Have you ever run out of filament mid print? This calculator will estimate the length of the remaining filament, based on 3 measurements of your spool.

Works for 1.75mm and 3mm filaments. All measurements are in mm. The resulting length is in m.

D - Outer diameter of the remaining filament
d - Diameter of the inner coil
W - Inner width of spool

How it works: Calculates the volume of the filament on the spool and then divides by the area of the cross section of the filament. I have accounted for imperfect alignment of filament on the spool with a conservative factor of 0.8.