Does anyone know how I can estimate aproximate weight of a rainbow trout if I know the length? Earlier this summer I caught a 25 incher and today a 23 incher and I'm wondering how heavy they were. I have inches marked on my rod so it's easy to determine length without harming the fish before release. I know there is a formula for length and girth but obtaining the girth isn't practicle, so I'm looking for something that uses only length realizing that this alone won't be the most accurate.