Here you can calculate the "Condition Factor" for your sea trout according to Fulton's formula. Fulton works with values between 1.00 and 1.20 for rising trout. If you want to read more about "Condition Factor", go to this website: R. Arrow. It describes a bit about why trout and salmon have different "Condition Factor" depending on the habitat.
The website mainly applies to sea trout. Salmon is usually longer per unit weight. You can use the calculation, but you get lower numbers, and then my comments are not completely correct. Typical values for rising salmon are between 0.90 and 1.05. Calculations of more than 50 salmon from Varde Å show an average of 0.95. It will be interesting to see what the "Condition Factor" will be when all 130 salmon are caught. In 2018, I have calculated an average for small salmon at 0.95 and for large salmon 1.05. This gives an average of 1.00 for salmon in Varde Å.
At R. Piil page for "Condition Factor" there is a table for normal length (cm) and normal weight (kg) for a sea trout with "Condition 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".
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 here for the Weight Determination formula.
Enter the weight of the fish in g and the length of the fish in cm. Use decimal period if required.
Then press Calculate condition factor. The unit of measurement is grams per cm³ * with a constant (100).
Let me give you an example of a good sea trout of 5.5 kg and 77 cm long. The "Condition Factor" is 1.205. It is a sea trout in very good condition, which is worth cold smoking. When this sea trout has been up to spawn and you then catch it, its "Condition Factor" will be 0.99. Since the length is constant, it must be the weight that has dropped to below 4.5 kg. Thus, sea trout have lost over 20% in weight. It has lost proteins is loose in the meat, and not worth eating. Should be released so you can enjoy it for next year, if you catch it. It will then weigh 9.1 kg and 91 cm long and with the same "Condition Factor" as before 1.205.