Slicers report filament use in two ways — length and weight — and converting between them needs the density of the specific material. Almost every calculator online uses 1.24 g/cm³ for everything, which is the figure for PLA, and is wrong by up to 30 percent for anything else.
The arithmetic
Filament is a cylinder. Its cross-sectional area is pi times the radius squared — 2.405 square millimetres for 1.75 mm filament, 6.379 for 2.85 mm.
A metre of 1.75 mm filament is therefore 2,405 cubic millimetres, or 2.405 cubic centimetres. Multiply by the material density and you have grams per metre: 2.98 g/m for PLA, 3.05 for PETG, 2.50 for ABS.
Divide 1,000 by that and you get the length on a one kilogram spool. PLA comes out at about 335 metres, PETG at 328, ABS at 400.
Why the density spread is so wide
The common filaments are not similar polymers. ABS is 1.04 g/cm³, PLA is 1.24, PETG is 1.27, and carbon-fibre PETG reaches 1.35. Polypropylene is 0.90 and actually floats.
That is a 50 percent spread from lightest to heaviest. A kilogram of PP is nearly 460 metres; a kilogram of CF-PETG is about 308.
Filled filaments are the ones to watch. Carbon fibre, metal powder and glass all raise density, and the fill fraction varies between brands — so a filled filament's density is a nominal figure rather than a precise one.
Spool weight is not filament weight
A one kilogram spool means one kilogram of plastic. The reel it is wound on adds 150 to 250 grams for a plastic spool, and some cardboard spools come in under 100.
So weighing a spool on a kitchen scale and subtracting 1,000 grams does not tell you how much is left. You need the empty spool weight, which the better brands print on the side.
If your brand does not, weigh the reel the next time you finish one and write it on the box. That one number makes every future estimate on that brand reliable.
Diameter tolerance shifts the answer
Filament is sold as 1.75 mm with a tolerance, commonly plus or minus 0.02 or 0.03 mm. Because area goes with the square of the diameter, a filament running at 1.78 has about 3.5 percent more material per metre than one running at 1.75.
That is enough to matter across a whole spool, and it is also why flow calibration is worth doing per brand rather than once per printer.
Measure your filament with calipers in a few places. If it is consistently off nominal, enter the measured figure rather than the label.
What to do with the number
The main use is deciding whether a print fits on what you have. A slicer estimate of 340 metres and a spool you believe has 300 left is a print that will fail overnight.
The second use is cost. Divide what you paid by the grams on the spool and you have cost per gram, which multiplied by the slicer's gram figure gives the real cost of a print — usually far less than people guess.
The third is shipping. Filament is dense and heavy, and knowing the weight of a box of spools before ordering avoids a surprise at checkout.