Majority of Thai is Buddhist and temple (or Wat) is a sacred place for us. You might have heard that some temples in Thailand have a very strict dress code. And, it is true. For visitors who are not familiar with the culture, here is my recommendation of what to wear when visiting temples in Thailand.
The strictest temple is Wat Phra Kaew and The Grand Palace. The other temples also require visitors to dress properly, but do not have such strict rules. Here is the photo that I took before entering Wat Phra Kaew and The Grand Palace. I hope it gives you an idea how to dress. No short pants and sleeveless. Your shoulders, arms, and knees must be covered.
Well, do not worry too much about what to wear when visiting temples because most of the strict temples provide sarongs for rent. But you might have to wait in a bit long line. My tip is to prepare short pants or comfortable clothes in your bag especially if the day that you are going to visit temples is very warm. Then, you can change it after visiting the temple. For girls, you can use scarf or shawl cover your shoulders and enter some temples.
Besides, please bear in mind that you have much chance to remove your shoes when visiting temples in Thailand. We usually walk around the temples with shoes, but when entering the building, we do remove the shoes. So, wear the shoes that are easily taken off too.
Hope this information helps. Enjoy traveling Thailand!