Infill in 3D Printing Explained: Density, Patterns & When to Use Each
Infill is the lattice the slicer hides inside your part — the structure you never see but always pay for in plastic and time. Get the density and pattern right and a print is strong exactly where it needs to be and light everywhere else. Get them wrong and you either waste hours filling a decorative model with plastic nobody will ever stress, or you snap a functional part that looked solid on the outside. This guide explains what infill is, how density and pattern change the result, and how to choose sensible settings for the shapes you export from this tool.
What infill actually is
Almost nothing you print is solid plastic. When a slicer processes a model, it builds the outer skin from a few walls (perimeters), caps the part with solid top and bottom layers, and fills the hollow space in between with a repeating internal pattern. That internal pattern is the infill. Its job is to support the top surface so it does not sag, give the walls something to brace against, and add stiffness and strength — all while using as little material as possible.
Because infill is hidden, it is the easiest place to save plastic. A printed cube at 20% infill is roughly four-fifths air on the inside, yet it feels solid and survives ordinary handling. That trade — mostly empty space, mostly the strength you need — is the whole reason infill exists.
Infill density: the single most important number
Infill density is the percentage of the interior volume filled with plastic. 0% is fully hollow; 100% is fully solid. Everything useful happens in between, and the relationship is not linear: the jump from 10% to 20% adds a lot of stiffness for little cost, while the climb from 60% to 100% adds a great deal of weight and print time for steadily smaller gains in usable strength.
That diminishing return is why experienced makers rarely print at 100%. Past roughly 50–60%, you are mostly paying in filament and hours rather than buying real-world durability, and for most parts the walls contribute more to strength than the infill does anyway. Here is a working set of starting points:
| Density | Best for | Strength | Print time & filament |
|---|---|---|---|
| 0% (vase mode) | Single-wall vases, cups, decorative shells | Very low | Fastest, least filament |
| 10–15% | Display models, figurines, props | Low | Fast, light |
| 20–25% | General-purpose / standard parts | Moderate | Balanced |
| 40–60% | Functional parts, brackets, jigs | High | Slow, heavy |
| 80–100% | High-stress, load-bearing, threads | Maximum | Slowest, most filament |
Treat these as a dial, not a law. 20% is the default in most slicers for good reason — it is a sensible middle ground for the average print. Move up when a part will be stressed, dropped, or threaded; move down when it is purely cosmetic and you want speed.
Common infill patterns
Density sets how much plastic goes inside; the pattern sets how it is arranged. At the same percentage, different patterns trade off strength direction, print speed, and material use. You rarely need to memorise them — the slicer default is usually fine — but knowing the character of each helps when a part has a specific job.
Fast, general-purpose patterns
- Grid — straight lines crossing at right angles, layer over layer. It is fast, simple, and the common default. Strong enough for everyday parts, with most of its strength in the two horizontal directions.
- Lines — parallel lines that alternate direction each layer. Even faster than grid because the nozzle makes fewer crossings, but weaker, since the lines are not bonded into a continuous lattice. Good for quick drafts and display pieces.
- Triangles — a lattice of triangles. Triangles resist shear well, so this pattern holds up to side loads better than grid, at a modest cost in print time.
Stronger and more uniform patterns
- Gyroid — a smooth, wavy three-dimensional pattern that is nearly equally strong in every direction. It uses material efficiently, prints reasonably fast because the lines rarely cross, and allows a part to flex slightly without cracking. A favourite for functional prints.
- Cubic — a 3D lattice of stacked cubes that distributes strength in all three directions rather than just the horizontal plane. A solid all-rounder for parts that see load from more than one angle.
- Honeycomb — a hexagonal pattern with an excellent strength-to-weight ratio. Very sturdy, but slower to print than grid because the nozzle changes direction often.
- Concentric — rings that follow the shape of the outer wall inward. It is flexible rather than rigid, which makes it the right choice for flexible (TPU) prints and for parts where you want the inside to give a little.
| Pattern | Character | Speed |
|---|---|---|
| Lines | Weak, very fast | Fastest |
| Grid | Balanced default | Fast |
| Triangles | Good shear resistance | Medium |
| Gyroid | Strong & uniform, slight flex | Medium |
| Cubic | Strong in 3 directions | Medium |
| Honeycomb | Excellent strength-to-weight | Slower |
| Concentric | Flexible, follows the wall | Medium |
How infill trades against strength, weight and time
Every infill choice spends three budgets at once: strength, weight/material, and print time. Raising density buys strength but adds weight and minutes in roughly equal measure. Switching to a denser-feeling pattern like honeycomb buys strength and resilience but costs speed, because the nozzle travels a more complicated path. The art is matching the spend to the job.
A practical way to think about it: decide what the part must survive, pick the lowest density that clearly meets it, then choose a pattern that suits the load. A wall hook that hangs straight down mostly needs vertical strength, so cubic or gyroid earns its keep. A display bust needs none of that, so 10–15% grid prints faster and lighter with no downside. There is no single "best" pattern — only the best one for what the part has to do.
Walls, top and bottom layers — the rest of the team
Infill never works alone. The walls carry most of a part's bending strength, which is why adding perimeters is often the cheapest way to stiffen a print. The top and bottom layers are solid skins that close the part off, and they lean directly on the infill: if density is too low, the slicer's lines are spaced too far apart to bridge, and the top surface dimples or sags between them — the classic "pillowing" defect. The fix is usually to add a top layer or two, or nudge infill up to 20–25% so the skin has closer support.
So the three settings move together. More walls and solid layers can let you run lower infill while keeping a part strong and its surfaces clean. If you are still choosing how thick to make those outer walls, our companion guide on wall thickness in 3D printing covers the nozzle-width rule that governs them.
How infill applies to the shapes you export here
The STL files you export from Free STL Shapes describe geometry only — the cube, sphere, pyramid, torus, or other shape — not how it should be filled. Infill is decided later, by your slicer, when you turn that mesh into printer instructions. The same STL can print hollow at 10% or near-solid at 90% without changing the file at all.
A couple of geometry-specific notes are worth keeping in mind. A thin-walled hollow shape such as a pipe or ring may have so little interior that infill barely applies — its strength comes from wall count, not infill. A solid shape like a chunky cube or pyramid, on the other hand, has a large interior where density really matters, so it is the place where a thoughtful infill choice saves the most plastic. Once your STL is ready, the slicer is where these settings live; for the full path from this tool to a finished print, see how to 3D print a shape.
Make the shape, then choose your infill — free
Design a clean cube, sphere, torus, or custom solid in your browser and export a print-ready STL in seconds. Density and pattern stay in your hands, set in the slicer — no sign-up, no install, nothing uploaded.
Open the STL generator →Frequently asked questions
How much infill do I need?
For most everyday prints, 20% is a reliable default. Drop to 10–15% for decorative models, raise to 40–60% for functional parts that take load, and reserve 80–100% for high-stress pieces and threaded holes. Going above 60% rarely pays off for the extra time and plastic it costs.
Which infill pattern is strongest?
There is no single winner, because it depends on the direction of the load. Gyroid and cubic are strong in all three dimensions, while grid and triangles are strong mainly in the horizontal plane. For an all-round functional part, gyroid is hard to beat; for pure speed, grid or lines.
Does more infill always mean a stronger part?
Up to a point. Beyond roughly 50–60% density the strength gains shrink while weight and print time keep climbing. For real-world stiffness, adding a wall or two is often more effective than pushing infill toward 100%.