Filament length

Measure what remains on a spool

Weigh the spool and subtract the empty spool's mass to estimate remaining filament. A guessed tare can make the difference between finishing a print and running short. The spool calculation reports whole parts from the entered material requirement; it cannot detect tangles or unusable sections.

Include supports and waste

Use the slicer's total filament requirement, including supports, brim, raft, and any purge material it accounts for. Leave a practical reserve rather than planning to use the final gram. Filled and foamed filaments can differ substantially from the density of their unmodified base polymer.

Using diameter and density in the conversion

The cross-sectional area of a round filament is π times diameter squared divided by four. Multiplying by length gives volume, and density converts volume to mass. Because diameter is squared, using the wrong filament size has a much larger effect than a small rounding difference. A density in grams per cubic centimeter also requires the length units to be converted consistently.

If 500 g of usable filament remains and each proposed print needs 80 g, six whole prints fit arithmetically, leaving 20 g. That remainder does not establish that the last print will finish if the slicer estimate excludes purging or startup extrusion. Gross spool weight must be reduced by tare before using this comparison. Keep a known empty-spool weight for the same spool design where possible. A material name such as PLA or PETG is only a starting density basis; fillers and foaming can change it. For critical material planning, compare the estimate with actual consumption from a representative print.

Formula

Mass per meter in grams = πd²ρ/4 when d is mm and density is g/cm³.