What is the formula for determining the weight of a fish based on its size?

The rule of thumb for fish weights of various body types are:

Pike (length X length X length) divided by 3500

Walleye (length X length X length) divided by 2700

Bass (length X length X length) divided by 1600

Bluegill (length X length X length) divided by 1200

Another generalized formula is:

(girth X girth X length) divided by 800

All of these will result in reasonable approximations.