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Published estimates range from 63 to 111 PET bottles for 1 kg of finished filament, but neither number is universal. The count depends on bottle weight, how much usable filament each bottle produces, and the conversion method. One 2025 study’s measured per-bottle yield implies about 106 bottles per kilogram, while that same paper separately estimates 63—an unresolved discrepancy worth keeping in view.
Why published bottle counts differ
A bottle count only makes sense when the material being counted is clear. One kilogram of empty-bottle feedstock is not the same as one kilogram of finished filament: plastic is removed or lost during preparation and processing, and the reported yields vary by study.
Bottle weight also varies. A 2024 study used bottles averaging 14.55 g, while a separate 2025 study used 1.5 L bottles averaging about 35 g. Comparing their counts without accounting for that difference would be misleading. The studies also use different processes—among them pultrusion, bottle-strip conversion and shredded-flake extrusion—and are not controlled head-to-head trials.
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| Source and method | Reported bottle or batch figure | What the figure means |
|---|---|---|
| Journal Online of Physics, pultrusion study, published approximately five months before October 2026 | 63 bottles; 23 minutes per 1.5 L bottle on average | The study’s estimate for 1 kg of filament. Its abstract snippet also gives operating conditions of 205 °C and 45 RPM. |
| Jurnal Otomasi Kontrol dan Instrumentasi, 2025 | About 35 g per empty 1.5 L bottle; 9.45 g and 7 m of filament per bottle; about 3.7 kg of bottles per 1 kg of filament | The paper also says 28 bottles amount to about 1 kg of plastic input, and estimates about 15 minutes per bottle or 7 hours for 28. Its separate 63-bottle estimate does not agree with the per-bottle yield calculation. |
| Semesta Teknika, 2024 | 14.55 g per bottle; 9.07 g of filament per bottle; 111 bottles per 1 kg of filament | The paper reports a 38% weight reduction and 69 bottles in 1 kg of bottle waste. It reports 1.7 mm filament and notes that nozzle design and filament quality need improvement. |
| Discover Mechanical Engineering, 2025, shredded-flake workflow | 12 recycled 1.5 L bottles → 200 g of PET flakes → about 30 m of usable filament | A batch result for that paper’s equipment and workflow, not a universal count per kilogram. |
The 63-bottle pultrusion figure is a study-specific estimate, not a standard. The 111-bottle result is also specific to its study’s bottle mass and yield. The 2025 study’s reported 9.45 g filament per bottle offers a direct way to calculate a count, but conflicts with that paper’s separate 63-bottle estimate.
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Calculate a count from usable filament yield
When a source gives usable filament mass per bottle, the calculation is:
Estimated bottles per kilogram = 1,000 g ÷ usable filament grams per bottle
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Using the 2025 study’s reported 9.45 g per bottle: 1,000 ÷ 9.45 = approximately 106 bottles. This is the arithmetic result of the per-bottle yield, not a reconciliation of the same paper’s separate 63-bottle estimate. The two claims should not be combined as though they were consistent.
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Do not instead divide 1,000 g by the bottle’s mass and call the result a finished-filament count. For example, the 2025 paper’s roughly 35 g bottle is a feedstock figure; the paper reports only 9.45 g of filament per bottle. The mass denominators answer different questions.
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What changes the result in practice
- Bottle mass and size: A count based on lighter bottles will be higher than one based on heavier bottles if usable yield is otherwise comparable. Size alone is not a reliable substitute for weighing bottles.
- Usable yield: Rejects, trimming and processing losses can reduce finished filament relative to bottle input. Use a measured output yield from the same process where possible.
- Conversion route: Strip-fed systems and shredded-flake extrusion have different equipment and preparation requirements. Published figures from different routes do not establish which is more efficient.
- Filament quality and diameter: A mass total does not prove that the output has consistent diameter or is suitable for a particular printer. The 2024 study reports 1.7 mm filament and identifies quality and nozzle design as improvement needs.
How bottle-to-filament workflows differ
Cut bottle strips
A strip-fed machine cuts a clean bottle into a continuous strip and pulls it through a heated nozzle. A university laboratory article describes 10 mm strips and a 1.75 mm nozzle. This is a specific machine design, not a general recipe or a guarantee of printer-ready filament.
Sustainable Design Studio’s Filament Maker V2 product page describes a bottle-strip workflow and advertises 1.75 mm output, bottles of 1 L or larger, up to 92 g per hour, and 48 W power draw. These are manufacturer specifications, not independent test results; check the product page for current details.
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Shred bottles into flakes
The 2025 Springer paper documents a more involved lab workflow: remove labels, caps and rings; wash and air-dry the bottles; shred and sieve them; then dry the flakes at 140 °C for five hours before extrusion. The paper explains that residual moisture can cause hydrolytic degradation. Treat those temperatures and times as that study’s protocol, not a universal home-processing instruction.
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For this route, the useful comparison is not merely how many bottles go in. Check the bottle mass, usable output mass, preparation losses, drying requirements, filament diameter and dimensional control, and processing time. The cited studies do not provide a controlled comparison across all these factors.
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What to check before estimating a project
- Weigh a representative sample of clean, empty bottles rather than assuming a standard mass.
- Find a yield stated as usable filament mass per bottle or per known mass of bottle input; keep feedstock and output denominators separate.
- Confirm whether the machine accepts strips or flakes and what preprocessing it requires.
- Check the reported filament diameter and whether the process controls it consistently enough for your printer.
- Account for the process time and any rejected output. Published processing times are tied to individual machines and studies.
Recycled PET filament should not be assumed to behave like commercial PETG. The university laboratory article says printer configurations should match PET, while the 2024 study flags quality limitations. A filament’s material and diameter need to suit the printer and its settings.
Costs and scope of the available figures
An Indonesian Journal of Laboratory article gives a source-specific comparison of Rp 200,000–275,000 per kilogram for commercial filament and an estimated production cost of about Rp 60,000 for recycled PET filament. These are figures reported in an Indonesian context, not current global prices; the publication date is not resolved in the available source details. They do not establish what a home setup will cost.
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