FDM / FFF design rules for manufacturable parts
All core FDM design rules in one table: wall thickness, hole diameter, overhang, bridging, clearance and fillets for 0.4 mm and 0.6 mm nozzles.
Fused deposition modelling builds parts one layer at a time by extruding a molten filament bead through a nozzle. Every design rule you will read about FDM, from minimum wall thickness to maximum overhang angle, traces back to this one fact. A layer is a real physical bead with a width, a height and a bond to the layer below it. If you design as if the printer were a black box that magically turns a STEP file into a solid part, you will get warped, weak or unprintable geometry. If you design with the bead in mind, the same printer produces strong, accurate parts in one shot.
This guide is the entry point to our full design rules library. It gives you every core value in one table for both nozzle sizes we run in production, 0.4 mm and 0.6 mm, and links to the dedicated guides where each topic is explained in depth. Keep this page open while you model, and treat the deep dives as the place to go when a specific feature needs more thought.
Why layer-based manufacturing sets the rules
Three consequences of the layer-based process explain nearly every limit in the table below. First, extrusion width: the nozzle diameter defines the smallest bead the printer can lay down cleanly, which sets minimum wall thickness and minimum feature size. Second, layer adhesion: bonding between layers is always weaker than the strength within a layer, which is why orientation and part strength are inseparable from wall design. Third, gravity and unsupported material: a bead extruded into thin air sags or fails to bond, which sets the limits on overhang angle and bridge length.
None of these limits are arbitrary safety margins. They are the point at which the physical process stops behaving predictably. Designing inside them is what separates a part that prints correctly the first time from one that needs several iterations.
The full FDM design rules table
| Rule | 0.4 mm nozzle | 0.6 mm nozzle |
|---|---|---|
| Minimum wall (functional) | 0.8 mm (2 walls) | 1.2 mm (2 walls) |
| Recommended wall | 1.2 to 2.0 mm | 1.8 to 3.0 mm |
| Minimum feature width | 0.4 mm | 0.6 mm |
| Minimum embossed/engraved detail | 0.4 mm width, 0.3 mm depth | 0.6 mm width, 0.4 mm depth |
| Minimum hole diameter | 2.0 mm (print as-is) | 2.5 mm (print as-is) |
| Maximum unsupported overhang | 45 degrees from vertical | 45 degrees from vertical |
| Maximum bridge | 8 to 10 mm unsupported | 10 to 12 mm unsupported |
| Minimum gap between parts | 1.0 mm | 1.2 mm |
| Minimum clearance (sliding fit) | 0.2 to 0.3 mm per side | 0.3 to 0.4 mm per side |
| Minimum internal fillet | 0.4 mm, 1 to 2 mm for stress points | 0.6 mm, 1 to 2 mm for stress points |
These are starting points, not guarantees. Material matters: PLA tolerates thinner walls than ASA, and PA12 needs extra clearance because it absorbs moisture and swells slightly. Part geometry, size and orientation shift the safe values in both directions. Where a value in this table depends heavily on a secondary factor, we say so in the linked guide.
Nozzle diameter and extrusion width
Every wall, rib and detail on your part is built from beads whose width is tied to the nozzle. Designing wall thicknesses as multiples of that width avoids gap-fill artefacts and weak, partially-filled walls. We cover the mechanics of this, including why a 1.5 mm wall on a 0.4 mm nozzle is a bad idea, in our dedicated guide on nozzle diameter and extrusion width.
Wall thickness in depth
The minimum wall values above are floors, not targets. A 0.8 mm wall on a 0.4 mm nozzle prints, but it has only two perimeters and almost no room for infill, which limits its load capacity. Our guide on minimum wall thickness for FDM breaks down absolute minimums per material, single versus multi-wall behaviour and the failure modes of thin walls under load.
Small details, holes and overhangs
Embossed logos, engraved text, small pins and thin ribs all run into the same resolution limit as wall thickness, but with their own failure patterns, particularly for recessed detail that can fill in or fuzz over. Our guide on the smallest usable FDM details covers minimum sizes for text, teeth, latches and similar features. Overhangs and bridging deserve their own attention too: printing without support is often possible if you respect the 45 degree rule and bridge length limits, or if you split and reorient the part instead of fighting the geometry.
Layer height ties it all together
Layer height is the one parameter you actively choose per job, and it interacts with almost every rule above: finer layers improve small-hole roundness and surface finish but add print time, coarser layers save cost but exaggerate stair-stepping on sloped surfaces. Our guide on choosing layer height covers the tradeoffs between 0.10 mm and 0.30 mm in detail.
Frequently asked questions
- Do these design rules apply to every material?
- They apply as safe starting points for the roughly 20 filaments we run, including PLA, PETG, ABS, ASA, PC and PA12. Some materials, particularly the nylons and PC, are less forgiving at the thinnest end of the range, so we tighten the minimums in practice for those.
- Should I design for 0.4 mm or 0.6 mm nozzle from the start?
- For prototypes with fine detail, design for 0.4 mm. For production parts that are mostly structural with few small features, designing walls as multiples of 0.6 mm often gives a stronger, faster and cheaper part.
- What happens if my design is below the minimum wall thickness?
- The slicer may skip the feature entirely, print it under-extruded, or fill it as a solid rib that behaves differently than intended. We flag these areas during review and propose a corrected wall thickness before production starts.
- Can I ignore the overhang and bridging limits if I use support material?
- You can, but support adds cost, print time and post-processing, and it leaves witness marks on the surface it touches. We recommend designing around the limits first and using support only where reorientation is genuinely not possible.
- How accurate are these numbers compared to what your machines actually achieve?
- They reflect what we see hold up reliably across materials and geometries in our shop, not theoretical printer limits. Actual achievable values can be tighter for simple, well-oriented geometry, which is something we discuss during file review.
Have your part reviewed before production
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Read next
Designing Around Nozzle Diameter and Extrusion Width
Extrusion width is derived from nozzle diameter, and matching wall thickness to it is one of the highest-leverage design decisions in FDM.
Minimum Wall Thickness for FDM Parts
The wall thickness that prints and the wall thickness that survives real use are two different numbers, and the gap between them depends on material and orientation.
The Smallest Usable Details in FDM Printing
Fine detail in FDM has a hard resolution floor set by the nozzle, and recessed features fail earlier than raised ones.
Choosing Layer Height for FDM Parts
Layer height is the one parameter you choose per job, and it trades off strength, surface, accuracy and cost against each other every time.
Wall Thickness for FDM Parts: The Complete Guide
Wall thickness is the single design decision that most affects strength, cost and print reliability in FDM parts.