The airlines use the [Timatic database][1] to determine what documents, if any, must be provided by an embarkinig passenger. Airlines are very careful about this, as passengers refused entry at a destination must be returned to the passenger's point of departure at the airline's expense.

Timatic is available on the web [here][2]. 

Entering your nationality, destination, and stopover point yields this report:

[![Timatic Report][4]][4]

The report indicates that if you're traveling on a single ticket on Thai Air Asia or Thai Air Asia X, and your onward flight is within 12 hours of your arrival, then you do not need a Transit Visa. If you are traveling on another carrier (those listed in the "TWOV" or "Transit Without Visa" section), or on two different carriers, then you do need a Transit Visa. 


  [1]: https://en.wikipedia.org/wiki/Timatic
  [2]: https://cms.olympicair.com/timatic/webdocsI/spdbmainv.html
  [3]: http://
  [4]: https://i.sstatic.net/LKgqf.jpg