Relative Weight Calculator


Relative weight is an index used by fisheries biologists to determine if a fish is in good condition, fat, or thin. It is calculated by dividing the weight of a fish by the expected weight for a fish of the same species at the same length growing rapidly with plenty of food. In the scientific and management literature, this index is usually multiplied by 100 to put it in whole units or percentages. A relative weight below 80 is very thin, around 90 is average, and greater than 100 is considered fat. In a well-managed fishing pond, the fish would typically have relative weights around 90. For a more detailed description of how to weigh and measure fish and how to interpret relative weights, see Alabama Cooperative Extension publication ANR-1193, “Relative Weight: An Easy-to-Measure Index of Fish Condition." For example, a largemouth bass that is 16 inches long and weighs 2 pounds would have a relative weight of 89%, which is average.

Here is a calculator you can use to determine the relative weight of largemouth bass, bluegill, redear sunfish (shellcrackers), channel catfish, or black crappie. You need to either use metric units with the total length of the fish in millimeters and weight in grams or English units with the length of the fish in inches and weight in pounds and ounces. Measure the fish from the tip of the nose to the end of the tail squeezed together. Digital or spring scales are convenient ways to weigh fish.

Bluegill on measuring board

Rusty Wright, Fisheries Extension Specialist, Alabama Cooperative Extension System
Mark Bransby, Information Technology Specialist, College of Agriculture, Auburn Univeristy
Sam Bartlett, Information Technology Student Developer, ACES-Ag Information Technology

ANR-2039 Relative Weight Calculator.

Alabama Cooperative Extension System The Alabama Cooperative Extension System (Alabama A&M University and Auburn University) is an equal opportunity educator and employer. ©2013 by the Alabama Cooperative Extension System. All rights reserved.

The formulas used in the calculator as well as for other fish species can be found in:

Neumann, R.M., C.S. Guy, and D.W. Willis. 2012. Length, weight, and associated Indicies. pages 637-676. in A.V. Zale, D.L. Parrish, and T.M. Sutton, editors. Fisheries Techniques, 3rd edition. American Fisheries Society, Bethesda, Maryland.

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