There's so many variables with fish weight, it's pretty near impossible to guesstimate a crappies weight. Time of year being the number one. A 15 inch crappie with eggs in April will weigh much more than in July. A black crappie the same length will weigh more than a white crappie will but the white crappie grow quite longer than the black do. A shorter black CAN weigh more than a longer white. Best advice is to get a digital scale and a good ruler too. Some guys guesstimate length of their fish too.