327.36 m of 1.75 mm PETG, at a density of 1.27 g/cm³. In 2.85 mm the same kilogram gives 123.43 m.
What makes PETG worth a separate answer from PLA is not the number. It is that the number can be trusted.
A narrow density band means a dependable figure
PETG runs from 1.23 to 1.29 g/cm³ across mainstream spools. That puts the honest range at 322.29 to 338.01 m — a spread of under five percent, end to end.
PLA's band is wider, and matte and filled PLA grades run wider still. So a PETG spool is one of the few where the published length is close to what you will actually get, whatever brand is on the box.
The thing that does move the number
Moisture, and it moves the mass rather than the length. PETG is hygroscopic: a spool left in a humid room absorbs water, and that water is on the scale when you weigh it.
Weigh an open PETG spool that has been sitting for a month and you may find it heavier than the day it arrived, while containing the same filament. That is why the weigh-and-subtract method for estimating what is left is less reliable on PETG than on PLA, and why a dried spool is the one to calibrate against.
Working backwards from a print
If your slicer says a part needs 3.05 g of PETG, it needs one metre of 1.75 mm stock. That ratio — grams per metre — is the one worth remembering, because it converts between the two things you can actually measure: what the slicer reports and what you can see left on the reel.
At 2.85 mm the ratio is 8.1 g per metre, because the cross-sectional area is 6.3794 mm² against 2.405 mm².
That conversion is also the sanity check on your slicer. If its filament profile still holds PLA's density while a PETG spool is loaded, every mass it reports is out by the ratio between the two — small, consistent, and enough to leave you short at the end of a long job.
If your spool states a density, use it rather than the 1.27 assumed here — the length and weight converter takes whatever you give it. The stored band for the polymer is on the PETG page.