Here's a "general" rule of thumb that you can use:

Casual or 4-yard kilts have pleats that are a bit wider (1-1.5") and the depth is minimal. For the "approximate" yardage... take your hip measurement and write it down (since it's larger than your waist). For every 10" of hip, that's about 1.25 yards of material to use. Yes, that's single length from waist to hemline.

More traditional kilt have narrower pleats (.75-1.25") and the depth their is more substantial. For the "approximate" yardage... take you hip measurement and write it down (since it's larger than your waist). For every 10" of hip, that's about 1.75 yards of material to use. Again, that's single length from waist to hemline.

This is a "rule of thumb" that my teachers have given to me, and it hits right-on about 90% of the time. As Matt said however, the size of the tartan design and the pleating to stripe or sett will make a slight difference.