April 03, 2014
By Rob Neumann
Several methods are available for estimating the weight of a fish based on length and girth measurements. Without girth measurements, however, weight can be estimated using only length from length-weight relationships. These formulas can be developed for a fish species in a water body, state, or region, or for a species in general.
Species-specific length-weight formulas, called standard-weight equations, are used by fishery biologists to estimate body condition, or the relative weight or plumpness of a fish, compared to a standard weight for a fish of that length. These formulas are often created using thousands of fish from hundreds of populations across the range of the species, so they describe a general growth form of that species.
Following are standard weights based on the formula for largemouth bass. The weights given are for fish in "above average" body condition. Fish weights are often variable for a given length, so you can make a judgment about how fat your fish is and make an adjustment.
Length (inches) Weight (pounds) Length (inches) Weight (pounds) 12.0
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0.88
21.5
5.94
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12.5
1.01
22.0
6.41
13.0
1.15
22.5
6.90
13.5
1.30
23.0
7.41
14.0
1.46
23.5
7.95
14.5
1.64
24.0
8.52
15.0
1.83
24.5
9.11
15.5
2.04
25.0
