You probably catch a sea trout from time to time, which you want or need to release, but want to determine the weight of this trout before. Guessing the weight is OK, but a more accurate weight would be better. Running around with weights and measuring tapes is too cumbersome. A measuring tape similar to the one you take your waistline with is enough to also determine the weight of the fish.
You also spare the fish by not having to lift it out of the water to weigh it. Take the length in cm and the circumference in cm in the thickest place on the "Belly". Enter these two numbers in the table below and tast "Calculate the weight". You get a very reliable weight of a sea trout.
On Fulton's page for "Condition Factor" there is a table for normal length (cm) and normal weight (kg) for a sea trout with fitness factor 1.0 when it is a rising fish. Of course, there are variations in the "Condition Factor" depending on where the sea trout has lived and consumed its food. The large "herring eaters" get a high "Condition Factor".
Enter the length of the fish in cm and the belly size of the fish in cm. Use decimal period if required.
The Danish Sport Fishermen's Association has also published a small booklet with measurements and weights for salmon and sea trout. It is found here on the web.
Click on the image to get to the Fultons formula.
A German website Fliegen Fischen describes how to determine the weight of a sea trout based on the length and circumference of the "Belly". They claim the following formula should be an expression of weight:
V weight in grams, L length in cm, O circumference in cm and 28,9 a constant.
It is indeed! Try it on your own fish and see if you agree.