How much filament is left on a spool?

Weigh it: scale reading minus tare

The scale shows the gross weight: filament plus the spool it's wound on. The empty spool's weight is called the tare. Subtract one from the other:

1,180 g on the scale − 230 g empty spool = 950 g of filament left

Weigh the whole spool as it sits on the shelf, without the box or a desiccant pack. A scale that reads to 1 g is plenty.

Finding the empty-spool weight

• Some makers print it on the box or the spool's label.
• When you finish a spool, weigh the empty one before it goes in the bin. Spools of the same brand and line usually weigh the same, so that number serves for the next one.
• Plastic, cardboard and refill-style spools differ a lot, so don't borrow the tare from a different brand.

Turning meters into grams

A slicer or printer often reports filament as length. For 1.75 mm filament, grams = length × the filament's cross-section × its density. One meter of 1.75 mm filament is about 2.4 cm³, so:

• PLA (about 1.24 g/cm³): roughly 3.0 g per meter, so a 1 kg spool holds about 335 m.
• PETG (about 1.27 g/cm³): roughly 3.05 g per meter.

Density varies by brand and by filler, so the scale stays the final word.

Keeping the number right between weigh-ins

Nobody weighs a spool after every print, so the number in between has to come from somewhere. In Spoolroom:

• You enter the scale reading and Spoolroom subtracts the tare for you. Add a spool from the catalog and the empty-spool weight is often filled in already.
• Each print takes its grams off the right spool, from the sliced file or from a printer that logs its own prints. Every deduction, weigh-in and correction is kept as an entry you can read back.
• The next weigh-in wins. A spool you haven't weighed in a few weeks is marked drifting, one running low is flagged on the home screen, and planning a print tells you whether the shelf can cover it.