The formula to calculate the weight for a Smallmouth Bass that is 20 inches long and has a girth of 15 inches is as follows. Smallmouth Bass have a shape factor of 1100. Largemouth Bass have a shape factor of 1050.

Smallmouth Bass:

WEIGHT = LENGTH x GIRTH² / SHAPE FACTOR

WEIGHT= 20 x (15²) / 1100
WEIGHT= 20 x 225 / 1100
WEIGHT= 4500 / 1100
WEIGHT = 4.09 pounds

Largemouth Bass:

WEIGHT = LENGTH x GIRTH² / SHAPE FACTOR

WEIGHT= 20 x (15²) / 1050
WEIGHT= 20 x 225 / 1050
WEIGHT= 4500 / 1050
WEIGHT = 4.29 pounds