Infill Density Explained: How Much Do You Actually Need?

Infill is the most over-specified setting in 3D printing. People ask for 80% because the part matters to them, pay roughly double, wait roughly twice as long, and end up with a part that's barely stronger than it would have been at 30%.

This article explains what infill actually does, why it's usually the wrong lever for strength, and what density to choose for each kind of part.

What infill is

A printed part isn't solid. It has a shell, the outer walls, called perimeters, plus solid top and bottom layers, and inside that shell is a lattice at whatever density you specify.

Infill of 20% means the interior is 20% plastic and 80% air. Infill of 0% means the interior is hollow, though the part still has its perimeters and top and bottom skins.

That last point matters more than people expect. Even at 0% infill, an FDM part is roughly a quarter solid, because the perimeters and skins are always there. This is why dropping from 20% to 0% saves much less than the numbers suggest.

The counterintuitive part: infill is a poor strength lever

that surprises most people. For the most common load case, bending, the infill barely matters. The perimeters do almost all the work.

The reason is the same as why an I-beam is shaped like an I. When you bend a beam, the material at the outer surfaces experiences almost all the stress: the top is compressed, the bottom is stretched, and the middle experiences almost nothing. Material near the neutral axis in the centre contributes very little to stiffness.

Infill is exactly that near-useless middle material. Perimeters are the flanges doing the work.

So when a part isn't stiff enough, the instinct to raise infill is usually misdirected. Adding perimeters is more effective, and often cheaper.

A part with four perimeters at 20% infill will typically be stiffer than the same part with two perimeters at 50% infill. It will also be lighter, print faster and cost less. If you want a part stronger in bending, ask for more perimeters, not more infill.

When infill matters

There are real cases where density is the right lever:

Compression through the body. If the part will be squeezed, a spacer, a foot, a bushing carrying an axial load, the infill is directly in the load path and density matters.

Threads tapped into the material. A tap cutting into 15% infill finds mostly air. For tapped holes you want 60%+ locally, or better, a heat-set insert.

Machining after printing. If you'll drill, mill or face a printed part, low infill means the cutter breaks through into voids. High infill or solid regions are needed where material will be removed.

Impact absorption. Denser infill dissipates impact energy through more material.

Screws driven directly into plastic. Self-tapping screws need material to bite into.

Mass or feel. Sometimes a part should feel substantial. This is a legitimate reason, just not a structural one.

What to choose

0-10%, display and fit checks. Concept models, form studies, parts that exist to be looked at or offered up to an assembly once. Cheapest and fastest. Don't put load on these.

15-25%, the default, and the right answer for most parts. Functional prototypes, enclosures, brackets under light load, jigs, fixtures. Strong enough to handle, mount and test. 20% is our standard and it's correct far more often than anything else.

30-50%, load-bearing parts. Brackets carrying real weight, parts under repeated stress, anything where failure has consequences. Above about 50% you're into diminishing returns fast: the curve flattens hard.

60-80%, tapped holes, machining, compression. Specific technical requirements rather than general strength. Expensive and slow.

100%, rarely correct. Solid infill is enormously expensive in material and time, and it's more prone to warping because there's more material contracting. Even parts that seem to demand solid usually do better at 60% with extra perimeters. Reserve it for small parts where absolute density matters.

The cost curve

Infill affects price through both material and machine time. Roughly, on a typical part:

Infill	Relative material	Relative price
0%	0.24	~0.55×
15%	0.35	~0.75×
25%	0.43	1.00× (baseline)
50%	0.62	~1.35×
75%	0.81	~1.70×
100%	1.00	~2.05×

Going from 25% to 100% roughly doubles the price. Going from 25% to 15% saves about a quarter.

Note again that 0% isn't free. It's 55% of the 25% price, not 0%, because the perimeters and skins remain.

Infill patterns

Most slicers offer several lattice geometries. In practice the differences are smaller than the marketing suggests, but a few are worth knowing:

Grid and rectilinear, fast, fine for general use. The default for a reason.

Gyroid, a continuous curved surface with no sharp direction changes. Roughly isotropic, meaning it behaves similarly regardless of load direction, and it prints without the nozzle crossing its own path. Good for parts loaded from unpredictable directions. Slightly slower.

Honeycomb, excellent strength-to-weight in compression, slower to print.

Lightning, a sparse tree structure that exists only to support the top skin. Extremely fast and cheap, with essentially no structural contribution. Good for display models.

For most parts, the pattern matters far less than the density and the perimeter count. We choose it per job and it isn't usually worth specifying.

A worked example

Take a bracket, 110 cm³ solid volume, printed in PETG.

At 20% infill with 3 perimeters: about 41 cm³ of plastic, 55 grams, roughly ₹817. Strong enough for most mounting applications.

At 60% infill with 3 perimeters: about 82 cm³, 97 grams, roughly ₹1,269. Twice the material for maybe 25% more bending stiffness.

At 20% infill with 5 perimeters: about 48 cm³, 61 grams, roughly ₹900. Stiffer in bending than the 60% version, at two thirds the price.

That third option is the one to choose, and it's the one people almost never ask for.

How to decide, quickly

Ask one question: how will this part be loaded?

• It won't be loaded → 10-15%
• It will be handled and mounted but not stressed → 20%
• It will carry weight or take repeated force → 30-40%, and ask for extra perimeters
• Something will be threaded or machined into it → 60% locally, or use an insert
• It will be squeezed along its axis → 40-60%

Then, if it's a bending load, add perimeters before you add infill.

What we do

The estimator defaults to 25%, which is a sensible middle. The slider is there because the right answer varies, and the price updates live so you can see what each change costs.

If you tell us the load case when you submit a quote, we'll set perimeters and infill to suit it rather than accepting whatever the slider was left on. That advice is free, and on a load-bearing part it's worth more than any discount. A part optimised for its actual load is usually both stronger and cheaper than one where the infill was turned up out of caution.