Filament Length & Weight Calculator
Convert 3D printer filament weight into length (and cost) from the spool diameter and material density — know how much of a spool a print actually used.
How it works
Cross-section A = π · (diameter ÷ 2)² · Volume = weight ÷ density · Length = Volume ÷ A · Cost = price × weight ÷ spool weight
Filament is a solid cylinder, so its length depends on both how thick it is and how dense the plastic is. A gram of dense PETG is a shorter strand than a gram of light ABS, and 2.85 mm filament runs far shorter per gram than 1.75 mm because its cross-section is more than twice as large. Slicers report grams; this converts that back to metres so you can tell whether a print will fit on what is left of a spool.
Worked example
A full 1 kg PLA spool (density 1.24 g/cm³, 1.75 mm) has a cross-section of π · 0.0875² = 0.02405 cm². Volume is 1000 ÷ 1.24 = 806 cm³, so length is 806 ÷ 0.02405 ÷ 100 ≈ 335 m — the ~330 m figure printed on most PLA spools.
Frequently asked questions
Why does 2.85 mm filament give less length per kilogram?
Length is inversely proportional to the cross-sectional area, and area grows with the square of the diameter. A 2.85 mm strand has about 2.65× the cross-section of a 1.75 mm strand, so the same kilogram is roughly 2.65× shorter.
What density should I use?
Use the value for your material: PLA is about 1.24 g/cm³, PETG 1.27, ABS 1.04 and TPU 1.21. Manufacturers sometimes print the exact density on the spool or spec sheet; a small error changes length by the same percentage.
Does this account for the empty spool weight?
Enter the net (filament-only) weight for the spool — usually 1000 g for a 1 kg spool. The plastic reel itself typically adds 150–250 g, which you should not count as filament.
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This calculator is for educational purposes. Double-check important results before acting on them.