eSIM for Thailand: Bangkok, the islands and what happens on the ferry

Thailand pairs very cheap local data with very expensive roaming. The gap is among the widest anywhere: what a local operator charges for a month can cost the same for a single day in roaming. So this is a destination where connectivity is worth solving before departure, not in a queue at Suvarnabhumi.

Coverage around the country

• Bangkok — 5G everywhere, including the BTS and MRT.
• Phuket, Samui, Krabi — solid 4G across the resort areas, 5G in the main towns.
• Phi Phi, Lanta and the smaller islands — signal in the villages and on the beaches, gone in the bays between them.
• Ferries and longtails — coverage breaks up in the straits. Save tickets, bookings and maps offline first.
• The north: Chiang Mai, Pai — reliable in town, patchy on the mountain switchbacks.

How much to take

Thailand burns data faster than people expect: food delivery, ride apps, menu translation and a lot of video.

• A week on an island — 3 GB.
• Two weeks on the move — 5–10 GB.
• A month-long stay — start at 20 GB and compare against a local contract. Past a month a local SIM can genuinely be cheaper, and that is the honest answer.

About the validity period

The plan starts counting from the first connection to a network, not from the moment you pay. You can install the profile a week before departure and leave it off — the clock will not move.

In short

• The roaming-versus-local gap is unusually large in Thailand.
• Islands have coverage; the water between them often does not.
• A week usually fits into 3 GB.
• For a stay of a month or more, compare honestly with a local contract.
• Validity starts at first connection, not at purchase.