When preparing a 3d print you process it with a Slicer to convert it from a 3d model to a printable file. Current versions of slicers such as Cura 4.x and PrusaSlicer 2.x include both the length of filament needed for a print and the weight of the filament needed.
Earlier versions of slicers often only provided the length of filament needed for the print. This can be a problem.
It is easy to find the weight of filament on a reel by just weighing it and subtracting the weight of the reel. The following calculator can be used to convert between filament length and filament weight.