How Healthy Is My Fish?

With each fish at the end of their line, avid anglers often ask themselves a simple question: How healthy is this fish? One way that fisheries biologists answer that question is by looking at the relative weight of a fish. To calculate relative weight, you divide the actual weight of the fish by the expected weight for a fish of the same species and length, then multiply the result by 100.

For instance, say we have a 12-inch largemouth bass that weighs 1 pound (454 grams) and we want to determine how healthy it is.

For example, if a fish has a relative weight of 113, that means it weighs 13% more than expected for its size and is considered stout. In a well-managed body of water, most fish have a relative weight between 90 and 100. A value below 80 means the fish is very thin, while anything over 100 means it’s above average in condition.

To use the relative weight calculator found below, you’ll need to measure your fish from the tip of its closed mouth to the end of its pinched tail and record its weight. You can use either standard (inches, pounds/ounces) or metric (centimeters, grams) units, just be sure to use the same system for both length and weight.