Overstay: 500 baht a day, and the bans that start at 90 days

Staying past your permitted date is a criminal offence under section 81 of the Immigration Act B.E. 2522 (1979): imprisonment up to two years or a fine up to 20,000 baht, or both. In practice it is settled administratively at departure: 500 baht per day of overstay, capped at 20,000 baht (the cap is reached at 40 days).

The re-entry bans come from the Ministry of Interior's Order 1/2558, in force since 20 March 2016, and they run on two tracks. If you surrender yourself — that is, you present yourself at the border or airport and pay the fine: overstay of more than 90 days brings a 1-year ban from the date of departure; more than 1 year, a 3-year ban; more than 3 years, a 5-year ban; more than 5 years, a 10-year ban. If you are arrested and prosecuted instead, the tariff jumps: any overstay under 1 year brings a 5-year ban, and over 1 year a 10-year ban.

The folklore correction: an overstay under 90 days followed by voluntary departure carries no ban — but the fine still applies, repeat overstays can get your passport stamped as an immigration-law violator, and arrest at any point (including a routine police check far from the airport) moves you onto the harsher track. Voluntarily walking into the airport and paying is categorically better than being found.