Koh Samui Weather, Month by Month

Almost everything written about when to visit Thailand describes the Andaman coast and then applies it to the whole country. For Koh Samui that advice is not merely imprecise, it is inverted. This island keeps a different calendar from Phuket and Krabi, and a visitor who plans around the national version of the season will book the wrong fortnight.

There are two numbers that decide a Samui holiday, and this page carries both: how much rain falls, and what the sea is doing. They do not move together here, and that is the whole argument.

The island’s year in two bands

Dry and calm are not the same season

Look at October and December on the panel above and the point makes itself. October is the calmest sea Koh Samui gets all year, 0.24 metres off Chaweng, and it is also the second-wettest month at 270mm. December is nearly the reverse: 163mm of rain, which is manageable, over water at 0.64 metres, the roughest of the year.

The reason is that this coast faces east into the Gulf, and the northeast monsoon arrives in November and blows through to February. It brings the rain first and the swell afterwards, so the wind outlasts the weather. On the Andaman side the two arrive together and leave together, which is why a single “rainy season” works as shorthand over there and fails here.

The practical consequence is that a dry forecast in January is not a promise of a flat sea. It is the most common complaint from people who booked Samui for Christmas or New Year on the strength of a rainfall chart.

The four months to actually book

April, May, June and August are the only months when both bands are low: under 150mm of rain and under 0.35 metres of wave. That is the honest recommendation and it is an awkward one, because those months are exactly when British and European travel pages describe Thailand as being out of season.

They are out of season on the other coast. In June, Koh Samui takes 85mm of rain and the Andaman takes 240mm. In August it is 90mm against 248mm. The Gulf in June is drier than the Andaman manages in February, and that comparison is the one that reframes a summer trip to Thailand entirely.

July is the near miss, and it is worth naming rather than rounding away. Rainfall is the same 90mm as August, but the wave figure at Chaweng ticks up to 0.36 metres, just over the line this page draws at 0.35. In practice July is a good month here; it is simply not quite in the same class as the four beside it.

Month by month

January and February are dry and busy and the sea is at its roughest, 0.58 metres in both months. Rain is 83mm and 41mm respectively, and February is the driest month of the Samui year by a wide margin. If you are coming for the beach rather than the swimming, this is a good stretch. If you want flat water for a boat day, it is not.

March is the changeover. Rain is still low at 84mm and the sea has dropped to 0.43 metres, halfway between the winter swell and the summer calm. It is the first month of the year when the crossings to Koh Tao stop being a lottery.

April, May and June are the island at its best on the figures: 99mm, 113mm and 85mm of rain, with the sea at 0.30, 0.29 and 0.29. Songkran falls on 13 April and is worth planning around in either direction. May is the first month the Andaman is genuinely unusable, at 269mm, which makes it the clearest month of the year to be here instead.

July and August hold at 90mm each. The sea is 0.36 and 0.31. This is peak European holiday season and it is a much better time to be on this coast than on the other one, which is the opposite of what most itineraries assume.

September is the last clean month, 139mm and 0.35 metres, still under the dry threshold but visibly moving. The Andaman is at 293mm in the same month.

October breaks. 270mm across 26 rain days. The sea is flat, which is a genuine consolation for a boat trip, but there is no dry coast anywhere in Thailand this month and the Andaman at 315mm is no better.

November is the worst month of the Samui year: 313mm over 23 days. It is worth knowing that this lands within a couple of millimetres of Phuket’s own annual maximum. The two islands take almost the same beating, they just take it at opposite ends of the year, and in November the Andaman is drying out while this coast is not.

December recovers to 163mm and prices go to their annual peak from around the 20th. The sea is at its roughest, 0.64 metres. It is a perfectly good month for a beach holiday and a poor one for anything involving a small boat.

What this means if you are choosing between coasts

Nine months of the year Thailand has a coast that is working, and it is rarely both at once. From roughly April to September the answer is here. From November to March it is the Andaman. January to March both are dry, and October neither.

If your dates are fixed and you have not committed to an island yet, the two-coast comparison is laid out in full in our month-by-month guide, and the honeymoon version of the same argument, which is the strictest case because the dates are chosen by a wedding, is on our Thailand honeymoon page.

What we do not tell you here

There is no temperature on this page, and its absence is deliberate rather than an oversight. Koh Samui’s reanalysis grid cell is mostly water, and a cell that is mostly water reports a daily maximum several degrees below what a thermometer on the beach reads. Our data would say something like 25C for a January afternoon that is really near 29. Every other number on this site is one we are prepared to defend, so rather than publish a figure with a known bias we publish none, and note why.

The rain and the sea are the variables that change the holiday here anyway. The heat does not: it is hot every month, and the difference between the warmest and coolest month on this island is smaller than the difference between morning and afternoon.