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What infill does
A typical FDM print has an outer shell and an interior. Infill is the pattern printed inside that shell; it is not the same as the part’s walls, also called perimeters. Infill can brace top layers that would otherwise span open space, add support against compression, and affect the part’s weight, material use, print time, and appearance. Prusa Research describes supporting top layers as infill’s main purpose in its infill documentation.
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Infill density is the proportion of the interior occupied by that pattern. A higher percentage usually uses more filament and takes longer to print, but it does not guarantee a proportional increase in strength. The result depends on the pattern, material, part geometry, print orientation, and the direction and type of force the part will face.
Is infill necessary?
No—not for every print. A model that closes gradually toward its top may be printable with no infill, because the geometry can support the closing layers. Prusa says most models can use 10–15% infill and notes that above 30% is rarely needed as general slicer guidance, not as an engineering rule. The page does not state a publication date. Prusa does not recommend treating hollow printing as a blanket choice.
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Whether a particular part needs infill depends especially on its top surfaces and intended use. Sparse infill leaves more open space beneath a top skin, which can make the first top lines sag. A part expected to resist compression may also benefit from internal structure. Conversely, a decorative object with sloped surfaces and little mechanical load may not need much.
How to choose a starting density
Begin with the slicer’s profile rather than assuming one percentage suits every model. Prusa’s 10–15% figure is a useful general starting point for many models, not a specification for a particular part. UltiMaker’s 2025 guidance gives different typical ranges for stiff parts, assuming two to three perimeter walls and a strength-oriented pattern:
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| Material | UltiMaker’s typical infill range for stiff parts |
|---|---|
| PLA | 20–30% |
| PETG | 25–40% |
| ABS | 30–50% |
| Nylon | 35–50% |
| Carbon-fiber-filled PLA or PETG | 15–25% |
These are UltiMaker’s general recommendations, not independently validated requirements for every part or printer. Its guidance says many uses may balance strength and efficiency around 20–40%, while emphasizing that material, pattern, and loading affect the result. Do not combine the figures into a universal rule: use them only as starting points alongside the part’s geometry and purpose.
- Slice the model with its normal profile. Check the preview for top surfaces and the amount of infill beneath them.
- Look for unsupported spans beneath the top skin. If the first top lines bridge a wide gap, consider more top solid layers or more support near the top, as well as adjusting infill.
- Match the structure to the job. Consider whether the part is mostly decorative, must resist compression, or may be loaded from several directions.
- For non-critical parts, compare settings on a small test print. For safety-critical or load-bearing parts, generic percentage guidance cannot replace part-specific engineering and testing.
Choose infill by pattern and purpose
Patterns differ in print speed, material use, support beneath top layers, appearance, flexibility, and how they respond to different load directions. The names and behavior described below come from Prusa’s slicer documentation unless noted; a pattern alone does not establish a part’s strength.
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| Pattern | What it is suited to | Trade-off or qualification |
|---|---|---|
| Gyroid | A three-dimensional structure that Prusa describes as supporting in all directions; a potential general-purpose choice when loads may come from several directions. | Prusa describes it as relatively quick and as having a good strength-to-weight ratio. Actual results depend on the whole part and print. |
| Rectilinear | A speed-conscious choice; its lines alternate direction between layers. | Prusa calls it one of its fastest patterns. PrusaSlicer switches to rectilinear at 100% infill. |
| Adaptive cubic | Varies density, leaving larger cavities in the center and denser support nearer walls and top and bottom regions. | Prusa’s documentation reports about one-quarter less material than rectilinear for the implementation it describes; this is a slicer-specific claim. |
| Support cubic | Places denser infill near top layers to support them while saving material. | Prusa describes its purpose as supporting top layers, not improving mechanical qualities. |
| Lightning | Useful for models and rapid prototypes where internal strength is less important. | UltiMaker describes it as saving time and material; it is not a suitable default for parts expected to carry meaningful loads. |
Use the preview to check what a chosen pattern actually places beneath top surfaces. A pattern described as supportive in multiple directions is not a substitute for choosing suitable walls, orientation, and settings for the forces the part will face.
When strength matters, consider walls and orientation
More infill is not always the best first adjustment for a weak part. Prusa says model strength is mostly defined by the number of perimeters and recommends increasing them for a stronger print. Infill can help with compression resistance and internal support, but it cannot reliably compensate for poor layer bonding, an unsuitable print orientation, or an under-designed shell.
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Consider the expected force direction when orienting a part, then assess perimeter count and infill together. The best combination is specific to the geometry and material; generic slicer guidance is not a qualification for a load-bearing design.
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Top solid layers print over the infill. With sparse infill, those layers span a larger distance and their first lines may sag. Prusa suggests at least three top layers as a countermeasure. Depending on the model and slicer preview, you can also assess adding more top solid layers or increasing infill locally near the top.
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