From grams to meters
Filament is a long, thin cylinder, so its length is its volume divided by the area of its end. The volume is the weight divided by the density of the plastic. For a kilogram of 1.75 mm filament at 1.24 g/cm³, that comes to about 335 meters.
The calculator turns the other way too. Type meters, and it gives the weight, which helps when a printer or a slicer reports length and you want to know how much of a spool a print used.
Why the density matters
Each choice in the list is a product with the density its maker states on its data sheet. Polymaker lists its PolyLite PLA at 1.17 g/cm³ and Bambu Lab its PLA Basic at 1.24 g/cm³, and that gap is about 20 meters on a kilogram. Pick the closest product, or type the density from your own filament’s sheet.
1.75 or 2.85 mm
A 2.85 mm strand has about 2.65 times the cross-section of a 1.75 mm one. The same weight of it is about 2.65 times shorter, so check which diameter your printer takes before you compare spools.
Using it with a spool
To see what is left on a spool, weigh it and take off the empty spool’s weight first. The spool weight page does both steps and gives the length too.