How the remaining filament calculation works
A spool that looks half full can hold anywhere from 300 to 600 g depending on its hub size, and the eye is a poor judge. A scale is far better. Weigh the spool, subtract what the empty spool weighs, and what is left is filament. From there the calculator converts grams to meters and compares the result with the print you want to run.
The steps are:
filament grams = total weight − empty spool weightmeters = filament grams / grams per meter(about 2.98 g/m for 1.75 mm PLA)fits = filament grams ≥ print grams × 1.05
The 5% margin covers purge lines, slicer estimate error, a skirt you forgot, and filament near the hub that may be too tightly curled or tangled to feed well.
Measuring the inputs
Weighing the spool
A kitchen scale reading in 1 g steps is enough. Put the spool on flat, centered, and remove any desiccant packs, clips or spool holder parts that are not part of the empty weight you will subtract. If the spool is too wide for the scale platform, place a light bowl on the scale, tare it, then set the spool in the bowl.
Empty spool weight
This is the input that causes the biggest errors. Empty spools vary a lot by brand and material:
| Spool type | Typical empty weight |
|---|---|
| Cardboard 1 kg spool | 140–180 g |
| Plastic 1 kg spool | 200–260 g |
| Refill on reusable spool | Weight of the reusable spool only |
| Small 250–500 g spools | Often 80–150 g |
| Large 2–3 kg spools | Often 350–600 g |
These ranges are only a guide. The reliable method is to weigh an empty spool of the same brand and size once, write the number on a label, and use it from then on. Some manufacturers publish the empty weight, but measuring your own beats trusting a listing.
Print weight
Use the slicer's filament estimate in grams, including supports, brim and any purge or prime tower.
Worked example
A plastic spool of PLA weighs 612 g on the scale. An empty spool of the same brand weighs 210 g.
- Filament left: 612 − 210 = 402 g
- Length: 402 ÷ 2.98 ≈ 135 m
- Next print needs 380 g. With 5% margin: 380 × 1.05 = 399 g
- 402 g is above 399 g, so the print fits, but only just.
In a case this tight, a filament runout sensor or a plan to swap spools mid-print is wise. If the empty spool weight had been guessed at 180 g instead of measured, you would think you had 432 g and the margin would look comfortable when it is not.
Common mistakes
- Guessing the empty spool weight. A 50 g error is common and is about 17 m of PLA.
- Weighing with the holder attached. AMS inserts, spool adapters and clips add weight that is not filament.
- Forgetting moisture. Damp filament weighs slightly more, but the bigger issue is print quality. Dry hygroscopic materials before long prints.
- Trusting the last few meters. Filament near the hub is tightly curved and can be hard to feed through a long PTFE path. Plan for a little waste.
Practical tips
- Write the empty spool weight on the side of each spool or keep a list by brand.
- Weigh spools when you put them away. A quick note of remaining grams saves time when choosing one for a long print.
- For prints that will use most of a spool, consider splitting the job or enabling filament runout detection.
- Keep remnants under about 100 g for small parts, test pieces and calibration prints rather than starting a long job on them.
- If you use refills on a reusable spool, weigh the reusable spool once and reuse that number.
For other materials the meters figure changes with density. PETG runs about 3.05 g per meter at 1.75 mm, ABS about 2.50 g and nylon about 2.74 g, so 400 g of ABS goes noticeably further than 400 g of PETG. The fit check itself works in grams, which is what the slicer reports, so the density only affects the length shown, not whether the print fits.