About 335 metres, for 1.75 mm PLA at a density of 1.24 g/cm³.
For 2.85 mm filament the same kilogram is about 126 metres, because the cross-sectional area is roughly 2.65 times larger.
The inputs behind those numbers
A 1.75 mm filament has a cross-sectional area of 2.405 mm². One kilogram of PLA at 1.24 g/cm³ occupies 806 cm³, which is 806,000 mm³. Dividing volume by area gives 335,300 mm, or 335 m.
Density is the assumption that moves the answer. Real PLA spools run between about 1.17 and 1.26 g/cm³ depending on pigment load, which puts the honest range at roughly 330 to 355 metres. A matte PLA with a mineral filler can reach 1.35 g/cm³, which brings the same kilogram down to about 308 m — 8.1% less filament, and the usual reason a spool runs out earlier than expected.
Why this is not the same for every filament
Length per kilogram is inversely proportional to density, so the ranking runs opposite to the density table. Polypropylene at 0.90 g/cm³ gives around 460 m per kilogram. PETG at 1.27 gives around 327 m. Metal-filled PLA at 3.2 gives around 130 m — less than half the plastic length for the same money and the same spool.
If you need the figure for a material other than PLA, the length and weight converter will do it for any density and diameter, and the materials reference lists the density range for each one.
What a spool weighs is a different question
None of the figures above describe what a full spool reads on a scale, because filament is sold by the mass of the plastic and the reel it is wound on is uncounted extra. That has one practical consequence here: you cannot verify the 335 m answer by putting a spool on the kitchen scales unless you already know what the empty reel weighs, and reel weights differ enough between manufacturers to swallow the whole difference between a matte grade and a plain one.
The arithmetic for a part-used spool, and what happens when the reel assumption is wrong, is set out at how much filament is left on my spool; brand-by-brand reel weights are in the spool weights guide.
Quick reference
- 1.75 mm PLA, 1 kg: about 335 m
- 2.85 mm PLA, 1 kg: about 126 m
- 1.75 mm PLA, 500 g sample spool: about 168 m
- 1.75 mm PLA, 250 g sample spool: about 84 m