# 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)**

| Print | 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](/tools/filament-cost) does this maths as you type, and has these examples ready to open.