The method is three steps and one habit:
- Weigh the whole spool on a kitchen scale. Anything resolving to 1 g is enough.
- Subtract the empty reel's weight — the figure you measured, not one from a table.
- Divide by grams per metre for the material and diameter you are using.
The habit is step two. Weigh every reel when it runs out and write the number on it in marker. From then on that spool's estimate is exact, permanently.
How accurate this actually is
Take a spool reading 705 g on a 215 g reel: 490 g of filament, 164.29 m. Read it 5 g lighter — well within a cheap scale's error — and you get 485 g and 162.61 m.
So a five-gram error is worth 1.68 m. For deciding whether a spool will survive the night, that is irrelevant. For cutting a long print fine, weighing is the wrong tool entirely: start it on a fresh spool.
Where the real error lives
Not in the scale. In the reel weight.
Reel construction spans 140 to 250 g on 1 kg spools, so guessing wrong is a swing of up to 110 g — twenty-two times the scale error above. Every effort should go into knowing the reel, not into a better scale.
Practical notes
- Tare with the spool holder if it comes off with the spool. Weighing a spool still on its mount is the most common source of a wildly wrong answer.
- Weigh dry. A PETG or nylon spool that has absorbed moisture weighs more than the filament it contains. Weigh after drying, not before.
- Cardboard reels drift. They absorb humidity themselves, so a figure written on a cardboard reel two years ago is worth less than one written on a plastic one.
- Refill coils need no subtraction. Gross is net, and this is the one case where the obvious arithmetic is right.
Without a scale
Count the turns left and multiply by the mean circumference of the wound filament. It is crude, it ignores how the coil tapers, and it will get you within a rough band — enough to answer "plenty" or "nearly gone", which is usually the actual question.
Whichever route you take, the reel weight is the term that carries the error: get it wrong by fifty grams and every figure downstream is wrong by fifty grams. The length and weight converter assumes a 215 g plastic reel, and that is a starting assumption rather than a measurement of yours.