Koh Samui and Koh Tao in December: Dry Season or Washout? 11 Years of Rain

Averages 2015-2025 · Source: Open-Meteo, CHIRPS · How we measure

The trap: most guides treat December on the Gulf islands as one thing, a wet month on its way to dry season. Eleven years of daily rain say otherwise: it is a coin flip between a near-dry month and a washout, and the first two weeks decide most of it.

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What is December really like on Koh Samui and Koh Tao?

It depends on the year, more than on the island. Here is every December from 2015 to 2025 for the three Gulf islands, with three Andaman places beside them. The big number is rain in millimetres, the small one the number of rainy days.

Read down the Koh Samui column. December 2016 brought 519 mm on 17 rainy days. December 2019 and December 2021 brought 63 mm each, on 4 rainy days. Koh Tao tells the same story: 411 mm in 2016, 30 mm in 2019. The three Gulf islands mostly rise and fall together: 2016, 2018 and 2024 were wet on all three, though 2020 hit Koh Samui (298 mm) far harder than Koh Tao (107 mm).

The Andaman columns swing less. Their wettest Decembers were 236 mm on Phuket and 297 mm on Krabi and Koh Lanta, all in 2016 as well, the year the whole peninsula got a soaking, and still well short of Samui's 519 mm.

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How wide is the gap between a good and a bad December?

Here is the same record boiled down.

The averages (215 mm on Samui, 201 mm on Phangan, 156 mm on Koh Tao) hide the shape. On Koh Samui the middle year was 158 mm, but five of the eleven Decembers were 200 mm or more, and the driest two were 63 mm. There is no typical December: there is a dry year and a wet year, in about equal numbers. That is why a booking for "December on Samui" is a bet.

Koh Tao is the drier of the three on average, and has the lowest worst case (411 mm against 519 on Samui and 446 on Phangan). It still had three Decembers of 200 mm or more. If you are picking among the Gulf islands for December, Koh Tao is the better bet, but the margin is small next to the spread between years.

Is early or late December better?

Late December, clearly. Every Gulf island in the table gets about twice the rain, or more, in the first fifteen days than in the last sixteen: on Koh Tao an average of 114 mm against 42 mm, on Samui 146 mm against 70 mm. Rainy days follow the same pattern; the table shows the averages for each half.

This is an average, not a rule. In some years the second half was the wetter one, as on Samui in 2020, and in 2016 the first half alone brought most of the month's rain. But if your dates can slide, arriving around 15 December rather than 1 December improves your odds a lot, and it matches what we see in the January numbers on our Samui and Phangan guide, where the drying continues.

How does this differ from your Koh Samui page?

The numbers are lower, and the reason is the source. Our place pages use CHIRPS, a blend of satellite and rain-gauge data, which gives 318 mm for a typical December on Samui. Here we use the Open-Meteo archive for every place, because it gives the same daily method for Koh Tao, Phangan, Samui and the Andaman places, so the years and the islands can be compared. That source gives an average of 215 mm for Samui. Use the place page for the long-term average and this one for the spread between years and between islands. The archive is a model on a grid of about 10 km, so rain on a small island can differ from your side of it.

Where should I be if I need a dry December?

On the Andaman side, and further north still if you can. Koh Lanta averaged 118 mm in these eleven Decembers, with 6 of 11 under 100 mm; Phuket averaged 129 mm and Krabi 146 mm. Phuket's wettest December in the record, 236 mm, was less than the five wettest Decembers on Koh Samui. The Andaman coast is not dry (Phuket had 14 rainy days in 2024), but its bad years are milder. Our Koh Lanta, Phuket, Krabi or Koh Phangan guide compares them from December to March, and Thailand in December shows that Bangkok, Chiang Mai and Hua Hin are drier than all of them.

If your heart is set on the Gulf for diving or a particular island, the three coasts guide explains why the Gulf swaps with the Andaman through the year.

What would we do for winter 2026-27?

• Fixed in December, rain is the worry: base on the Andaman side, Koh Lanta first.
• Set on the Gulf: book from around 15 December, choose Koh Tao over Samui if the choice is open, and keep the first week refundable.
• Flexible: wait for January, which is clearly better; see our Samui and Phangan best-time guide.

These are eleven past Decembers, not a forecast for this one. We hold no ferry or sea-state data, so we cannot say when boats to Koh Tao stop running; ask the operator close to your dates.