What a 3D print really costs in filament
Ask ten people what a print costs and you'll get ten guesses. The filament part has an exact answer, and it takes three numbers.
1. Cost per gram
Divide the spool price by its weight in grams. A $25 spool of 1 kg is 2.5 cents a gram. A $20 spool of 500 g is 4 cents a gram, so a "cheaper" small spool can cost more per print.
2. The part weight, from your slicer
Don't use the model's solid weight. Slice it and read the grams in the print summary: that already counts walls, infill, top and bottom layers and supports. The same 80 mm box can weigh 18 g or 60 g depending on those settings.
3. Waste
Every print loses a little: the purge line, a skirt, the odd failed first layer. On a one-colour printer 5% is a fair allowance. Multi-colour prints are different: each colour change purges filament, and 20 to 40% is normal. Your slicer reports the purge weight; use it.
Putting it together
cost per part = cost per gram × part weight × (1 + waste)
| Spool | Part | Waste | Filament cost | |
|---|---|---|---|---|
| Everyday PLA part | $25 / 1 kg | 50 g | 5% | $1.31 |
| 20 keychains | $22 / 1 kg | 8 g each | 8% | $0.19 each, $3.80 total |
| Large PETG part | $30 / 1 kg | 450 g | 5% | $14.17 |
| Multi-colour part | $28 / 1 kg | 60 g | 35% | $2.27 |
What this number isn't
It's filament only. When you price a job, add machine time, electricity, wear parts, packaging and your own time on top. Filament is often the smallest part of the price.
Filament Cost does this maths as you type, and has these examples ready to open.