This calculator turns your own diameter readings into statistics and an area estimate. It replaces the old scored action picker: no score establishes spool tolerance, tells you that filament is good, or changes a printer setting.
Filament readings to diameter and area
Enter 5–200 measured locations, one per line. Use one diameter for a circular assumption or two perpendicular axes separated by a comma for an ellipse.
Example readings 1.74, 1.75, 1.76, 1.74 and 1.76 mm have a mean of 1.75 mm. Values are editable examples; entries stay in this page.
Run the calculation to see your result.
Record the sampling method
Measure several separate locations without squeezing the strand. Enter at least five locations; a larger, distributed sample is more informative than repeated measurements of one loose end. Five is this interface’s minimum, not a manufacturer tolerance standard. Record the instrument, sampling positions and conditions with the spool identity.
One value on a line treats the strand as circular. For an oval cross-section, put two perpendicular measurements on that same line, such as 1.70, 1.80. Under an elliptical approximation, area is π × axis 1 × axis 2 ÷ 4 and equivalent circular diameter is √(axis 1 × axis 2). A single diameter cannot reveal ovality, and two axes do not establish the complete cross-section of an irregular strand.
Read diameter and area separately
The tool reports mean, minimum, maximum and sample standard deviation of the equivalent diameters. It separately averages the measured areas. Area calculated from the mean diameter is generally not identical to the mean of the areas; the tool uses the latter for volume and length.
The five example readings average 1.75 mm but have a nonzero spread. That does not establish how the rest of the spool behaves. Compare your sample with the exact maker’s diameter tolerance and your measurement uncertainty rather than a universal good/bad cutoff. No material or tolerance preset is supplied.
The uncertainty field recalculates each axis at the entered plus/minus amount. The resulting area interval is a sensitivity scenario, not a statistical confidence interval or a claim about caliper accuracy. A display reading of 0.01 mm alone does not establish that the measurement is accurate to 0.01 mm.
Optional mass-to-length calculation
Enter both net filament mass in grams and exact material density in g/cm³. Empty density leaves length unknown. Numerically, length in meters is mass in grams ÷ (density in g/cm³ × mean area in mm²), after unit conversion. Exclude spool packaging from mass. Density and representative cross-section remain assumptions, so the result is an approximation.
Prusa’s filament calculator describes the separate use of filament mass, density and diameter for volume/length comparisons. This article’s sample statistics, ellipse treatment and uncertainty scenario are original geometry; no spool was measured for publication. Source checked October 15, 2026.
Keep geometry separate from calibration
Do not use a diameter summary as proof of drying needs, surface quality or mechanical faults. Keep your printer’s supported filament profile and calibration procedure. The flow helper handles a different measured wall test. For a slicer’s actual consumed mass, use the job cost calculator .