In 3D printing, infill percentage is the setting that controls how much material fills the inside of an object. It shows up as a percentage, where 0% makes a completely empty model and 100% creates a completely solid part. This single number is one of the most useful tools in your slicing software, directly controlling how your final print turns out.
The choice you make is always a trade-off. A higher infill percentage usually makes a stronger part, but it also takes much longer to print and uses more plastic, which costs more money. A lower percentage does the opposite, making lighter, faster, and cheaper prints that aren't as strong. Getting good at this setting isn't about finding one "perfect" number. It's about learning to adjust every print for what you need it to do, whether it's a lightweight display model or a strong mechanical part.
Understanding 3D Print Infill
To really understand infill, you need to see it as more than just "filling" and think of it as an internal support system. It creates the hidden structure that gives an object its strength.
A Simple Way to Think About It
Think of a 3D print like building a house. The outer walls of your print, called shells or perimeters, are like the outside walls of the house. They create the shape and provide a smooth surface. The infill, however, is the internal support structure—the network of beams and supports inside the walls. This internal framework can't be seen from the outside but is completely responsible for the house's ability to resist forces and stay rigid. Without proper internal structure, even a house with thick walls would be weak and collapse under pressure. Infill does this exact job for your 3D prints.
Important Terms
Your slicing software uses several related terms to control this internal structure. Understanding them helps you make better decisions.
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Infill Percentage: This sets how dense the internal support structure is. A low percentage creates a sparse, web-like structure, while a high percentage creates a dense, tightly packed one.
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Infill Pattern: This controls the specific shape of the internal structure. Common patterns include grids, triangles, and complex 3D shapes like gyroids. Each pattern has different properties affecting strength, speed, and flexibility.
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Shells/Perimeters/Walls: These are the continuous lines that form the outer surfaces of your print. The infill's main jobs are to support these walls from the inside and provide a foundation for the top solid layers.
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Top/Bottom Layers: These are the solid, horizontal layers that form the visible top and bottom surfaces of your object. The top layers print directly onto the infill structure, so enough infill is necessary to prevent them from sagging or collapsing.
[A visual cross-section in a slicing program would show these parts clearly: the outer lines are the shells, the internal geometric pattern is the infill, and the solid layers at the very top and bottom are the top/bottom layers.]
The Three-Way Balance
When you adjust the infill percentage, you are making a decision that balances three important factors: strength, speed, and cost. Understanding how these three elements work together is key to moving beyond default settings and getting better results for every print.
Part Strength and Durability
The most obvious effect of infill is on how strong the part is. A higher infill percentage creates more internal connections within the model. This dense network is better at spreading out stress and resisting forces from any direction, resulting in a stronger, more rigid, and more durable object. A part with 15% infill might feel light and crush easily in your hand, while the same part at 60% infill will feel solid and be very hard to break.
However, the relationship isn't perfectly straight. You will see diminishing returns. The increase in strength gained by going from 20% to 50% infill is often dramatic and very noticeable. In contrast, the strength gained from increasing infill from 70% to 100% is less significant and may not be worth the steep increase in time and material for many uses.
Print Time and Speed
There is a direct and often dramatic connection between infill percentage and total print time. The infill is typically the largest portion of any print job. To create the internal structure, the print head must make a complex series of movements across the entire internal area of the model, layer by layer. Doubling the infill percentage means the printer has to put down roughly twice as much material inside the part, which can drastically increase the total print time.
For a real example, consider a standard 4x4 inch cube. Moving from 20% to 40% infill might add an hour to the print time. However, going from 80% to 100% infill could add several more hours, as the printer must now trace nearly every square millimeter of the internal area. For large models, this can be the difference between a 10-hour print and a 24-hour print.
Material and Cost
Just as infill affects time, it directly impacts material use and, therefore, the cost of a print. Filament costs money, and infill is where most of it goes. A higher infill percentage uses more plastic, simple as that. While this might seem minor for a small figurine, it becomes a major financial consideration for large parts or when making multiple items.
To put this in practical terms, printing a large storage box with 40% infill might use 200g of a 1kg filament spool. By analyzing the part's needs and determining that 15% infill is enough, you could reduce material usage to just 80g. This not only saves money on that single print but also adds up to significant savings over the lifetime of your printer, allowing you to get more prints out of every spool.
Choosing Your Infill Percentage
With the theory covered, the practical question remains: what percentage should you use? The answer always depends on what the printed part will be used for. There is no universal "best" setting. The following recommendations provide a clear, actionable starting point for your projects.
Recommendations by Use
This table breaks down common use cases and provides a recommended infill range, along with the reasoning for that choice. Use this as a guide for your slicer settings.
| Use Case | Recommended Infill % | Reasoning |
|---|---|---|
| Display & Visual Models (Figurines, Busts) | 5-15% | Strength is not important. The goal is a fast, cost-effective print. Infill only needs to support top layers. |
| Standard Prototypes & General Use (Cases) | 15-30% | The "go-to" range. Offers a great balance of reasonable strength, speed, and material usage for daily items. |
| Functional & Mechanical Parts (Brackets, Gears) | 40-70% | Strength and durability are primary. These parts must withstand mechanical stress, load, or repeated use. |
| High-Stress & Load-Bearing Parts (Tools, Mounts) | 70-100% | For critical components where maximum strength is most important, and print time or cost are secondary concerns. |
Beyond Percentage: Infill Patterns
While infill percentage controls the density of the internal structure, the infill pattern controls its shape. This second, equally important setting is often overlooked by beginners but is a key tool for advanced print optimization. The shape of your infill can dramatically change a print's strength, weight, flexibility, and even the time it takes to print.
Why Infill Shape Matters
Different patterns provide strength in different ways. Some, like Grid, are two-dimensional and fast but offer strength primarily along the X and Y axes. Others, like Gyroid or Cubic, are three-dimensional structures that provide more uniform strength, meaning the part is equally strong no matter the direction of the force. The choice of pattern can mean the difference between a part that is rigid and one that has slight flex, or a part that prints quickly versus one that is slow but exceptionally strong.
Common Infill Patterns Compared
Most modern slicers offer a wide variety of patterns. Here is a breakdown of the most common ones and their ideal uses.
| Pattern Name | Shape | Best For... | Key Features |
|---|---|---|---|
| Grid / Lines | 2D grid of intersecting lines per layer. | Fast prints, general-purpose models. | One of the fastest patterns. Strong in two dimensions but can be weaker between layers. |
| Triangles / Honeycomb | Interlocking triangles or hexagons. | Functional parts needing high wall strength. | Excellent shear strength in the X/Y plane. Takes longer to print than Grid. |
| Gyroid | A unique, non-overlapping 3D wave. | Uniform strength, parts needing some flex. | A modern favorite. Provides near-equal strength in all directions. Prints fast and quiet. |
| Cubic / Octet | Stacked, offset 3D cubes or tetrahedrons. | Strong, functional parts needing 3D strength. | Creates a very strong internal 3D lattice. Offers excellent layer adhesion but is slower to print. |
Case Study: A 2025 Test
To show the real-world impact of these settings, we conducted a straightforward experiment in our workshop. We wanted to see the practical differences in strength, time, and material usage by changing only the infill percentage.
The Experiment Setup
For our test, we chose a standardized utility hook model, designed to hold a 5kg (approx. 11 lbs) weight. We used a standard PLA filament and kept all other slicer settings—such as layer height, print speed, and shell count—identical across all tests. The only variable we changed was the infill percentage, using a basic Grid pattern for consistency.
We printed three versions:
* Version A: 20% infill
* Version B: 50% infill
* Version C: 80% infill
Head-to-Head Results
The differences were clear as soon as the prints were finished.
Print Time & Material Usage:
- Version A (20%): 1 hour 15 minutes / 18g of filament.
- Version B (50%): 2 hours 5 minutes / 32g of filament.
- Version C (80%): 3 hours 10 minutes / 48g of filament.
The jump from 20% to 50% nearly doubled the material cost and added almost an hour to the print time. The jump to 80% more than doubled the time and nearly tripled the material usage compared to the baseline 20% version.
The Strength Test:
We then tested each hook by hanging the 5kg weight from it.
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Version A (20%): The hook held the 5kg weight but showed visible and significant bending under load. When a small, sudden additional force was applied, it immediately snapped at the curve. It was not reliable for its intended function.
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Version B (50%): This hook held the 5kg weight with no visible flex or strain. It felt completely rigid and solid. It handled moderate additional stress before finally failing, proving it was more than sufficient for the intended purpose.
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Version C (80%): The hook was extremely rigid, holding the weight with zero deflection. It was significantly more difficult to break by hand and far exceeded the strength requirements of the test. The extra time and material provided no practical benefit for this specific application.
The Key Lesson
This simple test provides a clear lesson: more is not always better. The 50% infill hook offered the perfect balance, providing all the necessary strength for the application without the excessive print time and material cost of the 80% version. The 20% version was only suitable as a visual model, not a functional part. This shows the critical importance of matching your infill percentage directly to the mechanical needs of your print.
Conclusion: Mastering Infill
Understanding and controlling infill is a gateway to a new level of skill in 3D printing. It transforms your slicer from a simple "prepare to print" button into a powerful tool for engineering the physical properties of an object. By moving beyond a one-size-fits-all approach, you can save enormous amounts of time, material, and money while producing parts that are perfectly suited for their job.
Your Key Lessons
As you move forward, keep these core lessons in mind:
- What is infill percentage in 3d printing is your primary control over the internal density of a print.
- Every infill choice is a balance between strength, print speed, and material cost.
- Select your infill percentage based on the print's final application, not a single "best" number. Use ~15% for visual models, 20-50% for general use, and 50%+ for truly functional, high-stress parts.
- Remember that infill patterns like Gyroid, Grid, or Cubic also play a massive role in a part's final strength and characteristics.
The best way to develop an intuitive feel for these settings is to experiment. Print small test cubes or other benchmark models at different percentages and with different patterns. Break them, feel them, and observe the results. This hands-on experience, combined with the knowledge in this guide, is the fastest way to transition from a beginner to an expert maker.