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What this tool does
The question every printer owner asks at the worst moment is whether the spool in the machine holds enough for the print about to start. This spool remaining filament calculator answers it from three weighings: the full spool weight, the empty spool weight, and the current weight. From those it gives the filament left in grams, the percentage of the spool remaining, and the length left in metres for your diameter and material, plus a simple bar showing the spool at a glance.
The tool runs in your browser and needs no account. It is the fastest reliable check before a long print, and it beats the two common alternatives: guessing from how the spool looks, which is hopeless on an opaque spool, and unspooling to measure, which wastes filament and invites tangles.
The formula, in plain words
The arithmetic has two steps. First, subtraction: filament left equals current weight minus empty spool weight, and the original filament equals full weight minus empty weight. A spool that weighed 1,000 grams full, with a 250 gram empty spool, held 750 grams of filament. If it now weighs 625 grams, 375 grams remain, which is exactly half. Second, conversion: grams become metres through the filament’s cross-section and density, the same conversion as the companion length and weight converter. For 1.75 mm PLA, 375 grams is about 125.7 metres.
The percentage is the figure to watch during a print farm day, because it is comparable across spools of different sizes. A spool at 50 percent holds half its prints, whatever its size. The length is the figure to compare against a slicer estimate, which is always given in metres or grams for the coming print.
A worked example
Walk through a real check. Your slicer says the next print needs 90 metres of 1.75 mm PLA, about 268 grams. You weigh the spool: 540 grams. The empty spool weighs 250 grams, so 290 grams remain, about 97.2 metres. The print fits, with 7.2 metres to spare, which is tight but workable if your extrusion is well calibrated. If the spool had weighed 500 grams, only 250 grams, about 83.8 metres, would remain, and starting the print would gamble on running out near the end. That is the decision this calculator exists to make boring.
Notice how small the margin is in the first case. A print that needs 90 metres and a spool holding 97 metres leaves less than ten percent headroom. Most experienced printers want at least fifteen percent headroom on an important print, because the first layers, the skirt, and any purge all consume filament the model estimate may not include. When the margin is thin, the safe move is a fresh spool, and the half-used one goes to a smaller print.
How to weigh well
Use kitchen scales that read to the nearest gram, and weigh the spool without the filament guide tube attached and without any loose end hanging off, since both add grams that are not on the spool. The empty spool weight is the critical number: makers sometimes print it on the spool, but the figure can include the box or a desiccant bag, so verify it once with an actually empty spool of the same brand. Keep a note of empty weights for the brands you use, taped to the shelf, and the whole check takes under a minute.
Weigh in the same conditions each time. A spool that has absorbed moisture weighs more, which makes the calculator overestimate the filament left, particularly for nylon, which can gain several percent in weight from humidity. If a spool has sat open for weeks, dry it before trusting the weighing, and remember that the density figures are typical values, so filled filaments will read slightly long.
Limits and honest warnings
The calculator assumes the spool weight is only spool plus filament. Cardboard spools vary in weight from unit to unit by several grams, and a spool with a broken flange or a taped repair weighs differently from a new one. For most purposes the error is small, but on a nearly empty spool a ten-gram error is a large fraction of what remains, so treat the last few metres as uncertain and never start a print that needs the very last of a spool.
Also remember that length remaining is not length usable. The last metre or two on a spool often will not feed cleanly, because the curve is tight and the end is usually bent or taped to the spool. Subtract a metre from the result for planning, keep the tail for test prints, and recycle the spool. This is an educational estimator, and your printer manual and filament maker’s guidance always govern temperatures, materials, and safety.
Related calculators
- Spanish version
- Filament Cost Calculator – Cost per Gram and per Print from Spool Price
- Print Time Estimator – Manual Print Time Without Opening a Slicer