What is the formula for converting inches of chest/heart girth measurements to pounds when measuring a calf at birth?

You could estimate weight using a heart-girth tape. These tapes for cattle are available in animal-supply magazines, such as Wiggins & Associates Inc. They sell for about $5. The tape is usually supplied with a chart that converts a tape measurement to a live weight. If you just want an estimate for your own use, this weight method would be acceptable. If you are using it to determine selling or buying weight, it is not accurate enough. Also, if you are a seed stock producer, this is not an accurate enough method for weighing your cattle. For more information on how to measure your beef cattle, go to http://beef.unl.edu/web/beef/miscellaneousquestions#framesize.