While what Steve says is true, kilting tartan woven by the main mills in Scotland is pretty consistent in weight and hand. If I order 14 oz from one and 12-13 oz from another, and 16 oz from a third, I know in advance pretty much what each one is going to be like. If I were ordering from an outlet or, as Steve says, ordering something other than wool tartan, I'd want to know in advance how they are calculating their weight or else simply order a sample.

Barb