"Will this spool be enough?" is one of the most common questions in 3D printing. The answer comes from four numbers: the model's volume, its surface area, the infill percentage and the material's density. Your slicer computes the exact figure, but you can get within 10–20% before slicing — which is what our calculator does when you drop in an STL.
Why volume is not weight
A 20 mm calibration cube has a volume of 8 cm³. Printed solid in PLA (1.24 g/cm³) it would weigh 9.9 g. Printed with typical settings — three walls, a few top and bottom layers and 15% infill — it weighs about 4.5 g, less than half.
That is because slicers do not fill the inside of a part. They print a solid shell and a sparse internal lattice called infill. So the weight of a print is:
grams = [shell volume + (solid volume − shell volume) × infill] × density
with the shell volume approximated as surface area × wall thickness. Wall thickness is the number of perimeters times the line width: three perimeters of 0.4 mm make 1.2 mm.
Shells matter more than you think
Small and thin parts are mostly shell. The planter in the calculator has 3 mm walls everywhere, so with 1.2 mm perimeters on each side there is very little room for infill: it weighs 48.5 g at 0% infill and only 59.9 g even at 100%. The 50 mm sphere is the opposite — a large solid lump — and its weight runs from 11.7 g at 0% to 81.1 g at 100%.
| PLA, 1.2 mm walls | 0% | 15% | 20% | 50% | 100% |
|---|---|---|---|---|---|
| 20 mm cube (8 cm³) | 3.6 g | 4.5 g | 4.8 g | 6.7 g | 9.9 g |
| 50 mm sphere (65.4 cm³) | 11.7 g | 22.1 g | 25.6 g | 46.4 g | 81.1 g |
| 70 mm planter (48.3 cm³) | 48.5 g | 50.2 g | 50.8 g | 54.2 g | 59.9 g |
So if you want a lighter, cheaper part, the right lever depends on the shape. For chunky parts lower the infill; for thin or detailed parts reduce the wall count or line width; for decorative items consider vase mode, which prints a single wall and no infill at all.
Density: same part, different weight
Materials have different densities, so the same STL weighs different amounts in each. PLA is about 1.24 g/cm³, PETG 1.27, TPU 1.21, PC 1.20, nylon 1.14, ASA 1.07 and ABS just 1.04. An ABS part weighs roughly 16% less than the same part in PLA — and since filament is sold by weight, a 1 kg spool of ABS prints more parts.
Each material page lists its density, typical price and meters per kilogram: PLA, PETG, ABS and TPU, for example.
Grams to meters
Some printers and spool trackers count filament in meters. To convert, divide the volume of plastic by the cross-section of the filament:
meters = grams ÷ density ÷ (π × (diameter ÷ 2)²)
For 1.75 mm filament the cross-section is 2.405 mm², so one kilogram of PLA is about 335 m and one gram about 33.5 cm. For 2.85 mm filament the same kilogram is only about 126 m. Our filament length calculator converts both ways for any material, and there are spool-length tables for PLA, PETG, ABS, TPU, ASA and nylon.
What the model doesn't include
- Supports. Overhangs need temporary scaffolding. Tree or organic supports typically add 5–15% to the weight; dense supports under large flat overhangs can add much more.
- Brims, skirts and rafts. Usually a gram or two, but a raft under a large part can be 10 g or more.
- Purge and wipe towers. On multi-material printers every color change purges filament. A small multicolor print can waste more plastic than it uses.
- Top and bottom layers. The shell estimate includes them through the surface area, which works well for most parts. Very flat parts with large top surfaces use slightly more than estimated.
The calculator's "supports & waste" percentage covers all of these. Five percent suits a simple part; use 15–30% for heavily supported or multicolor prints.
Will my spool be enough?
Spools are rarely exactly 1,000 g of filament — some have a little extra, and you may not know how much is left on a used one. Weigh the spool on a kitchen scale and subtract the empty spool weight, usually printed on the label or listed by the manufacturer (often 150–250 g for plastic spools). If the remaining filament is less than about 110% of the estimated print weight, load a fresh spool or use your printer's automatic spool switching.
Five ways to use less filament
- Lower infill on parts that are not structural; 10–15% is plenty for most decorative prints.
- Use gyroid or lightning infill for parts that only need to support their top surfaces.
- Reduce walls from four to two or three on decorative parts.
- Orient the part to need fewer supports, and switch to tree supports.
- Scale down: weight falls with the cube of the scale, so a model printed at 80% size uses only about half the plastic.