Infill density is a 3D printing setting that controls how much material fills the inside of a printed object. Expressed as a percentage from 0% to 100%, it determines whether your part is completely hollow, completely solid, or something in between. At 0%, the inside is empty. At 100%, it’s a solid block of plastic. Most prints land somewhere in the 10% to 50% range, where you get a useful balance of strength, weight, and print time.
How Infill Density Affects Strength
More infill means a stronger part, but the relationship isn’t perfectly linear. As you increase density, each additional percentage point gives you a little less return than the last. Going from 10% to 30% makes a dramatic difference in how much force a part can handle. Going from 70% to 90% adds strength too, but not nearly as much per gram of material used.
Testing on PLA parts with different infill levels shows this clearly through hardness measurements. Specimens printed at 20% infill scored 46 on the Shore D hardness scale, while 40% scored 56, 60% scored 61, and 80% scored 84. The pattern holds across different infill shapes: yield strength increases proportionally with the amount of material inside the part, regardless of whether the internal structure uses a honeycomb, gyroid, or grid pattern.
For most prints, densities above 40% offer marginal gains relative to the extra filament and time they cost. Prusa, one of the most widely used printer manufacturers, found through testing that 10% to 20% infill works well for the majority of projects and built those values into their default slicer profiles.
Choosing the Right Density for Your Project
The right infill percentage depends entirely on what the part needs to do. A rough guide:
- 0% to 15%: Decorative objects, display models, and anything where weight and material savings matter more than durability. These parts won’t handle much force, but they print fast and use minimal filament.
- 15% to 50%: Prototypes, enclosures, household items, and general-purpose parts that need moderate strength. This is the sweet spot for most functional prints.
- 50% to 100%: Load-bearing components, gears, tool handles, brackets, and structural parts that will face real mechanical stress. These prints take significantly longer and use much more material.
If you’re printing something decorative like a figurine or a vase, 10% to 20% is plenty. For a phone case or a shelf bracket, 30% to 50% gives solid results. Parts that will be bolted into assemblies, used as mechanical linkages, or subjected to repeated stress should start at 60% and go up from there.
Why Infill Supports Surface Quality
Infill isn’t just about strength. It also provides a scaffold for the top layers of your print. Without internal structure underneath, the top surface has to bridge across open air, which often leads to sagging, small gaps, or a defect called “pillowing” where the surface develops bumpy, uneven patches.
If you print with very low or zero infill, angled walls and flat top surfaces are especially vulnerable. The printer lays down plastic with nothing beneath it, and gravity pulls the molten material downward before it can solidify. Even a modest 10% to 15% infill provides enough support points to keep top layers smooth and gap-free.
Infill Density vs. Wall Thickness
One of the most common mistakes is cranking up infill density when you’d get better results by adding more outer walls instead. Wall thickness (sometimes called shell thickness) often contributes more to overall part strength than infill does, particularly for parts that need to resist bending or flexing. Studies comparing the two found that thicker shells provide better bending resistance, use less material, print faster, and carry a lower risk of warping than high-density infill.
That said, the best approach depends on the type of stress your part will face. If it needs to resist compression or crushing forces, like a support leg or a spacer, higher infill density with a strong internal pattern is the better choice. If the part needs to resist bending or snapping, like a clip or a lever, adding wall thickness gives you more strength per gram of material. For maximum mechanical performance, one study found that combining 60% infill with thick shells produced the best results overall.
Impact on Print Time and Material Use
Every increase in infill density adds both time and filament cost. A part printed at 50% infill can easily take twice as long as the same part at 15%, because the printer has to lay down far more internal passes on every layer. Material consumption scales roughly with density: doubling infill from 20% to 40% won’t quite double your filament use (since walls, top, and bottom layers stay the same), but the internal fill itself will use about twice as much plastic.
This is why starting low and testing is a practical strategy. Print your part at 15% or 20%, see if it’s strong enough for its purpose, and only increase density if it fails or feels too flexible. You’ll save hours of print time and significant filament over the course of a project, especially if you’re iterating through multiple versions of a design.