On the board of Khimen, the ticker for Iman Khodro Shargh, two numbers sat side by side on Monday 3 August 2026 (12 Mordad 1405). One was green and the other red. The symbol's last trade printed at 4,440 rials, 10 rials above the previous session's closing price, a gain of 0.23 percent. Yet the closing price recorded for that very same session was 4,400 rials, 30 rials below the previous day, a fall of 0.68 percent. One stock, one session, two numbers with opposite signs and a 40 rial gap between them.
Neither number is wrong, and the board is not broken. The two measure different things. What matters more is that the Tehran Stock Exchange main index sees only one of them: the closing price. So in the index calculation Khimen was a red symbol on Monday, even though the last person who bought it paid more than the previous day's close.
Background: a rule built to dampen volatility
The mechanism most shareholders reach for to explain this gap is base volume (حجم مبنا): the minimum number of shares that must trade in a session for the closing price to register the full price change of that day. The logic is straightforward. If the closing price were simply the last trade, a handful of small trades in the final minutes could move a number that anchors the main index calculation, tomorrow's price limit band, and the portfolio valuation of millions of people. Base volume shortens that lever.
The formula is a ratio. If a symbol's traded volume falls below its base volume, only that fraction of the price change is registered in the closing price. A symbol that trades half its base volume sees only half of its daily price change reflected in the closing price. Once traded volume reaches or passes base volume, the coefficient becomes one and the closing price registers the whole move.
But there is a dated caveat here, and without it the entire explanation misleads. Base volume rules have been revised several times in recent years, and today's situation is not what many shareholders carry in their heads. Based on trading system board data for Monday 3 August 2026, base volume for Tehran-listed symbols is set to 1 share. We checked this separately for three symbols of completely different size and sector, Bank Mellat (Vabmellat), Iran Glass Wool (Kapshir) and Mobarakeh Steel of Isfahan (Foulad), and in all three base volume equalled 1. The same figure repeats across the full market watch listing for other symbols.
The practical meaning is clear. Today the base volume brake is effectively off. Any symbol that trades even a single share immediately reaches the required floor and its coefficient becomes one. So the gap you saw on Khimen's board is not the work of the base volume brake. It is made by something else.
Monday's numbers
The table below puts four symbols side by side on the same day, Monday 3 August 2026. All prices are in rials, and all were taken from the trading system board at 16:00 that day.
| Symbol | Prior close | Last trade | Closing price | Gap |
|---|
| Khimen (Iman Khodro Shargh) | 4,430 | 4,440 (+0.23%) | 4,400 (-0.68%) | 40 rials, opposite signs |
| Vatoos (Toos Gostar Urban Development) | 7,540 | 7,740 (+2.65%) | 7,690 (+1.99%) | 50 rials |
| Afra (Afranet) | 10,610 | 10,300 | 10,330 | 30 rials, reversed direction |
| Vabmellat (Bank Mellat) | 1,165 | 1,199 | 1,199 | zero |
Three different patterns in one table: a symbol whose two numbers carry opposite signs, a symbol whose gap runs in the reverse direction, and a symbol with no gap at all. Daily symbol prices can be followed on the equity market prices page.
Where today's closing price actually comes from
When the base volume coefficient equals one, the closing price is nothing more than the volume-weighted average price of all trades that day: total trade value divided by the number of shares traded, rounded to the nearest tick. This can be verified with the board's own numbers.
Khimen traded 150,262,208 shares worth 661,832,540,420 rials across 3,208 trades on Monday. Dividing value by volume gives 4,404.5 rials, which rounds to exactly the recorded closing price of 4,400 rials. The same calculation gives 7,685 rials for Vatoos and precisely 15,830 rials for Kapshir, and both match those symbols' recorded closing prices. Three symbols, three times, the same result.
Now set the last trade beside it. The last trade is just one number, the price of the final parcel of shares that changed hands in that session. There might be 100 shares behind it or 10 million, and the board does not say which. So the difference between these two numbers is the difference between the volume-weighted centre of gravity of the whole day and the last snapshot of that day.
Drivers: what opens the gap
Intraday swing. Khimen ranged between 4,300 and 4,530 rials on Monday, and its first trade of the day printed at 4,300 rials, the low of the session. When a stock opens low, trades the bulk of its volume in the lower range, then climbs in the closing hours, the volume-weighted average stays low while the last trade rises. That is exactly where the gap is born.
Thin closing trade. A large gap usually means the last trade number rests on shallow volume. That is an independent data point in itself: the wider the gap, the less the last trade can be treated as representative of that day's true value.
Queues and price lock. On the other side, when a symbol trades the entire session at one price, the two numbers converge. Vabmellat was the perfect example on Monday: 5,179,357,680 shares traded across 10,418 trades, and both the last trade and the closing price stood at 1,199 rials, the ceiling of the permitted band. At that moment 1,204,935,994 shares of demand remained at that same price with the supply side empty, meaning a buy queue. When every trade happens at one number, the volume-weighted average is that number too, and the gap becomes zero.
The gap does not always point the same way. On Monday, Afranet was the mirror image of Khimen: a last trade of 10,300 rials against a closing price of 10,330 rials, meaning the last trade sat 30 rials below the closing price. Both were red against the previous day, but this time the number that entered the index was better than the last trade. So the gap is not a fixed bias and opens in both directions.
Why this matters for the main index
The Tehran Stock Exchange main index is calculated from symbols' closing prices, not from their last trades. This is not a technicality. It explains why the index number sometimes fails to move in step with a trader's felt sense of the session. Someone watching the board in the final hour sees the colour of the last trades, while the index sees the volume-weighted average of the entire day. If the market drifts weakly through the session and comes alive in the last half hour, the board looks greener than the index, and the reverse also holds.
On Monday the main index closed at 5,277,338 units, up 123,281 units or 2.39 percent against the prior session (as of 12:43, 3 August 2026). On a session that strong, a large share of symbols including Vabmellat and Kapshir locked at the top of the band and their two prices converged to a zero gap. That itself is a rule useful in board reading: a zero gap on a rising day usually signals a buy queue, not calm. We have opened up how these spreads form in our earlier coverage on the main index and the equal-weight index, and the basics of reading a board are covered in the Sahmino Academy lessons.
Bottom line
What happened on Khimen's board on Monday was this: a last trade of 4,440 rials and green, a closing price of 4,400 rials and red, and only the second number entered the main index. It matters because a shareholder's routine decisions, from reading portfolio returns to gauging tomorrow's price band, ride on a number most people never look at. The one thing to remember: the closing price is the volume-weighted average of the whole day and the last trade is only the final moment, and the size of the gap between them tells you how much the last trade number can be trusted.
What to watch
- The base volume column on the board of any symbol you follow. On 3 August 2026 that figure is 1 for Tehran-listed symbols, but the rule has changed several times before, and each fresh revision changes the relationship between closing price and last trade again.
- The size of the gap between the two prices as a gauge of closing-trade depth; a wide gap means lean less on the last trade.
- A zero gap alongside a buy or sell queue, which shows the symbol locked at the top or bottom of the band.
- Divergence between the main index and the felt sense of the session, especially on days when the market changes direction in the closing hour.
- The ratio of trade value to trade volume for each symbol, which reproduces the closing price and is a simple tool for verifying the board.
This report describes a mechanism and published data only, and contains no buy or sell recommendation.
