FDM Design Guidelines: Wall Thickness, Tolerances and Clearance
For FDM 3D printing, design walls as whole multiples of the extrusion width (on a 0.4 mm nozzle, about 1.2 mm or more for walls that carry load), leave clearance between mating parts rather than modeling them line-to-line, and expect about ±0.2 mm on a typical part from our machines. Holes come out slightly small, overhangs past about 45° need support, and the axis a part is printed along decides where it is strong. This guide is for engineers, product designers and maintenance teams preparing CAD for an FDM part. It covers the numbers to start from and the places where those numbers stop working.
Who this guide is for
You have a part in CAD and you want it to print right the first time. You might be a mechanical engineer sending a prototype housing, a maintenance lead replacing a bracket, or a product designer checking whether a snap fit will survive. You don't need to run a printer. You need to know what FDM does to your geometry, so the model you send is one that prints well.
What wall thickness should an FDM part have?
Short answer: make load-bearing walls at least about 1.2 mm thick on a 0.4 mm nozzle, keep them in whole multiples of the extrusion width, and keep walls even across the part.
An FDM printer builds a wall from perimeters, the outline loops it lays down on every layer. Each loop is roughly as wide as the nozzle, and a little wider in practice. A wall that is not a clean multiple of that width leaves the slicer choosing between a gap and an overlap. Prusa's slicer documentation notes that for a chosen number of perimeters and layer height, the slicer works out an optimal thin-wall thickness, and that changing the CAD wall to that exact value removes unnecessary perimeter overlap (Prusa Knowledge Base: Layers and perimeters).
Practical starting points for a 0.4 mm nozzle (recommendations, not rules):
About 0.8 mm (two perimeters): cosmetic walls, covers and display shells that carry no load. Hubs' general guidance is that walls thicker than 0.8 mm print reliably (Hubs: key design considerations).
About 1.2 mm or more (three perimeters or more): functional housings, brackets and anything with a screw boss or a clip.
2 mm or more: walls that take impact, clamp load or repeated flexing, and walls around threaded inserts.
Thicker is not always better. A very thick solid section takes longer to print, which affects price (see what affects 3D printing part price), and it can warp more as it cools. Large sudden changes in thickness are where warping and visible sink lines show up. Even walls with ribs for stiffness usually beat one massive block.
What tolerance can you expect from FDM?
Short answer: about ±0.2 mm is typical on our machines. That figure is a planning number, not a guarantee for every feature.
Tolerance on an FDM part depends on more than the printer. The material shrinks as it cools, and some materials shrink more than others. Long flat parts can bow. Features in the XY plane (across the bed) usually hold dimensions better than heights along Z, which are built in layer steps. Small holes and thin pins are the features most likely to come out off-size.
What this means for your drawing:
Call out the dimensions that matter. If two faces must mate, or a bore must take a bearing, say so. Most dimensions on most parts don't need it.
Don't expect machined-part tolerances from an as-printed surface. Where a fit is critical, the usual answer is to print the feature slightly undersize and drill, ream or sand it to final size. Hubs recommends the same for accurate holes (Hubs: how to design parts for FDM).
Print one before you print fifty. For a part that has to fit an existing assembly, a single test part checks the fit faster than any amount of tolerance math.
How much clearance do mating parts need?
Short answer: never model mating parts line-to-line. Leave a gap, and size it to the kind of fit you want.
Two parts drawn at exactly the same nominal size will not go together once printed, because each picks up a little material on its outer skin. Common starting points for parts printed separately, per side, to be confirmed with a test print in your material:
Press or tight fit: roughly 0.1 to 0.15 mm per side. Expect light sanding or a mallet.
Sliding fit (lids, slots, pins that need to move): roughly 0.2 to 0.3 mm per side.
Loose or easy-assembly fit: roughly 0.4 mm per side or more.
Print-in-place mechanisms, such as a hinge printed already assembled, need more gap than separate parts, because the two surfaces are printed next to each other in the same layer. Curved surfaces need extra care: CAD exports a curve as flat facets, so a cylinder in a hole can end up tighter in some spots than the nominal gap. Export the STL or 3MF at a fine resolution, or send STEP, which keeps the true curves.
Material changes the answer too. A flexible TPU clip and a rigid PA-CF fixture need different clearances for the same job. If you are still choosing a material, our materials page lists what we print. The PA-CF vs PPA-CF vs PPS-CF comparison covers the carbon-fiber grades.
Why do holes print undersized, and what should you do about it?
Holes printed vertically (their axis pointing up) tend to come out slightly small. The printer traces the hole as a loop of extruded plastic, and the inside of that loop ends up a little tight. Horizontal holes have a second problem: the top of the hole is an overhang, so it can sag or come out slightly pear-shaped.
What works:
Model the hole slightly oversize if it must take a fastener freely, or undersize and drill it if it must be accurate.
For horizontal holes, consider a teardrop or flat-topped profile so the top doesn't need support, or rotate the part so the hole runs vertically.
For threads, heat-set threaded inserts or a tapped hole in a thick boss usually outlast printed threads on a part that will be assembled more than a few times.
How does print orientation change strength?
Short answer: FDM parts are strongest along the layers and weakest between them. Orient the part so the main load runs along the layer lines, not across them.
Every FDM part is a stack of bonded layers. Pull a part apart along Z and you are testing the bond between layers. Pull it along X or Y and you are testing continuous extruded plastic. For a clip, hook or bracket, that difference decides whether the part bends or snaps.
Design moves that help:
Orient for the load, not just for appearance or to avoid supports. Note the load direction in your request so it can be planned around.
Add fillets where a feature meets a wall. Hubs notes that thin vertical pins under 5 mm in diameter are built from perimeters only, with no infill, and benefit from a small fillet at the base.
Keep snap-fit arms long and in plane with the layers so they flex along the extruded lines.
Split a part into two printable halves, then bolt, pin or bond them, when no single orientation suits every feature.
Overhangs, bridges and supports
FDM can print an overhang up to about 45° from vertical without support, and short bridges between two supports print cleanly. Hubs puts the practical bridge limit at about 5 mm before some sag or support marks appear (Hubs: how to design parts for FDM). Anything beyond that needs support material, which adds print time and leaves marks where it is removed.
Ways to design supports out:
Replace a flat overhang with a 45° chamfer.
Add a chamfer rather than a fillet on edges that sit on the build plate.
Put cosmetic faces up, away from supported surfaces.
Advantages and limitations of designing for FDM
Where FDM is a good fit: functional prototypes, jigs and fixtures, brackets, housings, replacement parts, and low-volume end-use parts in engineering plastics. It needs no tooling, so a design change costs a new file, not a new mold.
Where it is the wrong tool: features finer than the extrusion width, sealed fluid paths that must be leak-tight without testing, or tolerances tighter than an as-printed surface can hold without secondary machining. For those, another process or a post-machined feature is the honest answer. If you are deciding between printing and molding, our FDM vs injection molding guide walks through the trade-off.
A short pre-send checklist
Load-bearing walls are about 1.2 mm or thicker, with no sudden jumps in thickness.
Mating parts have clearance modeled in, sized for the fit you want.
Critical dimensions are called out, and you've decided which will be drilled or finished.
Overhangs are 45° or less, or you accept supports on non-cosmetic faces.
You've noted the main load direction and the material, or the environment the part lives in.
The file is STEP if you have it, or a fine-resolution STL, OBJ or 3MF, zipped.
Get a second look at your design
If you have a part in CAD and you're unsure about a wall, a fit or a material, send it. We quote the same day, there is no minimum order, and most parts ship 2 to 3 days after approval. Engineers sending models for the first time may want our guide to 3D printing for engineers. For iteration work, see our rapid prototyping service. Ready now? Upload your CAD file for a quote and tell us how the part has to fit.
Frequently asked questions
What is the minimum wall thickness for FDM 3D printing?
Walls of about 0.8 mm (two perimeters on a 0.4 mm nozzle) print reliably for non-structural parts. For functional parts, start at about 1.2 mm, and go thicker for parts that take impact or clamp load.
What tolerance does FDM 3D printing hold?
About ±0.2 mm is typical on our machines. Holes, thin pins and heights along the build axis are the features most likely to vary. Critical fits are usually printed slightly undersize and finished.
How much clearance should I leave between 3D printed parts?
As a starting point, about 0.1 to 0.15 mm per side for a press fit, 0.2 to 0.3 mm for a sliding fit, and 0.4 mm or more for an easy fit. Confirm with a test print in the actual material.
Why are my 3D printed holes too small?
The extruded loop that forms a hole squeezes slightly inward, and horizontal holes sag at the top. Model them a little oversize, or undersize them and drill to final size.
Which file format is best to send for a 3D printing quote?
STEP keeps true curves and is the most useful. STL, OBJ and 3MF also work. Zip the files and include the material, quantity and any critical fits.
Can you fix a design that isn't printable as drawn?
Often, yes. Thin walls, missing clearances and unsupported overhangs can usually be adjusted. If you don't have a model at all, our Idea to First Print service builds the CAD from a sketch, photo, measurements or the part itself.
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