Filament length to weight calculator

A spool tells you grams and a slicer tells you metres, and the two never appear on the same screen. Converting between them is only geometry — filament is a cylinder of known diameter, so a length of it has a volume, and a volume has a mass once you know the density. The catch is that last word. Density is a property of the pellets a particular company bought, not of "PLA" in general, and the datasheets prove it: PLA comes out at 1.24 g/cm³ on four sheets from four companies, but ABS is 1.04 at BCN3D, 1.05 at MG Chemicals and 1.1 at UltiMaker, and nylon is 1.14 at UltiMaker against 1.25 at Plexiwire. That is why the density here is a list of published figures with the manufacturer beside each one, and a box for the number printed on your own spool when none of them is yours.

How it is calculated

mass = π × (d/2)² × length × density

Take the diameter in millimetres, halve it, square it and multiply by π to get the cross-section in square millimetres. A metre of filament is a thousand millimetres of that cross-section, which is the same as one cubic centimetre per square millimetre of section — so a 1.75 mm filament carries 2.405 cm³ in every metre and a 2.85 mm one carries 6.379 cm³. Multiply the volume by the density in grams per cubic centimetre and the answer is grams. Nothing in that chain is rounded or fudged; the only number that can be wrong is the density you feed it.

Source: Filament is a cylinder, so its volume is π times the square of half its diameter times its length, and its mass is that volume times its density

Questions people ask

Why are 1.75 mm and 2.85 mm the only diameters?

Because they are the two that spool makers actually wind. There is no standards body behind either figure — no ISO number, no ASTM number, nothing to cite. What there is instead is the market: MG Chemicals lists "Diameter: 1.75 or 2.85 mm" on the front page of its datasheets, UltiMaker publishes its classic materials at 2.85 mm and its Method line at 1.75 mm, and BCN3D ships 2.85 mm. Some older machines took a nominal 3 mm filament, which in practice was the same 2.85 mm stock under a rounder name. If yours is genuinely different, the arithmetic above is one multiplication you can do yourself.

Which density should I use for ABS?

Whichever one your spool's maker publishes, and if you do not know who made it, know that the answer moves. BCN3D prints 1,04 g/cm³ under ISO 1183, MG Chemicals prints 1.05 g/cm³ under ASTM D 792, and UltiMaker prints 1.1 g/cm³ under ISO 1183. That is a spread of about six percent, which on a 1 kg spool of 1.75 mm filament is the difference between 400 m and 378 m — enough to decide whether a print finishes. The list above keeps them apart on purpose instead of averaging them into a single confident-looking number.

Do the datasheets agree with their own spool figures?

Not always, and it is worth knowing before you trust one. UltiMaker's Method ABS-R sheet gives a specific density of 1.04 g/cm³, a net filament weight of 650 g and a length of about 257 m at 1.75 mm; run those through the cylinder and 257 m of 1.04 g/cm³ filament comes to 643 g, about one percent light. Plexiwire is further out: its nylon sheet says 1.25 g/cm³ under ISO 1183 but also sells "400m = 1.1kg ±5%", and 400 m at 1.25 g/cm³ would weigh 1203 g. The weight and length on that sheet imply a density of about 1.14 — which is exactly what UltiMaker publishes for its nylon.

How do I find out how much is left on a part-used spool?

Weigh the whole thing on a kitchen scale, subtract the weight of the empty spool, and put the remainder in as grams. The empty weight is the number nobody has: it differs between cardboard and plastic spools and between manufacturers, some print it on the label and most do not, so the honest answer is to keep one bare spool and weigh it once. Guessing it is where most "how much filament is left" estimates actually go wrong — not in the arithmetic.

Does the diameter tolerance matter?

Less than you would think, because it is squared but it is small. A sheet quoting 1.75 ± 0.05 mm is quoting about ±3% on diameter, which is ±6% on cross-section at the extremes — but the tolerance describes wobble along the strand, not a spool that is uniformly fat, so it averages out over the hundreds of metres you are weighing. Density is the number worth getting right; diameter is not.

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