I disagree when Steve says the belt should be "fairly loose." I've seen a lot of guys wearing the kilt belts way too loose and they slide down the kilt and just look a mess.

The belt shouldn't bee too tight, either. If it's bunching up the cloth of the kilt, the belt is too tight (or more likely, the kilt is too big!).

In other words, the belt should be properly fitting. It should fit snug enough to stay securely in place, but not so tight as to be uncomfortable or distort the look of the kilt.

However, I will say that in all instances I have seen of the belt bunching up the fabric of the kilt, it was because the kilt itself was too big.

As to your original questions, though, I find most belts will have enough adjustment built in that if you just order according to your actual waist size, there is more than enough adjustment to allow for any girth the kilt itself may add.