Regular pants belts around 2" can look fine with a kilt. Less than that they can get lost in the pattern and look a bit funny. It all depends on the look you want. I prefer either a belt or sporran but not both since I think that both looks a bit busy especially with 3" belts with giant silver buckles. Some people love the look and think that an unbelted kilt looks like a skirt. If you're planning on wearing a jacket, you don't need a belt.