2.98 g, for PLA at 1.24 g/cm³.
The derivation is short enough to do in your head: 1.75 mm gives a cross-sectional area of 2.405 mm², a metre of that is 2,405 cubic millimetres, and at 1.24 g/cm³ that is just under three grams.
Diameter tolerance moves this more than you would expect
A spool sold as 1.75 mm is manufactured to a tolerance, usually printed on the box as two or three hundredths of a millimetre either way. Mass per metre follows the square of diameter, so that tolerance is amplified rather than passed through.
Run the same PLA at 1.79 mm and a kilogram gives 320.47 m instead of 335.28 m — 4.6% less filament for the same money, from a diameter difference you cannot see.
This cuts both ways. A consistently oversized spool also over-extrudes: the printer commands a length of filament and gets more plastic than the slicer's flow model expected, which shows up as bulging walls and elephant's foot long before it shows up on the invoice.
Which number your slicer is using
Your filament profile carries a diameter and a density, and it uses both to convert extruded volume into the gram figure it reports. If either is wrong, so is every mass estimate you have ever taken from it.
The diameter field matters least — most spools are close. The density field matters a lot, because it is routinely left at PLA's 1.24 while something else entirely is loaded. A matte PLA at 1.35 makes every estimate 8.1% wrong in one direction.
Calipers and a jeweller's scale beat any table
Take a caliper reading at three points a hand-span apart and average them. Then cut a known length — a metre is easiest — and weigh it on a jeweller's scale.
The measured grams per metre is worth more than any published figure, because it silently includes both the diameter your spool actually is and the density of the exact blend in front of you.
A caliper reading of 1.73 rather than 1.77 changes grams per metre by more than most people expect, because cross-section goes as the square of the diameter. That is why the length and weight converter asks for a diameter rather than assuming one.