An East Marvel guide. Updated November 2026.
You are not choosing the best beach in Thailand. You are choosing a sea. The Andaman is on the west. The Gulf is not one climate. Thailand runs all year. This page is which sea fits the month.
The rainfall figures are Thai Meteorological Department normals for 1991–2020. A week can sit either side of them. The rains shift by a week or two each year. Before you go, ask again whether the boat is running.
The west, the Andaman
Phuket, Krabi, Phang Nga, Phi Phi and Lanta are on this coast.
The rain is the southwest monsoon, from May to October. At the Phuket station the wettest month is September, about 350 mm. October is close, about 336 mm, and it has a few more rainy days. At the Krabi station October is the wettest, about 293 mm, and September is about 286 mm. In the worst weeks, boats are cancelled at short notice.
December to February is the calmest. February at Phuket is about 24 mm. March and April are hot, and quieter than December, except Songkran. April is drier than September, and it is not a dry month: the Phuket station averages about 135 mm. In May the sea turns. November is the start of the better months, not the first day of them. November at Phuket is still about 178 mm. In the opening weeks, the sea can keep you out of the water.
Keep one bay. Kata, Kamala or Bang Tao on Phuket, or Ao Nang in Krabi. Phi Phi is one day, only if the sea is flat, and only on the large ferry.
The Similan Islands and Surin are parks, not a season you can guess. This season both are open from 15 October 2026 to 15 May 2027. The department's standing season runs from 15 October to 15 May. The next year's closure list is a new notice. An open park is not a calm sea. Ask the operator about that morning.
Koh Chang, and the islands with it
Koh Chang, Koh Mak and Koh Kut take the southwest monsoon in full. At the Trat station, on that coast, a year averages about 4,900 mm. July alone is about 1,000 mm. May to October is the heavy stretch. Boats between these islands stop. The ferry from the mainland to Koh Chang mostly keeps running. In November the rain at Trat falls to about 85 mm, the sea settles, and the rooms are not yet at Christmas prices. December to February is the calmest.
The name means elephant, from the shape of the island. You do not go there to ride one.
Pattaya and Koh Samet stand in the same wind. They do not get that rain. Pattaya's wettest month is October, about 225 mm, against a year of about 1,100 mm. Its driest months run from November into April. Pattaya is about two hours from Bangkok, traffic depending.
Samui, Phangan, Tao
Koh Samui, Koh Phangan and Koh Tao sit on the other side of the Gulf. They take the northeast monsoon.
At the Ko Samui station the heavy rain is October to December. November is the worst, about 445 mm. October is about 295 mm and December about 263 mm. February is the driest month, about 64 mm. From February through September every month stays far under November. July is about 117 mm and August about 102 mm. Those are the months to use if the Andaman is in its rains: here you can usually still swim. May and June are a little hotter, with a few more showers. A normal January afternoon is about 29°C. May is not that month.
November on Samui is the sea that is raining. Mu Ko Ang Thong, the park off Samui, is listed as closed from 1 November to 15 December 2026. The park's own page says 1 to 30 November. The 2027 dates will be a new notice.
Chaweng is the busiest. For quiet, Lipa Noi or Choeng Mon. The Full Moon Party on Phangan is one night a month, not a season. It is loud and crowded. Book that night and the nights on either side. Koh Tao is for diving. A course takes the four nights. Choose the beach, or choose the course.
Choose the coast by the month
November to February. The Andaman, or Koh Chang. Samui is in its rains.
March and April. The Andaman, Koh Chang, or the Samui side. Hot. Songkran centers on 13–15 April 2027. In that week the streets are full of water, and the rooms are not the cheaper rooms of early April. Check that year's public-holiday dates before you lock the flights.
May to September. The Samui group. Leave the Andaman and Koh Chang.
October. No coast is in its best month. The west is still coming out of the rains. Samui is going into them. For the west, wait until November. For Samui, wait until January.
Hua Hin, about three hours south of Bangkok, is a drier case. December to April is the dry side, and December is about 12 mm. October is the wet month, about 250 mm. September is already about 130 mm, wetter than August at about 67 mm. May to August is showers. It is not Phuket's rain, and it is not Samui's November.
Twelve days is one sea. A flight can change coasts. A boat cannot change the monsoon.
The line to keep: November in the west, July on Samui. In October, do not book the sea yet.
What we recommend
November to April stays on the Andaman. The published route is Phuket, Phi Phi, and Krabi, with Phang Nga Bay. May to September uses the Gulf. That route is Bangkok, Koh Samui, and Chiang Mai: three nights, four nights, and four nights, and the flights are how it leaves the sea. Neither is an October booking.
Phuket, Koh Phi Phi & Krabi, 9 days