Weight of that length
Length of that weight
Length in a 1 kg spool
Weight per metre
The formula
The cross-section of the filament is π × (diameter ÷ 2)². In square millimetres that number is also the volume of one metre in cubic centimetres, so weight per metre in grams = cross-section × density. For 1.75 mm PLA that is about 2.4 × 1.24 ≈ 3 g per metre, or roughly 335 m per kilogram of filament.
Limits to know about
Densities are typical values. Filled filaments (carbon fibre, wood, metal) and different brands can differ, so check the datasheet for exact numbers. The 1 kg figure counts filament only, not the weight of the empty spool, and real diameter varies slightly along the roll.