This method of weight calculation is not accurate enough for record applications, but is close enough for most purposes. It works best on cylindrical shaped fish.
W = (L x G2) / K
W = weight in pounds
L = length in inches
G = girth in inches (squared)
K = 800 for fish with “normal shapes (bass, trout, etc.); 900 for long, cylindrical fish (pike, gar, barracuda, etc.)
Example: If you catch a rainbow trout that is 14 inches long and has a girth of 16 inches, you would estimate its weight by multiplying its length times the square of its girth, then dividing by 800:
Congratulations! You’ve just caught a fat 4 1/2 pound trout.
1. 14 x 16 x 16 = 3584
2. 3584 / 800 = 4.48
Information borrowed from Soucie’s Field Guide of Fishing Facts
Fly Rod & Line Balancing Chart
