Filament spool remaining calculator

Half a spool of filament carries no useful markings, so the only way to know what is left is to put the whole thing on a kitchen scale and take off the weight of the bare reel. That second number is the one nobody has. It is almost never printed on the label, and it is not a constant: across the spools whose tare Prusa and Bambu publish it runs from 140 g for Polymaker's cardboard reel to 330 g for Fiberlogy's — a 190 g spread, which on 1.75 mm PLA is 64 metres you either have or do not. Guessing it is where "how much is left" goes wrong; the arithmetic afterwards is exact. So this page carries the published tares with the brand beside each, takes the weight of your print from the slicer, and tells you whether it finishes.

How it is calculated

left = scale reading − bare spool · metres = left ÷ density ÷ (π × (d/2)² ÷ 1000)

The first half is a subtraction and the second is the same cylinder the rest of this category runs on: a metre of 1.75 mm filament holds 2.405 cm³, a metre of 2.85 mm holds 6.379 cm³, and dividing grams by density gives the cubic centimetres to spread over those metres. Prusa describes exactly this method for its own spools — weigh the spool with the filament on it, and the length follows from the spool's weight and the filament's density. Everything here is that sentence with the numbers exposed.

Source: Prusa Polymers, Check how much length of Prusament is left (20 December 2023) — Weigh your Prusament spool. Enter the measured weight (spool + filament), and the calculator will provide an almost exact length of the remaining filament based on the spool's weight and your filament's density

Questions people ask

Why do you not simply know what my spool weighs?

Because almost nobody prints it on the reel, and the figures that are published disagree by more than the amount you are trying to measure. The tares here come from configuration Prusa and Bambu publish themselves — PrusaSlicer carries a spool weight per filament profile, and Bambu sells its reusable spool with a stated 250 g — and they range from 140 g to 330 g. Two minutes with an empty spool and a kitchen scale beats every entry in that list, and you only ever do it once per brand.

Prusa publishes two different weights for the Prusament spool. Which is right?

We do not know, and that is why both are in the list. The vendor bundle that shipping PrusaSlicer downloads gives filament_spool_weight = 193 for Prusament PLA; the preset on the PrusaSlicer development branch gives 278 for the same filament. That is an 85 g gap, or 28 metres of PLA. Prusament spools have changed design over the years and one figure probably describes a spool the other does not, but neither file says which. Overture moves too, from 235 g down to 155 g. The rest of what we offer — Hatchbox, Devil Design, eSUN, Fiberlogy — reads the same in both files.

How accurate is the length?

As accurate as the density and the scale, and both are inputs here rather than hidden constants. A kitchen scale reading to 1 g is worth about 0.3 m of 1.75 mm PLA, which does not matter. Density does: PLA is 1.24 g/cm³ on four datasheets from four companies, but ABS is quoted anywhere from 1.04 to 1.1, and the wrong end of that shifts a kilogram of filament by 22 m. The diameter tolerance is squared but tiny, and it describes wobble along the strand rather than a uniformly fat spool, so it averages away over hundreds of metres.

Does "enough for the print" mean the print will actually finish?

It means the plastic is there, which is the question PrusaSlicer's own spool weight setting exists to answer: weigh a partially consumed spool before printing, compare it with the calculated weight of the filament plus the spool, and find out whether what is on the reel is sufficient to finish the print. What the slicer figure does not cover is the skirt, the purge at a filament change, and the last few metres fixed to the core that will never feed. If the answer comes out close, treat close as no.

What about the filament stuck to the middle of the spool?

It is inside the number and it should not be. Manufacturers fix the end of the strand into the core, so the final turns come off badly or not at all — weight you can measure and cannot print. Nobody publishes how much, because it depends on how the spool was wound and how it is mounted. Treat the last ten metres or so as a reserve rather than as stock, and start a long print on a spool that has more than the arithmetic says it needs.

Sources

The documents this page reads its numbers out of, linked so you can check them yourself.

1. Prusa Polymers, Check how much length of Prusament is left (20 December 2023) — Weigh your Prusament spool. Enter the measured weight (spool + filament), and the calculator will provide an almost exact length of the remaining filament based on the spool's weight and your filament's density
2. Prusa Research, PrusaSlicer vendor configuration bundle PrusaResearch 1.14.2 — [filament:Prusament PLA] filament_spool_weight = 193; [filament:Prusa PLA] and [filament:Fillamentum PLA] 230; [filament:Hatchbox PLA] 245; [filament:Devil Design PLA] 250; [filament:Esun PLA] 265; [filament:Overture PLA] 235; [filament:Fiberlogy Easy PLA] 330
3. M.G. Chemicals, PLA Technical Data Sheet ver. 2.00 (09 October 2018) — Dimensions for 1.0 kg Spool: 1.75 mm — 335 m, 2.85 mm — 126 m; Net Volume 806 mL. Dimensions listed above are approximations
4. M.G. Chemicals, ABS Technical Data Sheet ver. 2.00 (09 October 2018) — Dimensions for 1.0 kg Spool: 1.75 mm — 395 m, 2.85 mm — 149 m; Net Volume 952 mL
5. M.G. Chemicals, PETG Technical Data Sheet ver. 2.01 (01 May 2023) — Dimensions for 1.0 kg Spool: 1.75 mm — 327 m
6. Bambu Lab, Bambu Reusable Spool product specification — Specification: Low Temp (≤70℃) / High Temp (≤90℃), Packing Size 200 × 200 × 67 mm, Weight 250 g
7. Prusa Research, PrusaSlicer preset "Prusament PLA" — filament_spool_weight: 278, filament_density: 1.24, filament_diameter: 1.75, filament_max_volumetric_speed: 15
8. Prusa Research, PrusaSlicer preset "Polymaker PolyTerra PLA" — filament_spool_weight: 140, filament_density: 1.31
9. Prusa Research, PrusaSlicer preset "Overture PLA" — filament_spool_weight: 155, filament_density: 1.21
10. Prusa Research, PrusaSlicer preset "Elegoo PLA" — filament_spool_weight: 160, filament_density: 1.26
11. Prusa Research, PrusaSlicer source, definition of filament_spool_weight — Enter weight of the empty filament spool. One may weigh a partially consumed filament spool before printing and one may compare the measured weight with the calculated weight of the filament with the spool to find out whether the amount of filament on the spool is sufficient to finish the print

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