You best bet is to calculate the cross-sectional area or your typical weld, and then calculate the volume. Depending on the shape of your 'normal' fillet weld, it could vary by quite a bit. We used to do this in a piping shop I worked in... calculate lbs of rod for a 3" sch80 CS weld, lbs of filler and CF or shield gas for a 4" SS TIG weld, etc.