Do I need a visa for a layover in Bangkok without changing terminals?

No visa needed. However, going off on a slight tangent here, this might save someone some grief down the road. Last year me (Brit) and my wife (Thai) were flying Bangkok-Amsterdam- Glasgow with KLM. However, at the time of booking there appeared a much cheaper option, Bangkok-Paris-Amsterdam-Glasgow. We took that. On arrival at check-in my wife was denied boarding as she didn't have a Schengen visa for the Paris-Amsterdam leg. We had to re-book for the following day at a cost of around 20,000 baht.

No. You’re in transit.

International airline terminals are much like maritime law, no visas required if remaining within an international terminal.

No

I would like to share my experience

Last year I traveling from tokyo to Vietnam🇻🇳 transit 12 hours, then go to chiengmai.

I have a Japanese passport,

On checking counter in Tokyo Airport this American lady she trying to check to the same flight with me , she was asked , do you have transit visa ? , She said no,

Then she was told ,mom you can not get on the plane you have to have visa transit in Vietnam , she was almost crying 😢, with American passport you needs transit visas to Vietnam🇻🇳 but not Japanese passport .

It was Friday night I guess she have to staying tokyo for at least 4 more days, to get transit visa , so sorry for her .

So it was vetjet air ,

In this case, he was transit at Singapore 🇸🇬 , he shouldn't have no problem right?, with UKpassport ?

With Vetjet I had to have return tickets

Otherwise I couldn't get on the plan .

But with japanese airlines I can get on with one way ticket to Thailand 🇹🇭

So I guess depends on the airlines .

U don't need a visa for transit it will say connecting flight just proceed to that gate

No

No

Whether or not you are changing terminals doesn't matter. People arriving are never in the same area as people departing, and are forced to a certain direction. You cannot just go from one flight to another in the international terminal. You will have to either go through the international transfers desk if you have a single itinerary/ticket you are flying on, or you will have to go through passport control and enter Thailand and then check-in for your next flight if you are flying on different tickets and doing a self transfer.

If the 2nd one is the case, then whether or not you need a visa is completely dependent upon which passport you hold, which you did not include in your post.

Where are you getting confusing info from? Just curious. If British passport you don't need visa for Thailand.

No.