The shell (also called the perimeter or wall) of a 3D print is the set of solid outer lines the slicer lays down around each layer’s perimeter. It is the dominant factor in a part’s structural strength, surface quality, and bridging ability. This calculator converts a perimeter count and line width into the actual shell thickness in millimetres, flags shells below the practical structural minimum, and suggests how many perimeters reach it.
How it works
Each perimeter is one extruded line of plastic, and the lines stack side by side without gaps:
shell thickness = number of perimeters × line width
The line width defaults to the nozzle diameter. A 0.4 mm nozzle produces a line roughly 0.4 mm wide at default settings. Most slicers allow you to override this to between 100% and 120% of the nozzle diameter — a wider line lays down more plastic per pass, improving layer adhesion at the cost of slightly reduced fine-detail resolution. The calculator accepts an explicit line width so you can model the override accurately.
The structural minimum used to flag thin shells is approximately 0.8 mm — roughly two lines on a standard 0.4 mm setup. Below this, parts are fragile under any side load.
Worked examples
| Perimeters | Line width | Shell thickness | Structural? |
|---|---|---|---|
| 2 | 0.4 mm | 0.8 mm | Borderline |
| 3 | 0.4 mm | 1.2 mm | Yes |
| 4 | 0.4 mm | 1.6 mm | Yes, strong |
| 2 | 0.45 mm | 0.9 mm | Yes |
| 3 | 0.6 mm | 1.8 mm | Yes, strong |
For a standard 0.4 mm nozzle, 3 perimeters give a 1.2 mm wall — robust for most functional parts. For a 0.6 mm nozzle at 120% (0.72 mm line width), 3 perimeters give 2.16 mm — a very thick, strong wall in fewer passes.
Shells vs. infill for strength
Adding perimeters is almost always more weight-efficient than raising infill density when the goal is strength. Bending and impact loads concentrate at the outer surface — the shell — so thicker walls directly reinforce where stress is highest. A part with 4 perimeters and 15% infill is typically stronger and lighter than the same part at 2 perimeters and 50% infill, and it prints faster too.
Practical guidance
- Decorative objects: 2 perimeters is usually enough.
- Functional or structural parts: 3–4 perimeters; more for anything that takes repeated mechanical load.
- Thin walls or snap-fits: if the feature is only 1–2 line widths wide, the slicer may not fill the gap with infill at all — thickening the design slightly or increasing perimeters to overlap is more reliable than adding infill.
- Nozzle diameter vs. slicer line width: always check the slicer’s actual line width setting under wall/perimeter settings; it may differ from the nozzle diameter by 10–20%, which accumulates across multiple perimeters.
All calculations run locally in your browser; nothing is uploaded.