Filament Length from Weight Calculator
A spool of 3D printing filament is just a long solid cylinder, so its length follows directly from physics. Volume equals mass divided by density, and length equals that volume divided by the circular cross-sectional area. This calculator takes the filament mass in grams, the nominal diameter in millimeters, and the material density in grams per cubic centimeter, then returns the length in meters and the cross-sectional area. Because density varies by material, brand, and color, it stays a user-editable input rather than a hidden assumption, so you can match the figure on your manufacturer's data sheet.
Filament length formula
Volume (cm3) = mass (g) / density (g/cm3)
Radius (cm) = diameter (mm) / 20
Area (cm2) = pi * radius^2
Length (cm) = volume / area
Length (m) = length (cm) / 100
Diameter divided by 20 converts millimeters to centimeters and halves to a radius in one step. Because area scales with the square of the radius, an accurate diameter is the most important input.
Filament length context
- The math is exact for a solid cylinder; the only assumption is the density you enter.
- Use the resin density from your manufacturer's technical data sheet for best accuracy.
- Common nominal densities: PLA near 1.24, PETG near 1.27, ABS near 1.04 g/cm3.
- Subtract the empty spool weight before entering the filament mass.
- Diameter tolerance and moisture can shift real length by a few percent.
Filament length: frequently asked questions
How do I convert filament weight to length?
Filament is a solid cylinder, so volume equals mass divided by density. Length equals volume divided by the cross-sectional area, which is pi times the radius squared. With mass in grams and density in grams per cubic centimeter, you get volume in cubic centimeters, then divide by the area in square centimeters to get length in centimeters, and divide by 100 for meters.
What density should I use for PLA, PETG, or ABS?
Density varies by brand and color, so this calculator keeps it editable. Manufacturer technical data sheets list the resin density; common nominal values are about 1.24 g/cm3 for PLA, 1.27 for PETG, and 1.04 for ABS. Use the figure on your spool's data sheet for accuracy.
Does diameter matter much?
Yes, strongly. Area scales with the square of the radius, so the difference between 1.75 mm and 2.85 mm filament is large. Always enter the nominal diameter of your filament. A small measurement error in diameter has an outsized effect on the length estimate.
Why does my estimate differ from the spool label?
Labels often quote net weight, but actual density, diameter tolerance, and moisture can shift the real length by a few percent. Weigh the spool and subtract the empty spool weight for the most accurate input mass.
Can I use this to check how much filament a print needs?
Yes. If your slicer reports the print mass, enter it here to get the length, or enter the length-to-weight relationship to plan a multi-spool job. Pair it with a material cost calculator to budget the print.
Official sources
- National Institute of Standards and Technology: Office of Weights and Measures (mass, length, density units).
- RepRap project documentation: reprap.org (filament geometry reference).
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.