Filament is sold by weight, printed by volume, and measured on the spool by length. This converter moves between all three, and this page explains the arithmetic so you can sanity-check any answer it gives.
The relationship
A filament is a cylinder. Its cross-sectional area is π times the radius squared: 2.405 mm² for 1.75 mm filament, and 6.379 mm² for 2.85 mm. Volume is that area multiplied by length; mass is volume multiplied by density.
Put a kilogram of PLA through it and the volume field returns 806 cm³; divide that by the 1.75 mm cross-section and the length field returns 335 m. Switch the diameter field to 2.85 mm and the same mass becomes 126 m, because area goes as the square of the radius and the thicker filament therefore packs 2.65 times as much plastic into every metre.
Density is the field to get right
Diameter is fixed by the machine you own. Density is the field you can get wrong, and everything the tool returns — metres, cubic centimetres, grams per metre, prints remaining — moves inversely with whatever sits in it. Halve the density and every length doubles.
That makes the failure mode specific rather than vague. Leave 1.24 in the box while a matte or a glow grade is loaded and the converter over-reports your remaining filament by very nearly the percentage the filler added to the density — silently, because 1.24 is an entirely plausible number for something calling itself PLA. The per-material density table has the figure to use instead, and how many metres are on a 1 kg spool follows the ranking through in metres for the materials people ask about most.
Where a label gives no density at all, type the range rather than a point. Run the conversion once at the bottom of the material's stored band and once at the top: two answers that both clear the job mean you can start the print, and two that straddle it mean the spool is a gamble.
Reading the partial-spool output
Two fields drive that mode — the gross weight off your scale, and the weight of the empty reel. The tool subtracts one from the other, converts what remains, and if you also give it a per-print mass it divides again and reports a count of prints, which is usually the number you actually wanted rather than the grams.
The reel field is the one that carries the error, and it carries it undiluted: the result is wrong by exactly as many grams as your reel assumption is, with nothing downstream to damp it. How much filament is left on my spool shows what that does to the metres, and the spool weights guide has fallback figures by brand and by reel construction.
A kitchen scale reading to one gram is ample for this. The precision you are missing is never in the weighing.
Where this estimate goes wrong
Filament diameter tolerance. Good filament holds ±0.02 mm; poor filament wanders further. Because area depends on the square of the radius, a filament running 1.79 mm instead of 1.75 carries about 4.6% more material per metre than the calculation assumes. Measure with calipers in three places if precision matters.
Foaming filaments break the model entirely. A foaming PLA is roughly normal density on the spool and can be as light as 0.4 g/cm³ once printed, depending on how hot it was run. Spool density and printed density are genuinely different numbers for that material, and no single conversion covers both.
Sanity check
Weigh a finished print and compare it with the slicer's mass estimate. If the two disagree by more than a few percent, the density in your filament profile is almost certainly the culprit — and the materials reference has the real range for whatever you loaded.
Two questions this answers well
How many prints will this spool give me? Divide the spool's length or mass by the mass of one print, using the mass your slicer reports rather than the model's volume. A kilogram of PLA at 40 g a print is twenty-five prints, minus purge, prime lines and whatever fails.
Do I have enough for this job? Convert the remaining spool weight to a mass and compare it with the print's estimate plus a margin. A print that runs out at 90% has consumed nine tenths of the filament it needed and produced nothing, and there is rarely a graceful recovery.
Why we store density as a range
Because it is one. A single confident figure is easier to display and harder to trust: real PLA spools run 1.17 to 1.26 g/cm³, and the filled grades sit well outside that. Storing the spread and the basis of the figure lets you see how much the answer could move, which a single number hides.