Formula : W = (L * (G * G))/R, where:
Imperial calculations
W : weight in decimal pounds.
L : length in inches.
G : girth in inches (default is .58*L).
R : constant (800 for trouts, 900 for pikes).
Metric calculations
W : weight in kilograms.
L : length in centimeters.
G : girth in centimeters (default is .58*L).
R : constant (28850 for trouts, 32450 for pikes).
By this formula a trout of length 44cm (17.3") would
approximately weigh 1.0kg (2.2lb)