A kitchen scale, the tare, and a per-metre figure — the print's question answered.
Every spool lies a little. The label says 1 kg — new, sealed, that figure is true. Once the spool has been printing for a while, the label is a memory: the printer and the slicer both want to know how much filament is actually left, and the only honest instrument is a kitchen scale. The tools page carries the calculator; this is the math it runs.
Weigh the spool as it is. Subtract the tare — the empty spool's weight, which most brands print on the spool's rim or list on their spec sheets (the app keeps a weighed-tares list so you enter the number once). What remains is the filament's own mass.
The app's figures come from the material density: 1.75 mm PLA runs about 2.98 g per metre, which is why a kilo of PLA is about 335 metres. The 2.85 mm size carries about 2.6× the mass per metre of the same material — diameter squared, almost exactly: (2.85/1.75)² ≈ 2.65. Change the polymer and the figure changes again, which is why the calculator asks for material after diameter. The same maths in reverse runs the slicer's estimates: grams shown in a slicer's filament-usage line convert to metres (or to "can this print finish on what's left?") by the very same factor.
A print that dies at 90 metres of a 110-metre job wastes both the machine's night and the material on the plate — the failure the weigh-in exists to prevent. Weigh before the long job; write the spool's number on its rim (or let the app remember it); run the drying table from the drying guide if the filament has sat open. The scale turns "I think there's enough" into a decision.
Weigh it once, print it through. — the bench crew
