Since AD 1888, under the reign of King Rama V, Thailand has adopted the Thai Solar Calendar, which has some differences with the common calendar, also known as Gregorian Calendar.
Years are counted according to the Buddhist Era, that is 543 years greater than the Gregorian calendar used in western countries.
Converting between them it is as easy as adding or extracting 543 years, but why should you bother doing so, when you can have it done automatically by filling the form here? Just start typing the Thai year to obtain the Gregorian one, and vice versa.
Piece of cake!