The Black Watch duty uniform

[Graywolf]

If a period costume then, it depends on the year in question. In general, the earlier the period the higher above the knee. The Victorians started the lowering the hem of the kilt. Wouldn't want to offend the ladies after all. For kilts nowdays, the length should be at kneecap or top of knee. I prefer the old kneel on the floor, and if the kilt just touches or is slightly above the floor, then it's the right length.

[Pleater]

If you are ever going to be wearing your kilt out in the rain, and do not have a long enough cape or Inverness coat to stop the lower edge getting wet, you don't want it to be long enough to catch in the back of your knee or it will make the skin really raw really quickly.