Filament Remaining Calculator

In short The only reliable way to know how much filament is on a part-used spool is to weigh it and subtract the weight of the empty spool. Nothing else works: a spool that looks half full can be anywhere from about 30% to 60%, because the inner windings are a much smaller circle than the outer ones and hold far less filament per turn. Judging by eye consistently over-estimates. The whole calculation rests on the tare — the weight of the bare spool — and that is the number to get right. Many spools have it moulded into the plastic or printed on the label; if yours does not, weigh the next one you finish and write it on the side with a marker, because cardboard, plastic and metal-cored spools differ by well over a hundred grams and a generic figure is the weakest part of the arithmetic. Once you have the net weight, converting to length is the same geometry as any filament calculation, and the material density matters in the same way. The practical use is deciding whether to start a print. Compare what is left against the figure your slicer reports and leave a real margin — a print that stops three millimetres from the top has consumed all of that filament and all of that time for nothing, so anything inside about 5% is too close to risk. Running out mid-print is also where the very end of a spool fights the extruder, because the last turns come off with a tight curl.