Spool
Spool calculator: how much is left?
Weigh the spool on a kitchen scale, subtract the empty spool, and get the grams, metres and value still on it — without a filament sensor.
Getting the empty spool weight right
- Weigh the part-used spool and write the number down.
- Weigh an empty spool of the same brand and material; some manufacturers print that weight on the box. If you have none, buy the same brand once and weigh it before loading — it is the only unknown in the calculation.
- Enter gross and empty weight above. The difference is filament.
- If the result says you have more than the spool ever held, the nominal weight in field three is wrong (2 kg spool?) or you mixed up the two weights.
Worked example: a 1 kg spool that weighs 1,180 g
Gross 1,180 g, empty 220 g, nominal 1,000 g, PLA at 1.75 mm and $14.99/kg. The panel prints:
- Left
- 960 g
- Already used
- 40 g
- Of a full spool
- 96%
- Length
- 321.9 m
- Volume remaining
- 774.2 cm³
- Filament value left
- $14.39
Move one field at a time. At 2.85 mm the grams stay at 960 and the length falls to 121.4 m; with PETG (1.27 g/cm³) it is 314.3 m and $19.19; at 640 g gross it is 420 g, 42% and 140.8 m. The length to weight page turns those metres back into grams.
When this number is wrong
The subtraction is exact; the empty spool weight is the error budget, and every gram of it lands in the metres.
- Scale resolution. 5 g of error is 1.68 m of PLA, 10 g is 3.35 m, so a scale that rounds to 5 g spends the budget before you start. Check it against a litre of water.
- A guessed empty weight. Empty spools differ by brand, material and size, and this page publishes no table of them because we have not weighed enough. Weigh the product you bought: 40 g of error is 13.41 m.
- The nominal weight is a label. A 1 kg roll may hold slightly more or less, and above 100% either the empty weight is low or the roll was never full. If filament snaps mid-roll, weighing the rest still works.
Grams are measured, metres derived, so density enters only the length line. The cost-per-gram line rounds to whole cents ($14.99/kg prints $0.01/g); quote the value line. Yield is outside it too: the failure rate page turns scrap into cost, and cost per gram prices the remainder.
Frequently asked questions
Why does my 1 kg spool weigh more than 1 kg?
Because you weighed the spool too. Net weight is filament only; the empty spool accounts for the rest, which is what the empty-weight field is for.
How accurate is weighing the spool?
To about ±2 g on a kitchen scale, under 1% of a 1 kg spool — far better than guessing from how full the spool looks.
Can I use the spool width instead?
Estimating from the wrap diameter works, but badly: 5 mm of radius on a 200 mm spool is 10% of the filament. Weighing is easier and more accurate.
Does the diameter I select change the grams?
No. Grams come from the weight difference. The diameter and density only convert those grams into metres, which is why the length figure changes when you switch materials.
How many grams of a 1 kg spool actually end up in parts?
Fewer than 1,000. Every print lays down a skirt and a prime line, supports come off the part, and a failed print spends filament without producing anything to sell: at a 10% failure rate you must load 1,111.1 g to ship 1,000 g of parts. This page says what is on the roll, your scrap rate what comes off it.