Frequently asked questions
- How do you calculate how much filament is left on a spool?
- Weigh the spool, subtract the empty spool's tare weight to get net filament mass, divide that mass by the material density to get volume, then divide the volume by the filament's cross-sectional area to get length.
- How many metres are in a 1 kg spool of PLA?
- About 335 metres of 1.75 mm filament at the 1.24 g/cm3 density Prusa publishes for Prusament PLA. Polymaker publishes 1.17 g/cm3 for PolyLite PLA, which works out to about 355 metres, so the brand changes the answer by roughly 20 metres. A denser material yields fewer metres per kilogram, and 2.85 mm filament yields roughly a third of the length of 1.75 mm for the same mass.
- Why does the tare weight matter?
- Data sheets quote net filament weight, but a spool on a scale includes the spool itself. Empty spool weight varies widely between vendors and between cardboard and plastic construction, so it has to be subtracted before any length estimate is meaningful.
Filament Weight and Length Calculator
Enter what the scale says, subtract the empty spool, and get the net filament mass, the remaining length in metres, and the printable volume. Density and tare weight are both editable, because both vary by material and by brand.
Each preset is one named product's published figure, not an average. Change the density field for a different brand, or for any filled or composite grade.
Read this from the spool's own technical data sheet where possible.
Spool included, exactly as weighed.
Worked example: Prusa publishes its spool as two injection-moulded flanges at 81.5 g each plus a 39.5 g cardboard centre tube, so a Prusament spool tares at about 202 g. Other vendors differ, and a cardboard-cheeked spool is lighter. If the tare is not printed on the spool, weigh one after it runs out and keep the number for that brand.
How the conversion works
Filament is sold by weight and consumed by length, and density is the bridge between the two. Net mass divided by density gives volume. Volume divided by the filament's cross-sectional area gives length. The cross-section comes from the nominal diameter, so 2.85 mm filament yields roughly a third of the length that 1.75 mm does for the same mass.
The two inputs people get wrong are tare weight and density. Data sheets quote net filament weight, but the object on the scale includes the spool, and empty spool weight varies widely between vendors and between cardboard and plastic construction. Density varies by material and, more sharply, by filler: a carbon-fibre or glass-fibre grade does not share the density of the resin it is based on.
That is also why filled and composite grades get no preset here. PA6-CF, PET-CF, PLA-CF, glass-filled and wood-filled grades change density with the filler loading, and the loading is not standardised between brands, so any single figure offered for them would be wrong more often than right. For those spools, read the density off the data sheet and type it into the field above.
Published densities, and how far vendors disagree
| Material | Preset (g/cm³) | Source | Published spread | Metres per kg at 1.75 mm | Note |
|---|---|---|---|---|---|
| PLA | 1.24 | Prusament PLA data sheet | 1.17 – 1.24 | ~335 – 355 m | PolyLite PLA publishes 1.17, a 6% gap on the same material. |
| PETG | 1.27 | Prusament PETG data sheet | 1.25 – 1.27 | ~327 – 333 m | PolyLite PETG publishes 1.25. The tightest agreement of the group. |
| ABS | 1.12 | PolyLite ABS data sheet | 1.04 – 1.12 | ~371 – 400 m | Polymaker's own summary table lists 1.04 for the product whose data sheet says 1.12. |
| ASA | 1.13 | Polymaker ASA data sheet | 1.07 – 1.13 | ~368 – 389 m | Prusament ASA publishes 1.07. Pick the one matching the spool in hand. |
| TPU (flexible) | 1.22 | PolyFlex TPU95 data sheet | 1.20 – 1.24 | ~335 – 346 m | Published as a range, not a point value. Varies with shore hardness. |
| PC (polycarbonate) | 1.19 | PolyLite PC data sheet | 1.19 | ~349 m | Unfilled polycarbonate. Glass-filled PC grades are heavier. |
The spread column is the point of this table. Two mainstream vendors publish PLA at 1.17 and 1.24 g/cm³, which is a 20 m difference on a 1 kg spool, and ABS is worse: Polymaker's summary table gives 1.04 for the same PolyLite ABS its own data sheet puts at 1.12, a 29 m swing. A density picked off a generic chart is therefore worth roughly ±6% on any length estimate, which is the difference between a print finishing and stopping. The authority for a given spool is that manufacturer's data sheet for that exact material, and no chart can substitute for it.
Sources: Prusament PLA, Prusament PETG and Prusament ASA technical data sheets; Polymaker's data sheets for PolyLite PLA, PolyLite PETG, PolyLite ABS, Polymaker ASA, PolyFlex TPU95 and PolyLite PC, plus Polymaker's technical data at a glance table and Prusa's published spool component weights.
Related reading
- Filament density: convert spool weight to length, the arithmetic behind this tool and why net weight matters.
- PLA vs PETG vs ABS, with print temperatures, warping behaviour and enclosure requirements side by side.
- How to dry filament, with drying temperatures and times by material.
- Best filament for beginners, on what to buy first and how much of it.
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