Measure in Inches
1 Inch = 2.24 centimeter  1 English pound = 453 gram.
Measure the fish from tip of the nose to the fork of the tail (this will be called the LENGTH MEASUREMENT ).
Now measure around the fish in front of the Pectoral Fins (this will be called the GIRTH MEASUREMENT ).

Now take the girth measurement and square it (multiply it by itself  if the measurement was 25 inches then would be: 25 x 25= 625 ).

Now multiply the girth number squared (625) by the length measurement (lets say 125 inches) ( 625 x 125 = 78125 ).

Now divide by 800 ( 78125 : 800 = 97.66 ) so your fishes weight is approximately 97.66 lbs.