Overstaying your visa is illegal. If you overstay for a short period, you will have to pay a fine. However, if you overstay for more than 90 days then it is a serious offence and you will get deported from Thailand and banned from visiting again. The length of the ban depends on how long you have overstayed your visa.
If you are caught overstaying by an Immigration officer, then it can result in being taken to jail if you are not able to pay the fine or have a long overstay.
How much is the fine for overstaying?
The fine depends on the number of days you have overstayed. The minimum charge is 500 Baht per day. This goes to a maximum of 20,000 Baht for overstays of 40 days or longer.