Filament Remaining Calculator
Put the spool on a kitchen scale, pick the brand (or enter the empty spool's weight), and see exactly how many grams and meters of filament are left — and whether it's enough for the print you're about to start.
How it works
length left (m) = grams ÷ (π × (diameter ÷ 2)² × density)
Weight is the most direct way to know what's on a spool, because you can't see how many layers are wound underneath the outer one. The single piece of information you need beyond a scale is the empty spool's weight, and that's the one most people don't have — which is why the presets exist.
What about printers that estimate it for you?
Some do, and it is worth knowing how. Bambu's AMS reads the RFID tag on a Bambu spool as it passes the reader once per revolution, and compares that against the length of filament its odometer says it has fed. Length per revolution gives the effective circumference, which gives the spool's current diameter, which maps to the amount remaining — a full roll feeds far more filament per turn than a nearly empty one. A standard 1 kg roll is treated as 100% unless it is told otherwise.
It is a genuinely clever approach and it works. But it is geometry-derived rather than a measurement of mass, so it can drift; it assumes a standard roll; and it depends on readable RFID tags. Third-party spools, re-spooled filament and anything without a tag fall back to a figure you set by hand. Weighing sidesteps every one of those assumptions, which is why a scale stays useful even on a printer that estimates.
Worked example
A Hatchbox PLA spool reads 640 g on the kitchen scale.
- Filament left: 640 − 225 = 415 g (about 41% of the original kilo)
- Length: 415 ÷ 2.98 g/m = 139 m
- A print needing 380 g will make it with 35 g to spare — enough, but a mid-print runout would waste 380 g, so it's worth double-checking the tare on a reel you've weighed yourself.
The second method: measure the roll
A scale is the better answer, but it means taking the spool off the printer. If it is loaded in an AMS or sitting in a dry box, there is a geometric alternative that works in place.
Wound filament forms an annulus — a ring with the hub as its hole. Its cross-sectional area is proportional to the difference of the squared radii, and the length of filament in it is proportional to that area times the winding width. Compare the ring you have now against the ring on a full spool and everything awkward cancels:
grams remaining = fraction × grams on a full spool
The winding width cancels because it is the same ring. More usefully, so does the packing factor — how tightly the filament is wound and how much air sits between the wraps. That figure is genuinely uncertain, somewhere around 0.6–0.75 for typical machine winding, and any method that needs it inherits that uncertainty. Taking a ratio against the full spool removes it entirely, as long as the winding is similar, which for the same brand and machine it is.
What you need once, per brand: the outer diameter of a new spool. Measure it before you start printing and write it on the box alongside the tare weight. After that the method needs only a ruler across the roll.
Expect it to land within perhaps 5–10% of the weighed figure. Uneven winding, a partly collapsed roll, or filament wound to a different tension will all move it. If both figures are on screen the page shows the gap between them, which is a decent check on your assumed full diameter.
Empty spool weights
These are typical values compiled from manufacturer statements and community measurements. Spools change — several brands moved from plastic to cardboard between 2022 and 2025, and even the same design varies by 10–20 g batch to batch. The only exact number is the one you get by weighing an empty one.
| Spool | Approx. empty weight | Type |
|---|---|---|
| Bambu Lab reusable spool | 250 g | Plastic, two-part |
| Bambu Lab refill (core only) | 12 g | Cardboard core, no flanges |
| Prusament | 190 g | Plastic (the label often prints the exact tare) |
| Hatchbox, Inland, Elegoo, eSun (plastic) | 220 – 225 g | Standard black plastic reel |
| Creality, Sunlu | 200 – 215 g | Plastic |
| Overture, Polymaker, eSun (cardboard) | 135 – 145 g | Cardboard |
| Generic cardboard | ≈ 130 g | Cardboard |
A habit that pays off: when a spool runs out, weigh it, and write the number on the box or in a note. After a few months you'll have exact tares for every brand you buy, and the presets become unnecessary.
Turning grams into prints
Your slicer reports the grams a print needs. Compare directly with what's left, and keep a margin — the tare uncertainty alone can be ±15 g, and printers waste a little on purge lines and retractions the slicer doesn't count. If the margin is under about 10% of the print's weight, it's a gamble; either use a fresh spool, or plan to swap spools mid-print (most modern printers pause on runout, and most filaments can be spliced or simply changed at a pause).
To go the other way — how many grams a given length is, or how far 300 g will go — use the length ↔ weight converter, and to see what those grams cost, the filament cost calculator.
Frequently asked questions
How do I know how much filament is left on a spool?
Weigh the whole spool and subtract the weight of the empty spool. What's left is filament. A full 1 kg spool on a 220 g reel weighs 1,220 g; if yours reads 640 g there's 420 g of filament left. Divide by the grams-per-meter figure for the material (about 3 g/m for 1.75 mm PLA) to get the length.
How much does an empty filament spool weigh?
Between about 100 g and 260 g depending on the maker. Cardboard spools (Overture, Polymaker, eSun cardboard) are 130–150 g; standard plastic reels (Hatchbox, eSun, Inland) 210–230 g; Bambu Lab's reusable plastic spool about 250 g; Prusament about 190 g. These vary by batch, so weigh an empty one when you finish a spool and write it on the box.
How many meters are in 1 kg of filament?
About 335 m for 1.75 mm PLA, 327 m for PETG, 400 m for ABS and 344 m for TPU. For 2.85 mm filament divide by roughly 2.65. The calculator uses the density of the material you select.
Is there a way to check without weighing?
Some printers estimate it. Bambu's AMS reads the RFID tags on Bambu spools and counts how much filament is dispensed per spool revolution; because a fuller spool feeds more filament per turn, that ratio reveals the current spool diameter and hence what is left. It works well within its assumptions, but it needs readable RFID tags, so third-party spools have to be set manually. Weighing is independent of all of that and works on any spool.
How does the Bambu AMS work out remaining filament?
It combines two measurements: an odometer for the length of filament fed, and detection of the RFID tag passing the reader once per spool revolution. Dividing length fed by revolutions gives the effective circumference, and therefore the spool's current diameter, which maps to how much filament remains. A standard 1 kg roll is treated as 100% by default. This is an estimate derived from geometry rather than a measurement of mass, so it can drift, and it depends on the RFID tags being readable.
My scale only shows whole grams. Is that accurate enough?
Yes. One gram is a third of a meter of filament — nothing you'll notice. The bigger uncertainty is the empty spool weight, which can vary by 10–20 g between batches of the same brand.
Last reviewed September 8, 2026. Spotted an error or want another calculator? Tell us.