When pigs are little, you can just pick them up and stand on a bathroom scale to weigh them. If you can put up with the squealing, that is. But once they’re more than a few months old, new weighing methods are required!
It turns out that there’s a mathematical formula that can come up with an estimate of a pig’s weight, using just its length and girth measurements. It works across all breeds of pigs, over a wide age range, and is accurate to within about 3%.
The two diagrams above come from The Pig Site, and are very helpful in getting the measurements right.
The magic formula comes from the Old Farmer’s Almanac, and is:
W = G x G x L x 69.3
W is weight in kilograms
G is the heart girth measurement, in metres
L is the length measurement, in metres
I found the easiest way to take the measurements was with a piece of string, at feeding time. First pour the pigs’ food into their trough so they’ll be standing still and occupied. Tie a knot close to one end of the string to give you a starting point.
Measure the length first, because the pig will hardly notice. Hold the end knot at the base of the tail, and run the string along the back to the base of the ears. Pinch that point on the string between your thumb and forefinger, and tie a knot to mark it.
Now measure the girth, right up behind the front legs. Again, tie a knot to mark the measurement.
You can repeat the process with a new piece of string for each pig, but mine are all about the same size so I just measured one.
Kevin’s measurements were L=0.96m and G=0.83m.
W = 0.96 x 0.83 x 0.83 x 69.3 = 45.8 kg
So, we’re about half-way to the target weight for slaughter (80-100 kg).
In case you’re wondering how much meat you’d get from a certain sized pig, they have a dress-out percentage of roughly 72%. So a 90 kg live pig would yield a carcass weight of about 90 x 0.72 = 64.8 kg.