The apron and underapron do take more cloth than you'd think. The deep pleat at the left edge of the apron should have 8-9" in it, and the facing on the right hand side of the apron commonly takes 6" or so (although you can get away with less if you have to or if you don't want a fringe edge). If your hip measurement is 42 (as in the example above), the apron is going to be 21" or so across at the hips, with some flare toward the bottom. So, say and inch and a half of flare on each side, that makes 24" plus the 9" for the deep pleat plus 6" for facing. That's 39" total for the apron. Getting it down to just 36" means serious skimping somewhere.

The underapron takes just about as much, although you can skimp more on both edges if you have to.

Obviously for a smaller hip measurement, you have a little more leeway (although I wouldn't go less than 4 yards). But, the opposite is also true. If you're thinking of doing a box pleated kilt and your hip measurement is much more than 42", it doesn't hurt to have an extra half yard or even an extra yard of material. Matt's rule of thumb is 5 yards is good if the hip is over 45".

Cheers!

B