Fish University 102: How to estimate fish weight
If you have the girth and the short measurement, the math is easy!
Estimating the weight of a fish is pretty simple once you know how to obtain the numbers necessary. All that is needed is the girth at the widest part (usually at the dorsal fin) and the measurement known the “Short measurement.”
The short measurement is the distance in inches from the tip of the lower jaw to the inner fork of the tail, whereas the "Long measurement" is the overall length of the fish. The bill and tail are both added length that cannot be relied upon for estimating weight so scientists use the lower jaw as a way to be more accurate.
Just like some humans are taller than others, fish are the same. Sometimes a very long fish has little girth, and sometimes an extremely wide fish is relatively short. Generally speaking, to make the magical 1,000 pound mark, a fish needs to have at least 72 inches of girth (6 feet) and 156 inches in length using the short measurement (13 feet).
Once you have the information you need, simply plug it into the following formula and you should be able to calculate a fish’s weight within ten percent:
(G2 x S)/800
G=girth in inches (measured all the way around the fattest part – note: if you can only measure the fish on one side, simply take the full measurement from top to bottom and double it)
S=short measurement in inches
Using the measurements above to make a thousand pounds, the formula goes like this:
72 x 72 x 156 = 808,704
808,704/800 = 1010.88
Another Way to Estimate:
And here's another way to measure Blue Marlin that's a bit simpler and sometimes more accurate. Simply take the measurement around the ankle of the fish just ahead of the tail. 12" will be a 200 pound fish, and for each extra inch, add 100 pounds. To make that magical "grander" mark, the ankle will be about 20 inches.
Using these two techniques together will greatly enhance your ability to guesstimate the weight of a fish. Of course, the second method only works for larger Marlin, so use them as a guide. It's not usually something you think of bringing fishing, but next time you go you may want to toss your calculator and a printout of these calculations into your bag just in case.