How is remaining filament calculated from weight?

It subtracts the empty spool weight from the current total: remaining grams equals total weight minus empty spool weight. The empty weight is usually printed on the spool or listed by the manufacturer.

How does the diameter method work?

It computes the cross-sectional ring area of the wound coil from the outer and core diameters, multiplies by the coil width to get a bounding volume, applies a packing efficiency of about 90%, then converts volume to mass and length.

Which method is more accurate?

Weighing is far more reliable, accurate to your scale's resolution. The diameter method is a useful field estimate when you cannot weigh the spool, but expect roughly plus or minus ten percent error from coil packing variation.

Where do I find the empty spool weight?

Many manufacturers print the empty (tare) weight on the spool or in their data sheet. If not, weigh a known-empty spool of the same model, or note the full spool weight when new and subtract the stated net filament weight.

How does material density affect the result?

Density converts volume to mass for the diameter method and converts mass to length for both methods. PLA is 1.24 g/cm³, PETG 1.27, ABS 1.04. Selecting the right material keeps the length estimate accurate.

Filament Spool Remaining Calculator

Running out of filament three hours into a long print is one of the most frustrating ways to fail a job. This calculator estimates how much filament is left on a partly used spool, either precisely by weighing it or approximately by measuring the wound coil with calipers, then converts that to metres so you can confirm a print will finish.

How it works

Weighing (recommended). The most accurate method is pure subtraction:

remaining grams = current total weight − empty spool weight

The empty (tare) weight is often printed on the spool. The tool then converts grams to length using the material density and filament diameter: length = (mass ÷ density) ÷ strand cross-section.

Coil diameter (field estimate). When you cannot remove the spool to weigh it, measure the outer diameter of the wound coil, the core hub diameter, and the coil width with calipers. The wound filament forms a flat ring (annulus), so its bounding volume is:

volume = (π ÷ 4) × (outer² − core²) × width

Round strands wound side by side leave gaps, so the tool applies a packing efficiency of about 90% before converting volume to mass and length.

Example and tips

Suppose a spool reads 420 g on the scale and its empty weight is 180 g. That leaves 420 − 180 = 240 g — roughly 79 m of 1.75 mm PLA. If your slicer says the next print needs 60 g, you have plenty of headroom.

Weigh whenever you can; the diameter method is best reserved for spools you cannot easily remove. For long or expensive prints, always keep a 10–15% safety margin because slicer estimates and real consumption can differ slightly.