What is Infill? A Visual Guide

What's Inside Your Print?

When you look at a completed 3D print, it looks solid. Tap it and it sounds solid. But unless you've printed at 100% infill density, the interior is almost entirely air — supported by a geometric lattice of plastic that gives structure without using unnecessary material. This interior structure is called infill, and it's one of the most impactful settings in your slicer. Understanding it allows you to produce parts that are lighter, faster to print, and often just as strong as solid alternatives.

The concept makes intuitive sense when you think about how engineering structures work. A hollow box with internal webbing (think of corrugated cardboard or aircraft wing ribs) can be enormously strong whilst using a fraction of the material of a solid block. 3D printing infill applies the same principle: the outer walls provide the visible, structural shell, whilst the infill pattern provides the internal bracing that prevents the shell from collapsing and supports the top layers.

Infill Percentage: How Dense Is Dense Enough?

Infill percentage is how much of the interior cross-section is plastic (vs air). At 0% infill, the interior is completely hollow except for the structural walls — often used for vases and display items. At 15–20% infill, you have a light lattice that works for most decorative and low-stress functional parts. At 40–60%, you're producing parts that can handle meaningful mechanical loads. Above 80%, you're approaching solid — reserve this for bearing surfaces, heavily loaded parts, or situations where you genuinely need near-maximum density.

The practical sweet spot for most prints is 15–25% infill. Tests on common geometries show diminishing returns above 30% for most load cases — going from 30% to 60% infill increases print time significantly without a proportionate increase in strength for the typical failure modes (wall peeling, layer splitting). The outer wall count (perimeter count) often has a larger impact on practical strength than infill percentage. Increase wall count before reaching for higher infill.

Infill Patterns: Not All Lattices Are Equal

Modern slicers offer many infill patterns, each with different properties. Lines is the fastest to print but only strong in one direction. Grid is stronger than lines in two directions but creates stress concentrations at intersections. Gyroid is the community favourite for most applications: it's isotropic (equally strong in all directions), creates no stress concentrations, prints at consistent speed, and looks fascinating on a cross-section — a continuous, mathematically smooth surface that never self-intersects. Cubic provides excellent all-direction strength and is efficient to print. Honeycomb mimics nature's most efficient load-bearing structure and is excellent for compression loads.

For everyday functional prints in PLA+ or PETG, gyroid at 20% is an excellent universal starting point. For parts that primarily experience compression (like feet, spacers, and pads), honeycomb at 25–40% is often superior. For the fastest possible draft prints in PLA, grid or lines at 15% cuts time significantly without meaningful strength penalty. See our design for 3D printing guide for how infill choice interacts with part orientation and wall count in real-world design decisions.

Advanced Infill: Modifiers and Adaptive Density

PrusaSlicer and OrcaSlicer support infill modifiers — you can paint specific regions of a model to have different infill density from the rest. This lets you place 80% gyroid infill only around a stress concentration (a hole, a junction) while the rest of the part uses 15% — producing a strong part without wasting material. Adaptive infill (covered in our adaptive infill guide) automatically increases density near surfaces and thin features. These advanced techniques reward the time investment — learning them takes an evening, but applying them makes every functional print you produce smarter.