How do you work out a seat from a kilt?

[Barb T]

You can fold the kilt in half at the bottom of the fell so that it curves smoothly and the bottom of the fell is stretched out and then measure along a tartan line at the bottom of the fell from the fringe edge, across the apron, and across the pleats stopping at the pleat edge of the underapron. That will give you a number of inches.

But, you don't know how that # of inches actually relates to the hip measurement that the kiltmaker took. In the kilts that I make, that would be at least an inch bigger than the actual measurement I would get if I measured the person around the hips. I always add an inch to the fringe edge of the apron to make sure that the apron covers the underapron, and I commonly add several inches to the hips if someone has a big belly so that the front of the kilt hangs straighter (avoiding the tilted barrel look).

If you're gifting a kilt that is an "off the rack" inexpensive kilt, you'll be fine with an approximation, because they'll likely only ask for a waist measurement. If this is for an expensive custom-made kilt, I would strongly urge you to surprise the person with a picture and a gift certificate, and then do the proper measurements the way the kiltmaker wants them.