I've found this to be a very accurate method no length measurement required just heart girth. I weighed and measured a couple hundred and most were within 10 pounds of the measured estimate.
Heart girth is measured in inches using a cloth measuring tape. The tape is placed directly behind the front legs, wrapped snugly around the heart girth, and read directly behind the shoulders.
pig weight = 10.1709 x Heart girth (inches) minus 205.7492.
Using the above formula, we get the following:
40 inch = 200 pounds
42 inch = 220 pounds
43 inch = 230 pounds
44 inch = 240 pounds
45 inch = 250 pounds
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46 inch = 260 pounds
47 inch = 270 pounds
48 inch = 280 pounds
49 inch = 290 pounds
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50 inch = 300 pounds
51 inch = 310 pounds