Thai stocks fall 5% in five days, then slip below 1,500

On Friday, 30 October 2026, a single stock was the main reason the Thai stock index lost more than 26 points in one day. DELTA closed down 8.14% at 203 baht and dragged the SET index down 26.01 points to 1,518.59, Thansettakij reported. Two trading days later the index fell below 1,500, a round number people in the market use as a reference point.

Over the five trading days to Monday, 2 November, the index fell from 1,586.61 to 1,507.20, a loss of about 79 points or about 5%, according to an InnovestX analysis published by Kaohoon International. It pointed to two causes: selling in refinery and petrochemical stocks after the government changed how fuel prices are calculated, and a global sell-off in tech stocks. Then on Tuesday, 3 November, the index closed at 1,491.01, according to the Stock Exchange of Thailand's website before the market opened on the morning of 4 November.

The SET index is a scale where big companies weigh more

Before getting to why the index fell, it helps to know what the number 1,500 means. The SET index is a single figure that sums up the prices of all common stocks on the main board of the Stock Exchange of Thailand, measured against a base date of 30 April 1975, when it was set at 100 points, according to the exchange. An index of 1,500 therefore says Thai share prices overall are about 15 times higher than on that day.

What people often miss is that stocks do not carry equal weight. The index is weighted by market value, which is the share price multiplied by the company's total number of shares. Picture a scale holding both big rocks and grains of sand. Move a rock a little and the needle twitches, while hundreds of grains have to move together before you see anything. In the Thai market the biggest rock is DELTA, a company that makes power equipment and IT systems for AI data centres. In July 2026 it made up about 18.5% of the total market value, Thairath reported. One stock can therefore drag the whole index, as it did on 30 October.

Tech stocks fell with global markets, and DELTA's profit missed expectations

The first push came from abroad. That same week, tech stocks around the world were sold heavily as investors began to ask when the hundreds of billions of dollars big companies are pouring into AI will turn into profit. On 30 October US tech stocks lost more than $1 trillion (about 34 trillion baht) in a single day, which we covered in Tech stocks lose $1 trillion in a day over AI doubts. Thai stocks tied to electronics and data centres were sold along with them.

DELTA faced two pressures at once. Besides the global mood around tech, its third-quarter 2026 results came in about 10% below what the market expected, because of higher costs, a partial shortage of raw materials and extra tax it had to book, Krungthep Turakij reported. From 203 baht on Friday the share fell below 200 baht on Monday and stood at 194.50 baht at the end of the morning session on 3 November, down another 2.51%, Kaohoon International reported.

The government changed the refinery price formula, so refinery stocks fell

The second push came from home, and it is closer to anyone who fills up a tank than it may seem. The price at the pump starts from the refinery gate price, the price at which a refinery sells fuel before taxes, the oil fund levy and the station's marketing margin are added. Since 2004 Thailand's formula for the refinery gate price had been based on Singapore market prices, plus freight, insurance and fuel lost in transit, as if the fuel were shipped in from Singapore, even though most of it is refined in Thailand. A column in Thansettakij therefore called these "phantom costs".

From 29 October 2026 the Energy Policy Administration Committee removed those assumed costs and switched to actual costs plus a refining margin of no more than 2.4 baht per litre, InfoQuest reported. Bualuang Securities estimated the change would save about 10,000 million baht a year. Consumers gain, but refiners receive less. When future profits look likely to shrink, share prices fall straight away. On the morning of 29 October, PTT Global Chemical's PTTGC fell 8.33%, Thai Oil's TOP fell 4.60% and Bangchak's BCP fell 5.05%.

As for drivers, do not expect a big cut at the pump yet. The same column said the saving may be only a few satang a litre, and pump prices may not fall right away because taxes, the oil fund levy and marketing margins are still part of the price. Added up across all the fuel the country uses, though, the money is big enough for investors to sell refinery stocks across the whole group.

Falling world oil prices hit energy stocks again

The third push came on Monday, 2 November. World oil prices fell after reports that Iran might open the Strait of Hormuz, a key shipping route for the world's oil, according to Thai Enquirer's news summary. That sounds like good news for drivers, but for PTTEP, which earns its money drilling for and selling oil and gas, lower prices mean lower future income. PTTEP and PTT shares were sold together, and the index closed that day at 1,507.20, down 11.39 points.

This is why energy prices can move the Thai stock market so much. Energy companies are a large group in the market, so a rise or fall in oil prices changes expected profits at several big companies at once. Within a few days, energy stocks got two pieces of bad news in a row, one hitting refiners and the other hitting the companies that drill for oil and gas.

What to watch after the fall below 1,500

There is nothing special about 1,500 as a number. It is a round figure investors use as a reference point, known as a support level, a level where buyers have stepped in before and stopped prices from falling further. Thai Enquirer's summary put support for 3 November at exactly 1,500. With the index now closing below it, what many are waiting to see is where the selling stops.

There are three things to follow this week. The first is the result of the US midterm elections, which starts coming in on 4 November Thai time, and the same summary expected the market's upside to stay limited until the result is clear. The second is DELTA's share price, because it moves the index more than any other stock. The third is world oil prices and news about the Strait of Hormuz, which affect both energy stocks and what we pay for fuel.

If you hold a Thai equity mutual fund, open the fund's fact sheet, the summary of key information in its prospectus. It lists the stocks the fund holds most, so you can see how much of your money is tied to DELTA or energy stocks. This story explains why the market fell. It is not advice to buy or sell any stock.

FAQ

What is the SET index?

A figure that sums up the prices of all common stocks on the main board of the Stock Exchange of Thailand, weighted by each company's market value and measured against a base of 100 points on 30 April 1975.

Why can one stock like DELTA move the index so much?

Because the index is weighted by market value, and DELTA is the company with the highest market value on the exchange. In July 2026 it made up about 18.5% of the whole market.

Will the new fuel price formula make fuel cheaper at the pump?

The cut at the refinery gate may be only a few satang a litre, and pump prices also depend on taxes, the oil fund levy and marketing margins, so they may not fall right away.

Does this story recommend buying or selling shares?

No. It explains why the market fell. Investment decisions depend on each person's goals and how much risk they can take.

Sources

• Kaohoon International: InnovestX Highlights Buying Opportunity as Thai Stocks Turn Attractive After Recent Dip
• Kaohoon International: SET Index Sees Marginal Decline as Investors Monitor Energy Prices and US Midterm Elections
• Thai Enquirer: Thai Enquirer News Summary, November 3, 2026
• Thansettakij: DELTA นำตลาดร่วง 8.14% ฉุด SET ปิดลบ 26 จุด หลุดยืน 1,518.59 จุด
• Thansettakij: สะเทือนโรงกลั่น ถอดสูตร "ต้นทุนทิพย์" ใครได้ใครเสีย
• InfoQuest: หุ้นโรงกลั่น ปิโตรฯ ดิ่งยกแผงรับพิษนโยบายรัฐปรับโครงสร้างราคาน้ำมัน
• Krungthep Turakij: หุ้นไทยเปิดเช้านี้ร่วง 8.31 จุด DELTA กดดัน หลังผลงานต่ำคาด
• Thairath: DELTA Shakes Market!
• Stock Exchange of Thailand: SET Index overview
• Stock Exchange of Thailand: SET Index profile