FDM Infill Percentage Guide: 25%, 50%, 75% or 100%?
Andrew Ng2026-07-14T15:53:50+10:00What FDM Infill Percentage Should I Choose?
25%, 50%, 75% or 100%? Infill changes the internal density, weight, print time and price of an FDM part. Here is the practical choice for most customers.
Visual comparison of different FDM infill percentages. Solidium3D offers 25%, 50%, 75% and 100% infill.
In this article
Infill is the internal structure printed inside an FDM part. Customers often assume that a higher percentage automatically makes every part much stronger. In practice, the best setting depends on the geometry, material, wall thickness, print orientation and direction of the expected load.
For most prototypes and general functional parts, 25% infill is a sensible starting point. It keeps weight, production time and price under control while providing internal support. Choose 50% when extra rigidity is required, and reserve 75% or 100% for demanding applications where the extra density is genuinely useful.
The image above shows how the inside of a part becomes progressively denser as the percentage increases.
What Is FDM Infill?
FDM parts are normally printed with solid outer walls and a patterned structure inside. This internal structure is called infill.
The infill percentage controls how much of the interior is occupied by printed material. A lower percentage leaves more open space, while a higher percentage creates a denser internal structure.
Increasing infill generally increases weight, material usage, print time and price. It may also improve rigidity and compression resistance, although the benefit is not always proportional to the percentage selected.
25% vs 50% vs 75% vs 100% Infill
25% — Standard Infill
A practical and economical option for prototypes, covers, housings, display parts and many general functional components.
Recommended starting point for most orders
50% — Stronger General Purpose
Provides greater internal support and rigidity for brackets, mounts, fixtures and parts exposed to regular handling.
Best when extra stiffness is required
75% — Heavy-Duty Infill
A dense internal structure for components exposed to higher loads, repeated clamping or demanding functional use.
Best for demanding mechanical applications
100% — Solid Infill
The interior is printed fully solid for maximum density, added mass, machining allowance or specialised requirements.
Use only when a fully solid part is needed
| Infill | Internal density | Print time and cost | Typical use |
|---|---|---|---|
| 25% | Standard | Lowest of these options | Prototypes, housings, covers and general functional parts |
| 50% | Medium to high | Moderate | Brackets, mounts, jigs, fixtures and stronger prototypes |
| 75% | High | Higher | Heavy-duty parts, clamping components and demanding applications |
| 100% | Fully solid | Highest | Machining stock, added mass and specialised solid components |
Which Infill Percentage Should You Choose?
If you are unsure, choose 25%. This is suitable for many prototypes, enclosures, covers and moderately loaded functional parts.
Choose 50% when the part needs more rigidity, improved resistance to crushing or better support around internal features. It is a practical option for functional brackets, machine mounts, jigs and fixtures.
Choose 75% when the part will experience higher mechanical loads, repeated clamping or demanding service conditions. The extra density increases material usage and production time, so it should be selected where the application benefits from it.
Choose 100% when a fully solid interior is specifically required. Examples include parts that need extra mass, will be drilled or machined after printing, or must resist significant compressive loads.
For batch production, infill has a major effect on total machine time and material consumption. A small increase per part becomes significant across 50, 100 or 500 components.
Our Simple Recommendation
- Choose 25% for most prototypes and general functional parts.
- Choose 50% when additional rigidity and internal support are required.
- Choose 75% for higher-load or heavy-duty applications.
- Choose 100% only when a fully solid interior is specifically needed.
- If you are unsure, 25% is the most economical starting point.
Walls Can Matter More Than Infill
A higher infill percentage does not automatically produce the strongest possible part. The outer walls form the main structural shell, so increasing wall thickness or the number of perimeters can sometimes improve bending and impact resistance more efficiently than increasing infill.
Print orientation also matters because FDM parts are built one layer at a time. A part can still fail along the layer lines even when 100% infill is selected.
Geometry is equally important. Thin arms, sharp internal corners, narrow mounting points and poorly supported screw holes can remain weak regardless of infill percentage. Fillets, thicker sections and a more suitable load path may provide a greater improvement.
If your part has a critical load direction, mounting face or mechanical requirement, mention it in the quote notes so the printing requirements can be reviewed before production.
Does Higher Infill Make a Part Stronger?
Usually, but not in a simple one-to-one relationship. Moving from 25% to 50% can increase rigidity and internal support, but it does not necessarily double the strength.
Material selection, wall thickness, part orientation and geometry can have a greater effect on performance. For many applications, a properly oriented part with suitable walls and 25% or 50% infill can perform better than a poorly oriented part printed at 100%.
Infill Percentage and FDM Printing Cost
Higher infill uses more material and usually increases print time. This can increase the quoted price, particularly for large, thick or high-volume parts.
Moving from 25% to 50% does not necessarily double the total price because the outer walls, top surfaces, bottom surfaces and production setup remain similar. However, 75% and 100% can add substantial material and machine time to bulky models.
Our online 3D printing quote tool allows you to upload your model and compare the available 25%, 50%, 75% and 100% infill options before ordering.
FDM Infill Percentage FAQ
Is 25% infill strong enough?
For many prototypes, enclosures, covers and general functional parts, 25% infill is sufficient. The design, wall thickness, material and print orientation must also be suitable for the application.
Is 50% infill much stronger than 25%?
It normally provides more internal support and greater rigidity, but it does not necessarily provide twice the strength. The result depends on geometry, wall thickness and load direction.
Does 100% infill make a part unbreakable?
No. A fully solid FDM part can still fail at layer lines, thin sections, sharp corners or stress concentrations. Material and orientation remain important.
Which infill should I choose for a functional prototype?
Start with 25% for ordinary functional prototypes. Choose 50% when additional rigidity is required, 75% for demanding applications and 100% only when a solid interior is necessary.
Can I choose a percentage other than 25%, 50%, 75% or 100%?
Our standard online quote options are 25%, 50%, 75% and 100%. For specialised batch-production requirements, contact us before ordering.
Not Sure Which Infill to Choose?
Upload your 3D model for an instant quote. For most FDM parts, 25% is the most economical starting point.
Explore our FDM 3D printing service, review the materials guide, browse our works, or return to the 3D printing blog.