What Infill Should I Choose?

Infill is the internal lattice inside a printed part. Almost nothing is printed solid — the inside is a honeycomb, and the percentage says how dense it is. It is also the setting people obsess over, usually to no benefit.

Here is the short version: for most functional parts, 20 percent is right, and if you want it stronger, add walls instead.

Why infill matters less than you think

When a part bends, the stress is highest at its outer surfaces and near zero in the middle. That is why a scaffold pole is a tube — you can take out the middle and lose very little stiffness, because the middle was not doing much.

A printed part works the same way. The perimeters — the walls — carry most of the bending and tensile load. The infill mostly stops the walls buckling inward and holds the top surface up.

Which means adding infill is spending material in the place it does the least good. Going from a moderate infill to a heavy one adds a lot of print time and plastic for a modest strength gain. Adding two more perimeters costs far less and does more.

If someone tells you to print a bracket at heavy infill, ask for more walls instead. It will be stronger, lighter, faster and cheaper.

Sensible defaults

• 10 to 15% — display models, cosmetic parts, fit-check prototypes, anything that never takes a load. Cheap and quick.
• 20% — the honest default for the great majority of functional parts. Brackets, mounts, enclosures, fittings, jigs. If in doubt, this.
• 30 to 40% — parts under real, repeated load, or that take knocks. Diminishing returns start here.
• 50%+ — rarely worth it. At this density you are usually solving the wrong problem: the part probably wants redesigning, thickening or reorienting, not filling.
• Solid — small parts only, where infill is meaningless anyway. A clip or a spacer a few millimetres thick has no room for a lattice, so it prints essentially dense regardless of the setting. For anything larger, printing solid is slow, expensive, and it builds up internal stress that can warp the part or split the layers.

That last point is worth sitting with: fully dense is not automatically stronger. Beyond a point, more plastic means more shrinkage stress, more heat, and a part more likely to crack or lift.

Where infill genuinely matters

• Large flat top surfaces. The solid top layers have to bridge across the infill. Too sparse and the top sags between the ribs. This is often the real reason to go up a step.
• Compression. A part that gets stood on or clamped, loaded straight through, benefits from density in a way a bending part does not.
• Screws and inserts. Around a boss or a heat-set insert, density gives the fastener something to bite.
• Weight or feel. Sometimes you want a part to feel substantial. That is a legitimate reason.
• Watertightness. Not really infill — that is walls and top layers again. See is PETG waterproof.

What infill costs you

Infill buys strength with material and time, and both land on your invoice:

• Material rises roughly with the percentage.
• Print time rises with it too, and time is usually the bigger cost. Our cost guide breaks this down.
• Failure risk rises on big dense parts, from heat and stress.

Going from 20% to 60% on a decent-sized part is a substantial increase in both for a strength gain you would struggle to notice in use. Going from three walls to five costs a fraction of that and does more.

Patterns, briefly

You do not need to specify this — we pick it — but for interest:

• Gyroid — strong in every direction, prints fast, no crossing paths. A good all-rounder and our usual choice for functional parts.
• Grid / lines — fastest, weakest. Fine for models.
• Cubic / triangles — stiffer, slower.
• Concentric — for flexible parts in TPU.

The thing that actually decides strength

Not infill. Orientation. A part is far weaker across its layers than along them, and that difference dwarfs anything infill does. A bracket printed the wrong way up will snap at 60% infill where the same bracket printed correctly holds fine at 20%.

We orient every part around where the load goes, before infill enters the conversation. Read how strong are 3D printed parts — it is the more important half of this topic.

You do not have to choose

Genuinely — this is our job, not yours. Tell us what the part does, where it lives and what it holds, and we will set the walls, infill and orientation around that. Getting it wrong in either direction costs you money: too little and it breaks, too much and you paid for plastic doing nothing.

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