We use something called the relative weight formula, a method commonly applied in fisheries science.   This formula predicts the length and weight of a fish by using species-specific constants that reflect its natural shape and growth patterns.  These constants come from trusted sources, including data from Fisheries Techniques (3rd Edition) and the global species database FishBase.

For example, the formula generally takes the form:

Weight = a × (Length)^b

Here, a and b are species-specific coefficients, which vary depending on the fish’s body type. By plugging in the length of a fish, we can estimate its weight—or vice versa—with impressive accuracy.

To ensure realistic estimates, we apply the relative weight concept, where a relative weight of 0.9 is considered the average or healthy benchmark for most species. This means that if a fish’s weight closely matches what is predicted at 0.9 relative weight, it’s in good condition, so that's the estimate we provide.

However, the system has its limits. Any fish with a relative weight below 0.25 or above 2.5 is flagged as an error because these values fall outside the realistic biological range. Extremely low or high relative weights indicate a miscalculation or unrealistic input. By staying within these boundaries, we provide you with reliable and meaningful estimates for your catches.

Here's an example of a weight vs length curve for Tiger Musky. 

Image Credit: EDSClikes2hunt - Own work, Public Domain, Wikimedia Commons