Here is the standard formula for estimating the weight of a fish:
Multiply the length of the fish in inches by its girth squared. Then, divide by 800 to get the weight in pounds of an average-shaped fish (trout or bass).
For long, slim fish, such as northern pike or barracuda, divide by 900 instead of 800.
The formula is fairly accurate for most fish.
And, it's not too far off when tested on Jarrett Edwards' Colorado state-record largemouth bass, which was so fat, he said it "looked square."
If you multiply the length of Edwards' bass (22.37 inches) by its girth squared (20.6 inches x 20.6 inches) and divide by 800, you arrive at 11 pounds, 13 ounces - only 7 ounces more than its actual weight of 11 pounds, 6 ounces.