Running out of filament three hours into a long print is one of the most frustrating ways to fail a job. This calculator estimates how much filament is left on a partly used spool, either precisely by weighing it or approximately by measuring the wound coil with calipers, then converts that to metres so you can confirm a print will finish.
How it works
Weighing (recommended). The most accurate method is pure subtraction:
remaining grams = current total weight − empty spool weight
The empty (tare) weight is often printed on the spool. The tool then converts
grams to length using the material density and filament diameter:
length = (mass ÷ density) ÷ strand cross-section.
Coil diameter (field estimate). When you cannot remove the spool to weigh it, measure the outer diameter of the wound coil, the core hub diameter, and the coil width with calipers. The wound filament forms a flat ring (annulus), so its bounding volume is:
volume = (π ÷ 4) × (outer² − core²) × width
Round strands wound side by side leave gaps, so the tool applies a packing efficiency of about 90% before converting volume to mass and length.
Worked example: weighing method
A spool reads 420 g on a kitchen scale. The empty spool weight is printed on the label as 180 g.
remaining grams = 420 - 180 = 240 gFor 1.75 mm PLA (density ≈ 1.24 g/cm³), the strand cross-section is:
area = π × (0.0875 cm)² ≈ 0.02405 cm²
length = (240 g ÷ 1.24 g/cm³) ÷ 0.02405 cm² ≈ 8058 cm ≈ 80.6 mIf your slicer reports a print needing 65 g (about 21 m), that spool can complete the job with comfortable headroom.
Material densities at a glance
| Material | Density (g/cm³) |
|---|---|
| PLA | 1.24 |
| PETG | 1.27 |
| ABS | 1.04 |
| TPU (95A) | 1.21 |
| Nylon (PA6) | 1.14 |
| ASA | 1.07 |
| PC | 1.20 |
Selecting the correct material in the tool matters: using PLA density for an ABS spool would overestimate the length by about 20%.
Finding the empty spool weight
The empty spool weight (also called tare weight) is often printed on the spool hub, the side of the reel, or in the manufacturer’s product listing. If it is not, you can:
- Weigh a confirmed empty spool of the same brand and model — most manufacturers use the same reel across their range.
- Note the total weight when you buy a sealed 1 kg spool and subtract 1000 g (or whatever the net weight is) once you start printing.
- For a rough guess, most 1 kg plastic spools weigh between 150 g and 250 g empty, with cardboard spools on the lighter end and heavier plastic reels toward the upper end.
Tips for reliable results
- Weigh whenever you can; the diameter method is a useful field estimate but expect ±10% due to coil packing variation.
- For long or expensive prints, keep a 10–15% safety margin, since slicer estimates and real consumption can differ slightly due to retraction moves, purges, and support material.
- If your slicer reports filament in metres, multiply by the strand cross-section and density to convert back to grams for a direct comparison.
- Store partially used spools sealed with desiccant — a dry spool weighs slightly more than a moisture-laden one and performs better throughout the print.