Why length is approximate
The calculation models filament as a uniform cylinder. Real diameter has manufacturing tolerance and material density changes with polymer formulation, pigment, fillers, foaming, and recycled content. Spool labels and technical data sheets take priority over a generic preset.
The conversion model
Diameter is converted from millimeters to centimeters. The calculator finds the circular cross-section, divides mass by density to find volume, then divides volume by cross-sectional area to estimate length.
area cm² = π × (diameter mm ÷ 20)²
volume cm³ = mass g ÷ density g/cm³
length m = volume ÷ area ÷ 100Typical density presets
The included PLA, PETG, ABS, and TPU values are approximate defaults. “PLA+,” flexible grades, filled materials, and even different colors can depart from them. Editing the density field does not change the physical spool—it makes the assumption visible.
Use the result well
- For purchasing, prefer the manufacturer’s stated net mass and length.
- For a print, prefer the slicer’s grams because it knows the actual toolpath.
- For inventory, include purge, supports, startup material, and failed jobs.
- For pricing, pass the resulting grams to the 3D Print Cost Calculator.
For examples and allowance methods, read How Long Does a 1 kg Spool of Filament Last?