Measure the fish from the tip of the nose to the fork of the tail then around the fish in front of the pectoral fins. Now take the girth measurement and square it, then multiply this by the length measurement and divide the product by 800. This gives the fish it's weight. Example: Girth of fish 12 inches. Length of fish 30 inches; 12 times 12 = 144; multiply by 30 equals 4320; divided by 800 equals 5.4 pounds.