Spool Remaining by Weight Calculator
Weigh a part-used spool and find out how much filament is really left. Subtract the empty reel, and get the remaining length and the percentage of a full spool — so you know whether the next print will finish.
Presets are typical — use the exact tare printed on your spool if you have it.
Filament only, for the % left.
Formula & how it works
Remaining filament = spool weight now − tare. Grams per metre is π × (diameter ÷ 2)² × density, so length is remaining ÷ grams-per-metre.
Percent left is remaining ÷ full net weight × 100.
Worked example
A spool reads 480 g on the scale and its plastic reel weighs 230 g, leaving 250 g of PLA. At 1.75 mm and 1.24 g/cm³ that is about 2.98 g per metre, so roughly 84 metres — around 275 feet — remain. Against a 1,000 g full spool, that is 25% left.
Getting a reliable reading
The tare is the whole trick
Everything depends on subtracting the empty reel correctly. Manufacturers often print the spool weight on the side or list it online; if not, keep one empty spool of each type and weigh it once. A guessed tare is the main reason a "filament left" figure comes out wrong.
Use a scale with enough range
A near-full spool can top a kilogram once the reel is counted, so a small jewellery scale may not cope. A kitchen scale reading to a gram is ideal — precise enough to matter, with the capacity for a loaded spool.
Leave a safety margin
Because diameter and density drift slightly along a spool, treat the length as an estimate. If your slicer says a print needs close to what remains, start a fresh spool rather than gamble on the last few metres and a failed print near the end.