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 which is at the Dorsal Fin and the measurement known as 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.
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 calculator and you should be able to calculate a fish’s weight within 5%.