How much filament is left on the spool? Track it without a scale
The print is 14 hours in, the last layers are going down, and the spool runs dry. You had looked at it that morning and thought "that's plenty". It was not.
Knowing how much filament is left sounds like it needs a scale, a spreadsheet and the discipline to update both. Most people try that once, keep it up for two weeks, and go back to guessing.
The useful version is smaller: three levels, set by eye, on the spool you are already looking at.
Full, half, low
Every spool in your SpoolShare catalogue has a level: full, half or low. You set it with one tap on the spool's page, or in the edit form. That is the whole interface.
The level is drawn, not written. The winding on the spool graphic shrinks towards the hub - a full spool is fat, a half spool is a ring, a low spool is a thin band around the core. On the catalogue grid you can read the state of forty spools without opening one, and cards for low spools carry a small "Low" badge in case you scan for text rather than shapes.
Why not grams? A number you have to weigh for is a number you stop updating. Three steps you can judge by holding the spool are updated when you swap spools, which is when it matters. If you want gram-precise tracking that talks to your printer, that is what tools like Spoolman are for - and you can import a Spoolman export into SpoolShare to share the list with friends.
Estimating by weight, if you want a check
For the moment you are unsure whether "half" is really "low", a kitchen scale settles it. Weigh the spool with filament on it and subtract the weight of the empty spool - manufacturers usually list it, and it is often printed on the box. What is left is filament.
Compare that with the net weight you bought (typically 1 kg, sometimes 750 g or 500 g). Above two thirds is full; between a quarter and two thirds is half; below a quarter is low. Those are the same cut-offs SpoolShare uses when it imports remaining weights from Spoolman, so the two agree.
You do not need to do this often. Once per spool, the first time you doubt your eye, is enough to calibrate.
What low is for
A spool marked low shows a hint on its page: put it on the wishlist now? One tap copies the same brand, material and colour to your wishlist, while the spool itself stays in Available. You keep printing from it and the replacement is already on the list for the next order.
When it finally runs out, Ran out moves it to the wishlist; when the new one arrives, Got it brings it back, full. The level resets with it - it is a new spool.
Sharing the level
On a shared page, friends see the spool the same way you do, level included. "Can I borrow your galaxy black?" answers itself when the card shows a thin band around the hub. That is a better answer than a number, because nobody has to ask what the number means.
Keep it honest. Set the level when you take the spool off the printer, not from memory a week later. It takes two seconds, and it is the only maintenance this needs.
Open your catalogue and set a few levels - or sign up if you are new here.