This topic seems to come up now and again, and I wanted to know too. So I constructed a ruler. It is pretty simple minded, I just calculated the apparent volume of the material, split it into 8 equal volume cylindrical torus, and graduated a ruler referenced to the rim. How accurate is it? I think, better than a wild guess and less than weighing the spool. But you can measure in no time, even when the spool is on the machine actively feeding filament. Only about 50g left on this one I think....
The text is pretty small, I suppose I should have made the whole thing a little bigger. You need to print it at 0.09 to get the engraving to show. However, try this one, markings are .1 mm (1 layer?) deeper.
