We were told how to do this when we went along to Pig Paradise. Remember, the measurements are imperial.
1. Measure the length of your pig from between the ears to the root of tail
2. Measure the girth of the pig around the shoulders, tight to the pigs front legs
3. Multiply the length in inches by the girth in inches, then divide this by 12 if it is a lean pig, 11 if it is a medium pig and 10 if it is a fat pig. The answer is the weight in pounds.
Remember, this is a guide only.
At 160lb, a pig is ready for slaughter as a porker. Carcase weight is about 66% of liveweight; so a 160lb live weight will be 105lb deadweight, of which about 20-25lb will be head, trotters, tail and "unuseable" pieces (I accept that all part are useable for something but probably not in a "mainstream" market). Expect pigs kept extensively to reach this weigh about 30 weeks
For bacon, deadweight would be 140 - 170lb, 210 - 250lb liveweight.
Hope this is helpful.