It's pretty close.

www.bcfishingreports.com/fish_calculator.html


I've lenght, girth and weighed lots of fish with my digital Normak and found there is no perfect calulation. All fish, even within their own species vary alittle. I bought a Shimano spring scale at Walmart, which is adjustable, it's dead on.

Go to the sewing dept and they have small, plastic, retractable 5ft reel tape measures for about 2 bucks that you can carry in your vest