Filament usage
How to calculate filament usage before you slice
Volume times density answers the wrong question: it gives you the weight of a solid block. A printed part is walls, solid skins and a sparse interior. Here is the chain that gets from a model to grams — with a calculator that shows every step, so you can check it against your slicer.
The chain, in order
- Bounding volume. Length × width × height of the model, in mm³.
- Walls. Subtract the hollow interior: every wall is 0.4 mm of extrusion width, so three walls are 1.2 mm a side.
- Solid skins. 5 top and 5 bottom layers at 0.2 mm is 1.0 mm of solid plastic at each face. Thin parts are almost entirely skin, which is why small brackets weigh more than the infill percentage suggests.
- Infill. Whatever is left of the interior, at your infill percentage.
- Add-ons. Brim or skirt, supports, and a purge line. The brim is about a gram; supports can be a fifth of the part.
- Failure buffer. Divide by 1 − f. At a 10% failure rate you buy 1.111 g for every gram you sell.
Worked example: a 120 × 80 × 60 mm enclosure
| Line | Volume | Notes |
|---|---|---|
| Bounding volume | 576.0 cm³ | 120 × 80 × 60 |
| Walls, 3 × 0.4 mm | 50.4 cm³ | outer minus hollow |
| Hollow interior | 525.6 cm³ | −525.6 cm³ |
| Solid top and bottom skins | 18.3 cm³ | 1.0 mm each face |
| Infill at 20% | 101.5 cm³ | interior minus skins |
| Extruded plastic | 170.1 cm³ | walls + skins + infill |
| PLA mass | 210.9 g | × 1.24 g/cm³ |
| Buy, with brim and 10% failures | 235.5 g | 211.9 g ÷ 0.90 |
| Cost at mid PLA | $3.53 | $14.99/kg |
The naive answer — 576.0 cm³ × 1.24 = 714 g — is 3.4 times the real figure. That is the difference between a quote that wins the job and one that loses money on it.
Put your own dimensions in
The calculator runs the same geometry. Change the size, the wall count or the infill and watch the grams move; the PLA and PETG figures use the dataset densities and mid prices. With JavaScript off, the worked example above is the answer for the default inputs.
The two allowances people forget
Supports are printed at the same density as infill, so a support-heavy part can use half again the plastic of the same part reoriented and printed support-free. Rotate first, then estimate. The failure buffer is not pessimism: a 20% failure rate means you buy 1.25 g per gram sold, and if you leave it out your quoted price is 25% under cost on top of the margin you intended.
Cross-check against your slicer
The slicer already does this arithmetic with the real toolpath. If your number and the slicer's disagree by more than 10%, the cause is almost always one of three things: your wall count is not what you typed, your infill pattern is not a straight percentage of volume, or the model has hidden internal geometry. For grams already on a part-used spool, use the spool calculator; for length rather than weight, the length-to-weight page; for price per kilogram by material, the filament cost calculator.
Once you know the grams, the per-gram cost page turns them into money, and print time turns the volume into machine hours. If the print fails anyway, the cause is usually on the eight-symptom checklist — and the material choice that affects both weight and reliability is covered on best filament for beginners.
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Frequently asked questions
How do I calculate how much filament a print needs?
Multiply the extruded volume by the material density. The trick is the extruded volume: take the bounding volume, subtract the hollow interior, add back the walls and solid skins, then apply infill to what is left. A 120 x 80 x 60 mm box comes to 170.1 cm3 and 211 g of PLA, not the 714 g a solid-block estimate gives.
How much extra filament should I allow for failures?
Divide by (1 minus your failure rate). At 10% that is 1.111x, at 20% it is 1.25x. Buy that much more than the part weighs, and keep one spare spool of your main material rather than a fraction of a spool.
Do supports and brim count as filament usage?
Yes, and they are easy to forget. A brim is about a gram, but supports can be 10–20% of a part's plastic on an overhang-heavy model. In the calculator they are separate fields for exactly this reason.