2 Answers

Or you can buy a weight tape which is calibrated for either Beef, or Dairy cattle, and use it in the location called the "heart girth" pulled snugly.  It will show where to read it on the tape.  They also have them for Hogs.  Great little tools!  They may be up to 10% inaccurate, as some are heavy or lighter for their frame size.  These are available at feed stores, or try NASCO.com

Estimating weight of calves by measuring girth can be done by measuring the girth of calves at the back of their shoulders in the smallest circumference with a measuring tape tightly pressed.  It is recommended not to feed the animals and not to give them water for 12 hours. In the following link there is a table that indicates measured girths to estimated weights: 
http://www.piedmontese.org/GettingYourCattleWeightsWithoutaScale.htm.