In an earlier post is this thread I stated that the first 20,000฿ of interest earned was exempt from taxation. That is not correct. Perhaps it was in previous years.
Currently you have a choice of two options regarding taxation of interest.
Option A is let the banks take 15% withholding and then you don't include your interest in your taxable income. This option is only available if the total interest you received is 20,000฿ or less.
Option B is you include the total interest you earned in your annual income, calculate the tax owed, then reduce the calculated amount by the amount (if any) that the banks withheld from your interest payments.
Option A is better if your annual income puts you in a Thai tax bracket where the rate is greater than 15% ( >> 750K฿) .