The specific gravity of fish is approximately 1.15 so that about 25
cubic inches of fish weigh 29 of water or one pound.
So that if you divide the number of cubic inches in a fish by 25 you
will have his weight in pounds:
Formula: (Girth^2)/16 × length/2 × 1/25 = weight in lbs.
which simplified is Girth^2 × length over 800.
My friend says a more simple formula is:
(1/4)Girth^2 × 2 times length = weight.
Tarpon must be measured when first taken from the water in order to
obtain their true weight for they shrink when exposed to the air. A
heavy fish that is hung up over night will stretch inches in length by
the following morning. The length should be measured from the tip of
the closed mouth over the swell of the fish to a line drawn between the
flukes of the tail.