Weigh a part-used spool, subtract the empty spool, and see how many grams and metres of filament are actually left — and whether your next print fits.
Remaining filament = what the spool weighs now minus the empty spool. Empty spools typically weigh 130-150 g (cardboard) or 200-260 g (plastic); the exact figure is often printed on the spool or listed by the manufacturer. The length uses the same cylinder maths as the length-weight converter.
Weigh the spool on a kitchen scale and subtract the empty spool weight. A spool weighing 750 g with a 250 g core has 500 g of filament left, which is about 168 m of 1.75 mm PLA.
Commonly 130-150 g for cardboard spools and 200-260 g for plastic ones, but it varies by brand. Many manufacturers print the spool weight on the label or list it online; weighing a spare empty spool of the same brand is the most accurate.
Enter the grams your slicer estimates for the print. The tool compares it to what is left and shows the spare margin, so you can decide whether to risk it or start a fresh spool.