Always dose drugs as accurately as possible, weight tapes can be bought that are easy to wrap around your goats but until you get one use a regular tape that you would sew with and use this conversion chart.

Weight Chart for the large breed dairy goats taken from http://www.saanendoah.com
NOT for meat, hair, Pygmy or Nigerian Dwarf breeds.  A weight chart for Pygmy goats can be found at 
http://hometown.aol.com/goatlist/weights.htm

Measure around the heart girth, just behind the front legs. Pull Snugly