How many times have you wished you could find or had taken your scales to weigh a nice catch? Why not try this? Carry a measuring tape - a cloth one that can be bought in any sewing/craft department. Take the measurements as described below, and use the following Fish Weight Calculator.
If you really goof up and forget or misplace the measuring tape, just use a couple pieces of line to take your mesurements and get the actual numbers when you get home. Be sure to somehow mark one so you'll know which is length and which is girth.
Remember that no length/weight formula can ever replace an accurate scale. However, the following calculator should enable to you to get a fair estimate that's within about 10 percent of the actual weight.
Measure the length from lower jaw to the fork of the tail
Measure the girth at the widest part around the body