I use calipers and math to figure out how much filament is left on a spool.

For example, think of the filament as one solid ring of plastic. The spool is 60mm wide, the inner radius (of the filament, not the spool) is 28mm, and the outer radius is 40mm. Subtract the volume of the empty cylinder in the center from the solid cylinder of plastic, then multiply by 0.7 to account for packing density, and boom, you have a volume of filament that’s accurate to within a few percent.

For a quicker, less accurate method, think of the filament as a collection of individual circles wound around the spool. My example spool is 60mm wide, so that’s around 34 strands of filament, and the filament is stacked 12mm deep, so that’s around 6 strands of filament. 34x6=204, so the filament is wound around the spool 204 times.

The average radius of one circle around the spool is probably 34mm (right in between the inner and outer diameters), so good ol 2πr gives us an average circumference of 213mm. 213mm×204 windings is around 43,500mm of filament, or 43 meters.

It sounds very involved, but once you get the hang of it it’s very intuitive. You just have to know that a circle’s circumference is 2π times the radius, or π times the diameter, and multiply that by the estimated number of windings.