1.75 mm vs 2.85 mm: how much length is on a kilogram?

For PLA at 1.24 g/cm³: 335.28 m in 1.75 mm, 126.42 m in 2.85 mm.

The thicker stock gives 2.65 times less length for the same mass — not 1.63 times, which is what the diameter ratio alone would suggest.

The squaring is the whole point

Cross-sectional area is π times the radius squared, so the area ratio is the diameter ratio squared: 6.3794 mm² against 2.405 mm². Every consequence below follows from that one fact.

What it changes in practice

Extruder gearing. A given volumetric flow needs 2.65 times less linear feed at 2.85 mm, so the drive gear turns more slowly for the same output. That was the original argument for the larger diameter, and it mattered most on long Bowden paths where feeding thin filament reliably is hard.

Buckling. Thicker filament resists buckling between the drive gear and the hotend, which is why 2.85 mm survived longest on machines with a long path between the two.

Reading the reel. A 2.85 mm spool visibly empties faster per printed gram. Judging what is left by eye is a habit that does not transfer between the two diameters.

Which one you should buy

Whichever your printer takes — the diameters are not interchangeable, and 1.75 mm now dominates the market by a wide margin, with the widest material and colour range attached to it.

If you have a genuine choice, that market share is the whole argument. The mechanical case for 2.85 mm was largely solved by direct-drive extruders becoming standard.

One place the difference bites

Retraction distances do not carry across. A retraction tuned in millimetres of filament moves 2.65 times more plastic at 2.85 mm than the same figure does at 1.75 mm, so a settings profile copied from a forum post for the wrong diameter will either string badly or grind the filament flat. Volumetric settings transfer; linear ones do not.

Convert either diameter at any density in the length and weight converter.