 31 Agustus 2026 Menguat ke 6.525"). That 6,525.478 number gets repeated on the news every day, but it is not an average stock price, and it is not a simple average of how much every stock moved. It comes from one specific formula, and understanding it changes how you read every headline about "the market."
The IHSG is not a price. It is an index: a single number that tracks the combined market value of every stock listed on the Indonesia Stock Exchange (BEI), measured against a fixed starting point. The formula BEI itself uses is:
Nilai Pasar ("market value") is the combined value of all listed shares today. Nilai Dasar ("base value") is a reference number anchored to 10 August 1982, when the index started at exactly 100 with just 13 listed stocks. Multiply the ratio by 100 and you get the index level everyone quotes.
That base date is why the IHSG can sit above 6,500 today: the combined market value of Indonesia's listed companies has grown to many times what it was in 1982, and the index simply expresses that growth as a multiple of the original 100.
Here is the part most explainers skip. Nilai Pasar does not count every share a company has issued. It counts only the free float: shares actually available to the public, excluding founder stakes, controlling-family holdings and other locked-up blocks. A company can have a huge headline market cap and still carry a small IHSG weight if most of its shares never trade.
That mechanic has its own detail worth a full explanation on its own; see how free-float weighting can cut a giant company's real index weight by 80% or more for the fuller picture, including a real IDX example of the gap it creates.
If Nilai Dasar never changed, then every time a company issued new shares, its Nilai Pasar would jump for a reason that has nothing to do with the stock market actually going up, and the index would spike or crash on paperwork instead of price action.
To prevent that, BEI adjusts Nilai Dasar whenever an event changes the free-float share count for a reason unrelated to trading:
The adjustment is calibrated so the index level does not jump purely because the share count changed. A stock split, a stock dividend or bonus shares do not trigger a Nilai Dasar adjustment, because those actions multiply the share count and divide the price by the same factor: the company's actual market value does not change, so nothing needs correcting.
This is the part that actually explains why some headlines call out a handful of names as having "moved the IHSG" while dozens of other stocks that rose by a similar or larger percentage barely get mentioned. Look at the formula again: Nilai Dasar is one fixed number applied to the whole market at any given moment. That means the number of index points a stock's price move contributes depends only on the rupiah size of the change in its free-float market value, not on the percentage the stock itself moved.
Say two hypothetical companies both rise 5% on the same day:
| Company (illustrative) | Free-float market value | Rupiah change from a 5% move |
|---|---|---|
| Large-cap Company A | Rp500 trillion | Rp25 trillion |
| Small-cap Company B | Rp5 trillion | Rp250 billion |
Both stocks moved by the identical 5%. But Company A's move changes total Nilai Pasar by 100 times more rupiah than Company B's, because Nilai Dasar divides that same rupiah change for every stock equally, a 5% move at Company A's size pushes the index roughly 100 times harder than the same 5% move at Company B's size.
This is exactly why closing reports for 31 August 2026 named a specific handful of stocks, including Telkom Indonesia (TLKM), Bank Rakyat Indonesia (BBRI) and PT Petrindo Jaya Kreasi (CUAN), as the names that pushed the index higher that day rather than crediting "the market" broadly (StockWatch, "IHSG Akhir Agustus 2026 Menguat 0,11% ke Level 6.525,478 Berkat Saham-saham Ini"): a handful of large free-float weights can outweigh the direction of everything else combined.
Once you know the formula, "IHSG up 0.11%" stops sounding like "the average stock rose 0.11%." It means the combined free-float value of everything listed on BEI rose by 0.11% of Nilai Dasar, and that combined figure was likely driven by a small number of very large, heavily-weighted stocks rather than by the market broadly. A portfolio full of small-cap names can underperform a flat IHSG day, and a portfolio full of the largest banks and telcos can beat it, purely because of how much weight each holding carries in the formula, not because either portfolio was better picked.
The formula explains the mechanism; it does not tell you which stocks are behind today's move. Open the Market page in NetWort to see the sectors and names actually driving the IHSG right now, rather than assuming every stock's move counts equally toward the number in the headline.