I saw this post, and since it related to my quesiton, I decided to add here. I am basically trying to calculate geometry for two fields using model builder. The first field is predefined as 'Total Miles' to house total mileage for all stream segments in that field. The second is 'Need miles' that will house mileage for a fraction of the records. I'll determine what records using select by attributes, then calculate geometry (mileage). My final predefined field is the percentage of Need Miles to the Total Miles, which I can derive using Calculate field in model builder. I know, too much information. Basically, I just need to know how to Calculate Geometry in model builder? I can't seem to locate the function in stand-alone format to drag in to model builder, and assume there is another approach, most likely a simple one. Thanks in advance!