How to Tell How Much Filament Is Left on a Spool
5 min read · Updated November 2026
Running out of filament ten hours into a twelve-hour print is one of the most frustrating ways to lose a part. The fix is knowing how much is left on each spool before you start. Here are three ways to find out, from most to least accurate.
Method 1: Weigh the spool
This is the most reliable method. You need a kitchen scale and one extra number: the weight of the empty spool.
Filament left = current weight − empty spool weight
Empty spool weight varies a lot between brands. Cardboard spools are lighter than plastic ones, and some plastic spools are much heavier than others. Some brands print it on the label; if yours doesn't, weigh an empty spool the next time you finish one and write the number down for that brand.
Weighing is accurate, but it means taking the spool off the printer. That's fine for an occasional check and tedious for a whole shelf.
Method 2: Log what you print
Every slicer tells you how many grams a print will use. If you start with a full spool and subtract each print's grams as you go, you always know what's left without touching a scale.
This works well as long as you log every print, including the failed ones. Slicer estimates are usually close but not exact, so weigh the spool once in a while and correct your running total.
Logging has a second benefit: once you know which spool each print came from and what you paid for it, you know what every print actually cost.
Method 3: Measure the length
If you have no scale, you can work from length. For 1.75 mm PLA, one metre of filament weighs roughly 3 grams, so a 1 kg spool holds somewhere around 330 metres. Other materials are slightly different because their densities differ: PETG is a little heavier per metre, and ABS a little lighter.
Measuring the filament left on a spool by hand isn't practical, so this method is mostly useful for converting your slicer's length estimate into grams.
How to stop running out mid-print
- Check before long prints. If the job needs more grams than the spool has left, swap spools first.
- Set a reorder point. Reorder when a spool drops below about 15% of its starting weight, or earlier for colours you use often.
- Use a runout sensor if your printer supports one. It pauses the print so you can load a new spool.
- Keep spools labelled. A short ID on each spool, like PLA-001, makes logging quick.
- Store filament dry. Moisture causes stringing and weak prints, which waste filament on reprints.
Keeping track across many spools
With one printer and a few spools, a sticky note on each one works. With a shelf of colours and several printers, you need a system: one place that lists every spool, subtracts each print, flags low spools, and tells you what your shelf is worth.