Here is a chart that can be used to determine approximate weight without using a scale. Â This chart uses total length (tip of nose to tip of tail) and maximum girth (the fattest part of the fish) Â The length should be measured with a rigid tape or ruler or bump board and the girth should be measured with a soft flexible tape. Â Round to the nearest inch. Â If measured properly, this chart should get you within +/- 10%. Â For example if you measured a 40 x 25 you would come up with 31lbs +/- 3.1 lbs. Â Â Since most catfish have the same body style it should work for all species. Â Click the link below to view the chart.