Fish Weight Calculations
The fish weights are calculated with the formulas below. If a fish girth is not specified, the default value Fish Girth = 0.58 × Length is used.
Trout, steelhead, mackerel, snook, redfish, bonefish
Fish Weight = Length × Girth × Girth / 900
Salmon, bass
Fish Weight = Length × Girth × Girth / 800
Walleye
Fish Weight = Length × Length × Length / 3500
Pike, gar
Fish Weight = Length × Length × Length / 2700
Sunfish, bluegill
Fish Weight = Length × Girth × Girth / 1200
Source: https://www.fishswami.com/fish_weight_calculator