Choosing a Layer Height

Layer height is how thick each slice of a part is. It is the number people fixate on, usually in the belief that smaller means better. It does not. It means slower, and past a point it means weaker too.

The basic trade

Halve the layer height and you double the number of layers, so you roughly double the print time. Print time is one of the main levers on what a part costs, per how pricing works.

What you get for that doubled time is a finer stair-step on curved and sloped surfaces. On a vertical wall or a flat top, you get almost nothing — those surfaces look much the same either way. The improvement is concentrated on shallow slopes, where stair-stepping is at its most visible.

So the question is never "how fine can we go". It is "does this part have surfaces where a finer layer would actually show".

The practical range

With a standard 0.4 mm nozzle:

• 0.3 mm and up — coarse and quick. Layer lines you can see across a room. Right for big chunky parts, drafts, jigs, anything hidden. Also good for watertightness — squashing thick layers together closes voids, as covered in is PETG waterproof.
• 0.2 mm — the default, and it is the default for good reason. Sensible speed, perfectly acceptable finish, good layer bonding. Most functional parts want this and most of what we print is this.
• 0.15 mm — a visible step up in finish on curves for around a third more time. A reasonable compromise for a part on show.
• 0.1 mm — fine. Twice the time of 0.2 mm. Worth it for detailed or cosmetic parts, and rarely worth it for anything else.
• Below 0.1 mm — diminishing returns on FDM, honestly. Print times get long and the limiting factor becomes the nozzle width, not the layer height. If you need that level of detail, FDM is probably the wrong process.

Rule of thumb: layer height wants to sit somewhere between about 25% and 75% of the nozzle diameter. Too thin and the nozzle drags through what it just laid; too thick and the layers do not key into each other.

The bit that surprises people: thinner can be weaker

More layers means more layer welds — more of the weak planes that make FDM anisotropic. And each thin layer has less mass, so it cools faster and reheats the layer below less effectively, which makes each individual weld poorer.

Thicker layers, printed hot, bond better. That is why a strong part is often a 0.25 or 0.3 mm part, not a 0.1 mm one.

This runs directly against the instinct that fine equals good. If the part's job is to hold something, a thicker layer is likely the better choice — and orientation matters far more than either, per print orientation and strength.

What actually improves the finish

If a part is going to be sanded and painted anyway, layer height barely matters — filler primer fills a 0.2 mm step about as easily as a 0.1 mm one. Paying for a fine layer height and then sanding it flat is paying twice. See painting and finishing.

Bigger levers on how a part looks:

• Orientation. Getting the cosmetic face vertical or on top beats any layer height.
• Avoiding supports on show faces. Support scarring is far uglier than a layer line — see supports and overhangs.
• Nozzle diameter. A smaller nozzle resolves finer horizontal detail, which layer height cannot touch.
• Steeper or flatter slopes. A 15° face stair-steps badly at any setting.

Variable layer height

Slicers can vary the layer height through the part — fine where a surface is shallow, coarse where it is vertical. It gets you most of the finish for a fraction of the time cost. We use it where a part's geometry rewards it.

So what should you ask for

Nothing, usually. Tell us what the part is for and we will pick:

• Functional, hidden, structural — 0.2 or 0.3 mm. Faster and stronger.
• On show — 0.15 mm, with the good face oriented properly.
• Fine visible detail — 0.1 mm, and expect to pay for the time.
• Big and rough — 0.3 mm and up.

Where FDM is the wrong process

If the detail you need is genuinely fine — crisp sub-millimetre features, sharp text a couple of millimetres tall, miniature figures — no layer height rescues it. The limit is the nozzle: a 0.4 mm bead cannot draw a 0.2 mm feature, however thin the slices. That is a resin job, and we do not offer resin. We will tell you that rather than sell you a soft-edged compromise.

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