Generally, most people dress casual. The weather can be hot and humid so it's best to wear light, cotton fabrics. Shorts, flip flops, t-shirts etc and plenty of sun cream!
Hills country
It is always advised to take some warm clothes as the temperature can drop a few degrees and the evenings can be particularly chilly.
Temples
If you wish to enter into any of the numerous Buddhist and Hindu temples in Sri Lanka, it is best to observe a few rules: woman travellers should cover their shoulders and dresses should be long enough to cover their knees. Similarly men will need to wear long shorts or trousers.
Shoes must be left outside