For most 3D prints, 20% infill is the right starting point. It provides enough internal structure to support top layers, handles light everyday use, and keeps print times reasonable. From there, you adjust up or down based on what the part actually needs to do: decorative pieces can drop to 10-15%, functional parts typically land between 20-40%, and heavy-duty structural components push into the 50-60% range.
But the percentage alone doesn’t tell the whole story. Your choice of infill pattern and wall count can matter just as much as density, sometimes more.
Quick Reference by Part Type
These ranges cover the vast majority of prints:
- Display models, figurines, prototypes: 10-15%. These parts sit on a shelf. They need just enough internal support to prevent the top surface from sagging.
- Everyday functional items (phone stands, cases, enclosures): 20-30%. This is the sweet spot for parts that get handled but aren’t load-bearing.
- Brackets, hooks, jigs: 40-60%. Parts under real mechanical stress need the extra material.
- Parts with threaded inserts: 25% minimum. Heat-set inserts need enough surrounding plastic to grip into, so going below 25% risks a weak bond that pulls out under load.
Below 20%, you risk a print defect called pillowing, where the top layers sag into the gaps below and create a bumpy, uneven surface. Around 20% density is typically enough to support the top surface cleanly.
Why Higher Isn’t Always Better
Yield strength does increase proportionally as you add more infill. That relationship is consistent across different patterns and materials. But every percentage point you add also increases print time, filament use, and weight. A part at 80% infill takes dramatically longer to print than one at 40%, and the strength gain per gram of material shrinks as you go higher.
For most people, pushing past 60% makes little practical sense. You’re better off strengthening a part through other means (more on that below) than cranking infill to 80 or 100%.
Your Infill Pattern Matters as Much as the Percentage
Not all infill patterns perform equally at the same density. A 30% gyroid part is often as strong as or stronger than a 40-50% grid part while using less material. Choosing the right pattern lets you print faster and lighter without sacrificing strength.
Gyroid is the most popular choice for strength-critical parts. It creates a wavy, three-dimensional structure with no flat “hinge” points where stress concentrates. It provides balanced performance in tension and compression across all three axes, making it close to equally strong no matter which direction force is applied. If you’re printing functional parts and aren’t sure which pattern to pick, gyroid at 15-25% handles most situations well.
Cubic infill works similarly, creating a repeating lattice of box-like cells that distributes stress through vertical, horizontal, and diagonal paths. This makes it much less prone to splitting along layer lines than simpler patterns like lines or grid. Triangles are another solid option for parts loaded primarily in one direction.
Grid and lines patterns are fine for decorative prints or quick prototypes, but they concentrate stress along specific axes. Under load, they fail more predictably and at lower forces than three-dimensional patterns at the same density.
Adding Walls Can Beat Adding Infill
One of the most effective ways to strengthen a print isn’t raising infill at all. It’s adding more wall perimeters. Research from Chiang Mai University confirmed that increasing wall count raises rigidity in the same way that increasing infill density does, and the outer shell of a print bears a disproportionate share of mechanical loads.
Think of it like a cardboard box: the corrugated interior matters, but the stiff outer panels do most of the structural work. Going from 2 walls to 4 walls on a part with 20% infill often produces a stronger result than keeping 2 walls and bumping infill to 50%, and it usually prints faster because walls are simpler toolpaths than complex infill geometry.
Parts with heat-set threaded inserts are a good example. The University of Wisconsin’s ME Design Lab recommends not just 25%+ infill but also extra wall loops around insert locations, because the walls are what the insert actually grabs into.
Special Case: Flexible Filaments
Flexible materials like TPU follow the opposite logic from rigid plastics. TPU relies on empty space inside the part to flex. If you fill the interior with 100% plastic, it becomes a solid block that barely bends regardless of the material’s rated flexibility.
For flexible prints, drop infill to 10-15% and limit walls to 2 perimeters. Pattern choice is critical here: avoid grid and triangles, which create rigid internal triangles that resist compression. Gyroid or cross 3D patterns have curving, squiggly paths that allow the part to compress and spring back in all directions. Combining 2 walls with 10% gyroid infill can make a 95A hardness TPU phone case feel noticeably soft and squishy, even though the raw material is relatively stiff.
If your TPU print came out feeling like hard plastic, too much infill is almost certainly the reason.
Practical Starting Points
If you’re still unsure, start with 20% gyroid infill and 3 walls. Print a test piece and see how it performs. If it flexes too much under the loads you need, try adding a wall or two before raising infill. If it’s a decorative piece that printed fine but took longer than you’d like, drop to 10-15% on the next version.
For parts that genuinely need to be strong, a combination of 25-30% gyroid infill with 4-5 walls will outperform a 60% grid infill with 2 walls in almost every real-world loading scenario, while printing faster and using less filament. The strongest prints aren’t the ones with the most plastic inside. They’re the ones where material is placed where it actually carries load.