Weigh the spool, subtract the weight of the empty reel, and convert.
A worked case: the scale reads 480 g, the reel is a 160 g cardboard one, so 320 g of PLA remains. At 2.98 g per metre that is 107.29 m, and at 25 g per print it is twelve prints.
The step everyone gets wrong
Subtracting a kilogram instead of subtracting the reel.
A spool sold as 1 kg holds a kilogram of filament; the reel is extra. A brand-new PLA spool on a typical 215 g plastic reel reads 1,215 g on the scale. Weigh it, subtract a kilogram, and you will conclude you have 215 g left on a completely full spool.
Reel weight is the whole accuracy problem
Assume the wrong reel and the error is enormous. The same 700 g reading gives:
| Assumed reel | Filament left | Length |
|---|---|---|
| 140 g cardboard | 560 g | 187.76 m |
| 250 g plastic | 450 g | 150.88 m |
A 110 g swing — nearly forty metres — from an assumption about packaging. That is more than most single prints consume.
What to do about it, once
Weigh your reels empty and write the figure on them in marker. It takes seconds and it is permanently correct for that reel. Prusa prints the weight on every label, which is the behaviour worth rewarding.
Two other cases worth knowing:
- Refill coils have no reel at all. The gross weight is the filament weight, and the naive arithmetic is correct for once.
- Cardboard reels gain weight in humid storage. They absorb moisture themselves, so a badly-stored cardboard reel weighs more than it did new and biases the estimate low.
When weighing will not do
A scale accurate to the nearest 5 g is fine for "will this print finish". It is not fine for cutting a spool close on a long job — for that, print the job in two parts, or start it on a fresh spool.
Type your scale reading and your reel figure into the length and weight converter and it will do the subtraction and the conversion together. If you do not know your reel, the spool weights guide has the fallback ranges.