How filament length and weight relate
Filament is a long cylinder of plastic, so its weight per meter depends on two things: how thick it is and how dense the material is. Slicers often report length in meters, spools are sold by weight, and your kitchen scale reads grams. This calculator moves between those units so you can check whether a print fits on a spool or compare a slicer estimate with what you have.
The formula is the volume of one meter of filament multiplied by density:
grams per meter = π × (d / 2)² × 100 × density
Here d is the filament diameter in centimeters (1.75 mm is 0.175 cm), 100 is the length of one meter in centimeters, and density is in grams per cubic centimeter. Grams are meters × grams per meter, and meters are grams ÷ grams per meter.
For 1.75 mm filament, one meter has a volume of about 2.405 cm³. At PLA's density of 1.24 g/cm³ that gives roughly 2.98 g per meter, or about 335 m per kilogram.
Densities used
| Material | Density (g/cm³) | 1.75 mm g/m | 1.75 mm m/kg |
|---|---|---|---|
| PLA | 1.24 | 2.98 | 335 |
| PETG | 1.27 | 3.05 | 327 |
| ABS | 1.04 | 2.50 | 400 |
| ASA | 1.07 | 2.57 | 389 |
| TPU | 1.21 | 2.91 | 344 |
| Nylon | 1.14 | 2.74 | 365 |
| PC | 1.20 | 2.89 | 346 |
| PLA-CF | 1.29 | 3.10 | 322 |
These are typical values. Brands vary, and filled materials such as wood, glow, silk or metal-filled PLA can differ noticeably. If the manufacturer lists a density on the spool or data sheet, use that.
Measuring the inputs
Diameter
Nominal diameter is 1.75 mm or 2.85 mm (often sold as 3 mm). Good filament holds within about ±0.02 to ±0.03 mm. To check, measure with calipers at several points along a meter and at different angles, since filament can be slightly oval. Because weight scales with the square of diameter, a spool at 1.78 mm carries about 3.5% more plastic per meter than one at 1.75 mm.
Length or weight
Take length from your slicer's filament estimate. For weight, use a scale that reads to 1 g and remember to subtract the spool, which is a separate calculation.
Worked example
Your slicer says a print needs 42 m of 1.75 mm PLA. At 2.98 g per meter that is 42 × 2.98 = 125 g. Going the other way, if you have 300 g of PETG left, 300 ÷ 3.05 = 98 m.
For 2.85 mm PLA the cross-section is much larger: π × 0.1425² × 100 ≈ 6.38 cm³ per meter, so about 7.91 g per meter and only about 126 m per kilogram. Mixing up diameters gives errors of more than 2.5 times, so double-check which one your printer uses.
Common mistakes
- Using the wrong density. ABS is about 16% lighter than PLA, so a kilogram of ABS is noticeably longer. Assuming PLA for everything misleads you on lighter materials.
- Including the spool weight. Weighing a full spool and converting gives a length that includes cardboard or plastic.
- Trusting nominal diameter blindly. Off-spec filament changes grams per meter. It also affects flow, so it is worth measuring a new brand.
- Comparing slicer settings that disagree. If your slicer's filament density or diameter setting is wrong, its gram estimate will be off. Set them to match the spool.
Practical tips
- Set accurate density and diameter in your slicer's filament profile. Its weight and cost estimates then line up with this calculator.
- When a print is close to the end of a spool, convert the remaining weight to meters and compare with the slicer's length, leaving a margin of at least 5%.
- For filled filaments, look up the maker's density. Carbon fiber, wood and metal fills vary a lot between brands.
- Keep a note of the density you used for each brand so your numbers stay consistent between jobs.
One last point on accuracy: the figures here are good to a few percent, which is enough for planning and costing. If you need better than that, for example when splitting a spool exactly between jobs, weigh a measured length of your actual filament. Cut one meter, weigh it on a scale that reads to 0.1 g, and use that grams-per-meter value directly.