The weight of a running yard is still the same - even though the weight and width might be given in metric - so 4 yards of 16 oz tartan, 54inches wide, weighs 4lb.

Whatever weight of fabric is left after cutting to length, making the waist band and loops, is to be deducted, and the weights of buckles, reinforcing materials, straps, and even a tiny bit for the thread, are added.

For a fully tailored kilt made for a tall man you could get a finished weight of over 4 lb, even though there would usually be a bit of material left over. For someone really tall you might use every bit of material and add something for the waistband.

I sometimes use DPM material which has a 'direction' to the pattern, or a pinstripe, where the fabric is cut or torn across from selvage to selvage almost every bit of the material is used. Usually the only leftover is from straightening the edges where it was cut from the bolt.