Yardage is an issue only if you want to make it an issue. The largest kilt I have personally made was for a 67" waist. I used a grand total of 6 yards.

Now, mind I was making a box pleated kilt and I wanted fairly wide pleats. They were something like 2.5" or 3" wide, if I recall. Six yards what what was required for that. Normally, on a box pleated kilt for a gentleman with a hip of under 46" I would only use 4 yards. If the hips are over 46" I'll order 5 yards of cloth to make the kilt (or 6, as in the case above). I don't always need all 5 yards, but I want to make sure I have them there if I need them.

For a standard "8 yard" knife pleated kilt, you don't want big wide pleats like the box pleat. That's just not the style. So, could you, at a 50" seat, have a kilt made from 8 yards of cloth? Sure, but there would be fewer pleats, and wider pleats, than you might otherwise expect. It would affect the look of your kilt. Most people, wanting that style of kilt, have in mind pleats of a certain narrowness. To maintain that look, you very well might need to add another yard of cloth.

But a lot of this depends on the tartan, as well. Depending on the size of the sett, you might need only 6 yards for an "8 yard kilt." Other tartans might take 9 yards for an "8 yard kilt."

I encountered this recently with a kilt that Lochcarron made for a customer of mine. His tartan was the red Gordon. His hips were something like 42", fairly standard. Lochcarron phoned to say they only had 7 yards of the cloth on hand (he wanted an 8 yard kilt). I didn't think one yard short would be such a big deal, until they told me that normally his kilt would be 9 yards in that tartan. Even though his hips were not large, because the tartan pattern was so large, the pleats were extra deep and they needed 9 yards.

Ultimately, in that case, the customer chose to have the 7 yard kilt made, but the pleats were noticably wider than they would have been otherwise. He chose not to make an issue of the yardage, and got a kilt that he liked. But then again he wanted the kilt for casual wear. Had he been purchasing it for use in a pipe band, or to dance in, or just wanted to look of the very narrow pleats, he would have needed 9 yards.

I hope this clarifies things a bit for you!