Do not overstay in Thailand!

The maximum overstay fine in Thailand is 20,000 baht (approx US$450). Foreign children are subject to fines for overstaying in Thailand only when over the age of seven.

You can pay your overstay fine at the airport, but if you are caught by Immigration before you reach the airport, you will be jailed and deported until you can pay your fine and show a one way ticket back to your home country. 

If you overstay by one day, there is no charge but only at the airport. Land borders charge for even the first day’s overstay. After that the fine is 500B per day.  

Paying the fine is simple. When going through Immigration, they will notice the overstay and take you to a desk to pay, it should take about 5 minutes in all. Fines can also often be paid in advance at any Immigration bureau. You will not be jailed it you voluntarily clear up your overstay.

Warning - Note that overstaying is technically breaking the law and while there is little problem if you 'surrender' yourself to Immigration at the airport, it can be a very different story if you are stopped by the police beforehand for whatever reason and are found to have overstayed - no matter for how long. This may well lead to you being detained in an Immigration Detention Center for a few days while your case is processed. Needless to say, this is an experience you can do without, as conditions inside have been described by Amnesty International as 'cruel and degrading' and 'seriously overcrowded'.

Though the chances of ending up in this situation are very small, you would be advised to not overstay your visa if at all possible. If you do overstay for any reason, obviously you should steer well clear of any kind of activities that might attract the attention of the authorities.