Filament tools: spool weight, length and density
Use a scale reading to estimate how much filament remains before starting another print. The filament calculator returns net mass in grams, remaining length in metres and material volume in cubic centimetres. Enter your spool's measurements and published density; the calculation runs in your browser without a signup.
Editable density, spool tare and filament diameter, with published product references.
Gather the four inputs
Gross weight is the scale reading with the filament still on its spool. Tare is the weight of that empty spool. Subtract tare once to obtain net filament mass. If you already know the net mass, enter it as the weight and set tare to zero. Do not use a full spool's advertised net weight as the weight of a partly used spool.
Diameter describes the filament strand, not the reel. Select the nominal 1.75 mm or 2.85 mm size that matches the spool. Density is the mass per cubic centimetre given in the exact product's technical data sheet. The material selector fills in a named product's value, and the separate density field lets you replace it for another grade. Read how density and tare affect spool length for the assumptions behind those inputs.
The formula, with units
Treat the strand as a solid cylinder. Divide net mass by density to obtain volume, then divide that volume by cross-sectional area to obtain length. Convert the diameter from millimetres to centimetres before calculating the area, and convert the resulting length from centimetres to metres at the end.
- Net mass (g) = gross weight (g) − tare weight (g).
- Volume (cm³) = net mass (g) ÷ density (g/cm³).
- Area (cm²) = π × (diameter in mm ÷ 20)².
- Length (m) = volume (cm³) ÷ area (cm²) ÷ 100.
Worked examples: 100 g, 200 g and a partial spool
Prusament PLA's technical data sheet lists a density of 1.24 g/cm³. Using that figure and 1.75 mm diameter, each gram represents about 0.335 m. Therefore 100 g of net filament is about 33.5 m, 200 g is about 67.1 m, and 1 kg is about 335.3 m. These are calculated estimates from that product's density.
For a separate example, suppose the scale reads 850 g and the empty spool weighs 200 g. The remaining filament is 650 g. At the same density and diameter, that gives about 524 cm³ and 218 m, matching the calculator's rounded default output. The 200 g tare is an example input; replace it with the figure for your own spool. At 2.85 mm diameter, the same 650 g gives about 82 m because the thicker strand uses more volume per metre.
Use the estimate for the right question
Compare the result with your slicer's material estimate in the same units, including supports, brims and purge material where applicable. The calculator estimates material quantity; it does not model the print or guarantee that a job will finish. An uncertain tare or a density copied from another grade carries through to the result.
Reel outer diameter, width and hub dimensions are separate measurements for checking a holder or dry-box fit. They cannot be inferred from net filament weight. For storage and moisture recovery, use the filament drying chart; for a first purchase, read the first-spool guide. Keep the product's density and empty-spool weight with your material notes so the next calculation starts with the same documented inputs.