This measurement is only a approximation and varies by as much as 4lbs to the heavy side in larger fish. For a quick and dirty measurement it works great. The formula Fish2much mentions is the one on I use. Here is a link to my calculator.
http://www.steelheader.net/Toppages/weight_calculator.htm

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Marty
www.steelheader.net
marty@steelheader.net