In Thai, the bathroom is called hong nam. Another common way to say it but in writing (that is often put on toilet signs) is sukha.
Occasionally, the word for male and female will be added to the toilet signs, such as hong nam ying meaning female toilet, or hong nam chai which means male toilet.
Similarly for signs: sukha chai which is male toilet, and sukha ying which is used for female toilet.
Here are some useful questions:
Where is the toilet? = hong nam yu nai…(kha/krap)?
May I use the toilet? = Kor kao hong nam noi dai mai (kha/krap)?
Kha/Krap is used at the end of sentences to make it more polite. Kha is for female speakers, while krap is used by male speakers.
If there are no public toilets around and someone lets you use a toilet, it is a nice gesture to say thank you, in Thai:
Khop khun kha/krap.
It’s good to know that toilet paper and soap are normally not provided at public toilets, in Thailand.
Thus, it’s a good idea to bring along toilet paper or wet wipes, as well as hand sanitizer or soap.
In addition, small coins will be handy when you use public toilets, which sometimes will charge 3-5 baht.
Most gas stations offer their toilets free of charge.