Enter the length, the girth and select a class of "general body shape" of the fish, in any of the designations; also select a specific body structure for the type fish (for example, is the body size average, large or small for the length of that specific TYPE fish, such as a bass, or salmon). If you do not know the specific body size for that specific fish, leave the selection as Average. If you do not know the girth and can't measure it, the shape selection will insert one for you. The accuracy will be better if you enter the CORRECT girth. Click on the Estimate button and the weight will be estimated in pounds and decimal fractions, grams and kilograms; each weight is independent of the others. Embellish as needed... The estimated weights are based on the classes of salt water fish; use the estimator for fresh water fish weight if needed... Please be aware that each class uses a different formula for estimation. In some classes, girth is NOT used; those classes beginning with * do not use girth!! If there are other fish you feel we should add to the estimator, please contact us.