How to Tell How Much Filament Is Left on a Spool
A spool that looks half full can run out halfway through a long print. Here is why eyeballing filament fails, how to weigh a spool with the right tare, how to track grams per project, and how to know when to reorder before a print stops mid-layer.
You slice a twelve-hour print, glance at the spool, and decide there is plenty. Eight hours later the extruder is pushing air and the part is ruined at layer 600. Almost everyone who prints regularly has a story like that. Filament is sold by weight, consumed by weight, and reported by your slicer in grams, yet most of us judge what is left by looking at it. The reliable answer is a kitchen scale, the weight of the empty spool, and a running note of what each print used.
Why empty-looking spools still surprise you
Eyeballing a spool is harder than it seems, for a few reasons:
- The core hides the truth. Wide cardboard and plastic cores make a thin layer of filament look like more than it is. The last 100 g on a large hub can look nearly identical to 250 g.
- Spools are not all the same size. A 1 kg spool from one brand can be wider or narrower than another, so a habit you built with one maker misleads you on the next.
- Winding is uneven. Filament stacks unevenly across the width, so the visible outer layer does not tell you how deep the rest goes.
- Memory drifts. You remember the spool was “about half” two weeks ago, but not the three small prints and the failed calibration cube since then.
Runout sensors help, but they only tell you when it is already too late for a single-piece print. Knowing the number before you hit print is what saves the job.
Weighing and tare tips
A basic digital kitchen scale that reads to 1 g is enough. The trick is knowing what to subtract:
- Find the empty spool weight. Many brands list it on their site or the box; common values range from about 120 g for cardboard to 250 g or more for thick plastic. Better still, weigh your own empty spools once and write the number on the side with a marker.
- Weigh off the printer. Pull the spool off the holder and set it flat on the scale. Weighing on the holder, or with filament still loaded into the extruder, adds tension and error.
- Subtract, do not guess. Total weight minus the empty spool is your filament left. A 1 kg spool that weighs 430 g with a 180 g core has 250 g remaining, not “a bit under half.”
- Weigh new spools too. Advertised 1 kg is often 1,000–1,020 g of filament. Weighing at the start gives you a true baseline instead of a rounded label.
If you reuse empty spools for refills, keep a short list of core weights by brand. It takes five minutes once and makes every future weigh-in accurate.
Track usage per project
Weighing tells you where you are now. Tracking per print tells you where you will be. Your slicer already estimates grams for every job, so the habit is simple: after each print, note the project, the spool, and the grams used, then subtract it from that spool’s remaining weight.
A few details make the log more useful:
- Log failed prints as well. A spaghetti failure at 40% still used 40% of the filament. Skipping it is how your numbers drift away from reality.
- Add purge and brims. Multi-material purge towers and big brims add up. If your slicer reports total grams, use that figure rather than just the model.
- Reweigh now and then. Every few weeks, put the spool on the scale and correct the logged number. Estimates and reality differ by a few percent, and a quick check keeps them honest.
Over time the history pays off in other ways: you see what a batch of parts really costs in material, which projects eat the most filament, and which colours you burn through faster than you thought.
Know when to order before a print fails
Once you know the grams left, the decision before each print is easy. Compare the slicer estimate with the remaining weight and leave a margin — 10–15% or at least 20 g — for purge, tolerance, and scale error. If the job does not fit, swap spools now or split the print, rather than babysitting a runout at 3 a.m.
For colours and materials you use often, set your own reorder point. If your go-to black PLA drops below about 300 g, order the next spool; shipping times are rarely shorter than the time it takes to finish a few more prints. Specialty filaments like silk or glow are worth a higher threshold, since they are the ones most likely to be out of stock when you need them.
If you would rather keep this on your phone than on scraps of paper, Spool Log tracks each spool’s remaining weight down to the gram. Log a print with the grams used, a project name, print time, and an optional photo, and the app subtracts it and shows what is left with a colour-coded ring that makes low spools obvious. It works on your device with no account, syncs through iCloud, and a one-time Pro unlock adds unlimited spools, full print history, photo logs, special finishes, and custom lists, with no subscription.