Remaining filament estimator using empty-spool weight: method and uncertainty

What the calculator does, and what it refuses to do

The tool applies one formula: estimated remaining mass = measured gross weight − empty-spool tare. Enter the scale reading for the partly used spool, then the corresponding tare. The form does not offer a list of saved tare values. It asks for neither density nor diameter and does not convert mass to length; a separate tool covers that conversion below.

Here is a deliberately hypothetical example, not a physical weighing: a gross weight of 735 g and an entered tare of 225 g give 510 g by subtraction. With a gross weight of 210 g and the same tare, the tool rejects the input instead of reporting zero. Those values do not satisfy the input rule; check the tare and weighings before interpreting the result.

Both fields accept finite, nonnegative numbers up to one million grams per field, and the tool rejects an input when gross weight is below tare. It returns one number in grams, with no calculated margin or range. You can then compare that estimate with the slicer’s planned mass while accounting separately for what your project is expected to consume. The display rounds to at most three decimal places; this formatting does not measure scale accuracy.

Where to get a tare you can trust for the spool in your hands

You can retain a measured tare from an empty spool of the same reference: brand, product line, generation and format. Record its configuration and weighing conditions. This provides a comparison value for your stock, but an identical reference does not establish that two spools have exactly the same mass. Distinguish a direct weighing of the actual empty spool from a value borrowed from another spool.

Always weigh in the same configuration. If the retaining clip, the bag, the desiccant pouch or an added label stays on the spool during the gross reading, your tare must include them too. Otherwise remove them both times. Any mismatch between the tare configuration and the gross configuration lands straight in the result, and the tool has no way to detect it.

If you do not know the tare for your reference, you can enter an estimate, but the tool does not quantify its possible error; record the value’s source separately and do not treat it as a measurement. Otherwise, wait until the spool is empty and weigh it in the configuration you intend to use. The sources on this page show differences between spool generations and filament products, not a universal tare. A suitable scale, discussed on the page about scales for filament spools, makes weighing easier.

Check the generation before reusing a tare

The Prusa Knowledge Base describes changes to its own spools when OpenPrintTag was added: the new spool is 3 mm slimmer, its paper core was replaced by a locking plastic core, and the spool is made from recycled polycarbonate. These design changes justify checking by generation; do not automatically transfer a tare measured on another version.

The same page states that old refills do not fit the new spools because they are wound on a paper core, and that Prusa recommends using the new refill only with new spools. For weighing purposes, this means a refill mounted on a reusable spool forms a combination whose tare depends on the carrier spool, not on the filament. Systems built around reusable spools and refills multiply these combinations.

The tool does not store tare values or display their provenance; it only receives the gross weight and tare you enter. Keep your own note with the brand, reference, generation and fittings included in the tare. A measurement from a similar spool remains an estimate until you verify it against the exact reference.

Limits of the weighing and estimate

The subtraction is applied to the two entered values, but the calculator does not check the scale or tare. It accepts no margin in grams or percent and shows no range. If you need to account for weighing precision, record that separately; do not mistake an external estimate for a tool output.

The scale reading covers everything weighed with the spool. A clip, label or residue left on the assembly can therefore contribute to gross weight; if those items are absent from the tare weighing, the difference remains in the result. The calculator cannot distinguish materials on the spool, so use the same configuration for both weighings.

The result is a calculated mass, not a direct measurement of filament alone or a guarantee that it will cover a job. Review the slicer’s estimate and the print’s starting conditions separately. If you need a safety margin, determine it outside this tool for your own project.

From grams to meters: that is a different tool

If your slicer reports consumption in meters, treat conversion as a separate step. The estimator on this page stops at grams on purpose. Turning a mass into a length requires the filament's density and its diameter, and both depend on the exact product. The Polymaker wiki gives a concrete example of that variability: the listed density is 1.17 g/cm³ for PolyLite PLA, 1.23 g/cm³ for PolyLite PLA Pro and 1.237 g/cm³ for Polymaker PLA Pro, three references that all carry the name PLA.

The same table lists 1.25 g/cm³ for PolyLite PETG and 1.3 g/cm³ for Polymaker PETG. A generic density by material family does not necessarily reproduce the published data for an exact product. These are manufacturer-declared product figures; they do not establish the real filament diameter for your calculation.

If you need a length, use the filament length and weight converter by material density, feeding it the remaining mass from this page and the density of your exact reference. Keeping the two tools separate lets you see at which step each uncertainty enters the calculation instead of stacking them without noticing.

Smart spools and automatic tracking: what changes and what does not

According to the Prusa Knowledge Base, OpenPrintTag is an open smart-spool standard that works offline by design. The tag is writable: material, color, key print settings and remaining filament are stored directly on it, with no dependence on a cloud service. The same page says a compatible printer or mobile app can update the recorded remaining value after a print, and that the label can be peeled off an empty spool and transferred to another spool under the manufacturer’s instructions.

Reading the tag, per Prusa, goes through the PrusaApp: open the menu, choose the NFC reader, tap scan and hold the phone near the tag. The article states that compatibility is guaranteed with supported devices, which is not the same as saying every printer reads the tag today. A spool from another brand without a tag, or a tag that was not updated after a job ran on a different machine, can be checked by weighing with a suitable tare.

The estimator on this page reads no tag. It stays useful precisely for spools that have none, for spools whose stored value is stale, and for physically checking a number a tag reports. Once you know how much filament a job really consumed, the print cost calculator uses that mass together with price and the other required inputs.

Sources and limits

• OpenPrintTag | Prusa
• Technical Data at a Glance | Polymaker

We organize manufacturer and community documentation; we have not measured these products ourselves. The manual for your exact model takes precedence over general guidance.