Step 1 - Girth measurement
Wrap the measuring tape around the widest part of the fish.
Take the measurement and square it.

Step 2Length
Take the tip of the tape to the top of the nose of the fish and extend it to the longest tip of the tail when pinched.
Multiply this length by the result of the girth x girth measurement.

Step 3Final calculation
Take your last result and divide it by 800. This will give you the weight of the fish.