Spool Remaining Calculator
A spool has no gauge on it. Weigh it, subtract the weight of an empty spool of the same type, and you have the grams left, what that is worth and how many more prints fit.
Weigh the spool
Subtract the empty spool and the rest is filament. Nothing is sent anywhere.
What this calculator works out
A spool has no useful gauge on it. The only honest way to know what is left is to put it on a scale and subtract the weight of the empty spool:
percent left = filament left ÷ net weight when full
value left = percent left × what you paid
usable prints = (filament left − tail) ÷ (model weight × (1 + waste))
The last line matters more than it looks. The final 20 to 40 grams of a spool usually will not feed — it tangles, snaps, or jams in the last few metres — so counting it as usable capacity makes every estimate of remaining prints optimistic.
How to weigh a part-used spool
Three steps, about a minute, and you only have to do the second one once per spool type:
- Put the whole spool on a kitchen scale and write down the number. Anything that reads to 1 g is accurate enough — a 2 g error is 0.09 of a cent on a $22 spool.
- Weigh an empty spool of the same make and size. Do this once, note it on the spool with a marker or in your phone, and reuse it for every spool of that type.
- Subtract. The difference is filament.
If you have never weighed an empty spool of your brand, do it before you throw the next one away. The number is specific to the manufacturer and the size, and reusing someone else's figure from a forum is how estimates drift.
Empty spool weights vary more than people expect
There is no universal figure. Cardboard cores, full plastic sides, and the wide-spool formats all weigh differently, and a 250 g empty spool on a 1 kg reel is 25% of the delivered weight. That is exactly the error that makes "price divided by 1 kg" wrong.
The table below is a starting point for what to expect, but make your own measurement the authority. A scale costs less than a spool.
| Spool format | Typical empty weight | If you assume 0 instead |
|---|---|---|
| Cardboard core, 1 kg reel | roughly 150 – 250 g | overstates filament left by up to 25% |
| Full plastic sides, 1 kg reel | roughly 200 – 300 g | overstates filament left by up to 30% |
| Small 250 g sample reel | roughly 60 – 120 g | overstates filament left by up to 48% |
The percentages in the right-hand column are the argument for weighing an empty one. On a small sample reel, forgetting the spool itself can nearly double your apparent remaining filament.
Worked example: a part-used 1 kg spool
742 g on the scale, a 218 g empty spool, a 1000 g reel that cost $22, and a 45 g model with a 12% waste allowance:
Nine prints, not ten. The difference is the 30 g tail the calculator holds back — 494 g of usable filament divided by 50.4 g per print. That is the number to use if you are deciding whether you have enough filament to accept an order tonight.
Important assumptions
The empty-spool default of 218 g is a placeholder. Replace it with the weight of a spent spool of your own brand before you trust the result.
The tail reserve is set at 30 g and is not adjustable on this page. Real tails vary with spool design and how the filament was wound; some spools unwind cleanly to the last metre, and some start tangling 60 g out. Treat the prints-left figure as an estimate that leans slightly conservative — that is the direction you want an estimate to lean.
Questions
How much filament is left on my spool?
Weigh the whole spool, subtract the weight of an empty spool of the same type, and the difference is the filament. A spool reading 742 g with a 218 g empty spool has 524 g of filament on it, which is about 52% of a 1 kg reel.
What does an empty filament spool weigh?
It depends on the format. Cardboard-core 1 kg reels are commonly around 150 to 250 g, full plastic-sided 1 kg reels around 200 to 300 g, and small 250 g sample reels around 60 to 120 g. Weigh a spent one from your own brand once — that is the only number that is right for your stock.
Can I estimate remaining filament without a scale?
No, not reliably. The slicer's estimate describes the model, not the spool, and there is no gauge on a spool. Scales that read to 1 g are cheap and turn a guess into a measurement you can reuse for every spool of that type.
Why subtract a tail I cannot print?
Because the last 20 to 40 grams of a spool frequently will not feed. It tangles, snaps or jams near the end, so counting it as usable capacity makes your prints-left figure optimistic. This calculator holds back 30 g for that reason.