Filament Length Calculator
Converts filament weight to usable length in meters by accounting for material density and filament diameter. Essential for knowing how much filament remains on a partial spool.
About this calculator
Filament is sold by weight, but slicers often report usage in meters, making it necessary to convert between the two. The formula derives length from the physical volume of a cylinder: Length (mm) = (filamentWeight − spoolWeight) / (materialDensity × π × (filamentDiameter / 2 / 10)²) × 1000. First, net filament weight is found by subtracting the empty spool weight. That net weight in grams is divided by the density of the material (g/cm³) to get volume in cm³. The cross-sectional area of the filament — a circle with radius equal to half the diameter converted from mm to cm — is then divided into that volume to yield length in centimeters, which is multiplied by 10 to convert to millimeters and then divided by 1,000 for meters. Common density values: PLA ≈ 1.24 g/cm³, PETG ≈ 1.27 g/cm³, ABS ≈ 1.04 g/cm³.
How to use
A partial spool weighs 340 g total; the empty spool weighs 190 g, so net filament = 150 g. Filament is 1.75 mm diameter PLA (density 1.24 g/cm³). Radius in cm = (1.75 / 2) / 10 = 0.0875 cm. Cross-section area = π × 0.0875² = 0.02405 cm². Volume = 150 / 1.24 = 120.97 cm³. Length in cm = 120.97 / 0.02405 = 5,030 cm. Convert to meters: 5,030 / 100 ≈ 50.3 m of filament remaining. Enter these values and the calculator returns the result instantly.
Frequently asked questions
What is the density of common 3D printing filaments for the filament length calculator?
PLA has a density of approximately 1.24 g/cm³, making it one of the most predictable materials to calculate. PETG sits at around 1.27 g/cm³, ABS at 1.04 g/cm³, and TPU varies between 1.20–1.25 g/cm³ depending on shore hardness. Nylon typically ranges from 1.08–1.14 g/cm³. Using an incorrect density value is the most common source of error in filament length calculations, so always check your manufacturer's datasheet for the exact figure.
How do I find the empty spool weight for my filament brand?
The most accurate method is to weigh your spool after it is completely empty using a kitchen or postal scale. Many filament manufacturers also publish empty spool weights on their websites or packaging — common values range from 100 g to 250 g depending on whether the spool is plastic or cardboard. If you are unsure, 200 g is a reasonable default for standard plastic spools, but even a 30 g error can translate to several meters of miscalculation on a nearly empty spool.
Why does 1.75 mm and 2.85 mm filament give different lengths for the same weight?
Thicker filament has a larger cross-sectional area, so the same weight of material produces a shorter length of strand. A 2.85 mm filament has roughly 2.65 times the cross-sectional area of 1.75 mm filament, meaning 100 g of 2.85 mm PLA gives you about 38% as much length as 100 g of 1.75 mm PLA. This is purely a geometric consequence of the cylinder volume formula and is why most modern printers have standardized on 1.75 mm — you get more printable meters per kilogram.