For Grand Palace and Temples in Bangkok and Bang Pa In Palace
Visitors to The Grand Palace must dress appropriately because The Grand Palace is a place of reverence for the Thai people. Inappropriate clothes for entry into The Grand Palace are as follows:
No sleeveless shirts
No vests
No short top
No see through tops
No short hot pants or short pants
No torn pants
No tight pants
No bike pants
No mini skirts
No pants skirts
No sleeping suit
For Temples outside Bangkok
Visitors to the temples outside Bangkok, for example in Ayutthaya (exclude Bang Pa In Palace), Sukhothai, Chiangmai could dress like when visit Grand Palace in Bangkok.
Nevertheless, Short sleeve shirts, and shorts that cover or below knees will be alright.