Filament Gage

FilamentGageScale estimates meters of 1.75 mm filament remaining on a 1 Kg Hatchbox spool. (199 mm OD, 80 mm core, 62 mm inside width)

FilamentGageScale2 estimates meters of 1.75 filament remaining on a 0.2 Kg MakerBot spool. (152 mm OD, 87 mm core, 32 mm inside width)

Simply put the tool on the spool flange and rest the scale on the filament. Then read off how many meters remain on the spool.