When a shop quotes "±0.2 mm tolerance," it sounds precise — but what does that actually guarantee, and where does it stop guaranteeing anything? Tolerances in 3D printing behave differently from machined parts, and misunderstanding them is one of the most common sources of surprise on a first build: the part measures right with calipers but won't drop onto the shaft, or the bolt holes line up but the bolt won't go through, or a 200 mm bracket comes off the printer a full millimeter short. None of that is a defect — it's the physics of how the part was made. This guide explains what tolerance means for a printed part, what numbers to expect from each process, why holes and shafts and big parts misbehave, and exactly what to tell us so the part fits the first time.
What tolerance actually measures
A tolerance is the allowed deviation from the nominal (designed) dimension. If you design a 50.0 mm feature and the shop quotes ±0.2 mm, you should expect the finished part to measure between 49.8 and 50.2 mm — and that's a band, not a target. A part at 49.85 mm is in spec; so is one at 50.15 mm. If your design only works if that feature is 50.00 ± 0.02, a ±0.2 mm process won't reliably give you that, no matter how carefully it's printed. The general-purpose machining standard most shops fall back on for unspecified dimensions is ISO 2768 — but it was written for machining, not additive manufacturing, which has its own standards work under ASTM/ISO committee F42.
The catch with 3D printing is that "the tolerance" isn't one number. It depends on the process (FDM vs SLA vs powder-bed), the material (ABS shrinks more than PLA), the feature size (a percentage tolerance means a 200 mm feature has 10× the absolute error of a 20 mm one), the feature type (a hole behaves differently from an outside edge), the orientation (a dimension along the layer plane is more accurate than one in the build direction), and even where the part sat on the build plate. A good quote accounts for all of that; Formlabs and Prusa both publish process-specific accuracy data worth comparing against whatever you're told.
Typical tolerances by process
| Attribute | FDM | SLA | SLS / MJF |
|---|---|---|---|
| Typical layer height | 0.1 – 0.3 mm | 0.025 – 0.1 mm | 0.08 – 0.12 mm |
| Min printable feature | ~0.4 mm | ~0.05 mm | ~0.5 mm (powder bed) |
| Dimensional tolerance | ±0.2 mm or ±0.2% | ±0.1 mm or ±0.1% under 100 mm | ±0.3 mm or ±0.3% |
| 10 mm feature, expected | ±0.2 mm | ±0.1 mm | ±0.3 mm |
| 200 mm feature, expected | ±0.4 mm | ±0.2 mm | ±0.6 mm |
| Hole accuracy (5 mm) | 4.7 – 4.9 mm (undersized) | 4.9 – 5.0 mm | 4.6 – 4.8 mm (undersized) |
| Surface vs. nominal | Layer-line stair-stepping on curves | Smooth out of printer | Slight powder texture |
| Big-part stability | ABS/ASA shrink up to 0.5 – 1.0 mm at 200 mm; PETG/PLA much steadier | Warp risk during post-cure on parts > 100 mm | Excellent — uniform powder-bed cooling |
| Best for | Functional parts > 50 mm, batch runs, structural use | Small precise parts, jewelry/dental, smooth finish | Industrial-grade nylon parts, complex small-batch geometry |
The plain-language version
- FDM: ±0.2 mm or ±0.2% (whichever is greater). A 10 mm feature: ±0.2 mm. A 200 mm feature: ±0.4 mm.
- SLA: ±0.1 mm or ±0.1% on parts under ~100 mm. Better small-feature fidelity, but large parts can warp during post-cure.
- SLS / MJF (industrial nylon): ±0.3 mm or ±0.3% — looser on large parts because of how the powder bed cools.
Why holes and shafts behave differently
An FDM nozzle lays down a bead of plastic with real width — about 0.4 mm on a standard nozzle. When the toolpath traces the inside of a hole, the bead's width eats into the opening; when it traces the outside of a peg, the bead adds to the diameter. On top of that, the plastic shrinks slightly as it cools, pulling holes a touch tighter and leaving pegs a touch fatter. The net effect is consistent and predictable: printed holes come out undersized, printed shafts come out oversized. In practice a 5.0 mm modeled hole often measures 4.7–4.9 mm, and a 5.0 mm modeled shaft often measures 5.1–5.2 mm. SLA does this much less — there's no nozzle width to compensate for — which is one reason small precision parts often go to SLA.
Design rules of thumb for FDM:
- Clearance / sliding fit: oversize the hole by 0.2–0.3 mm, or undersize the shaft by 0.1–0.2 mm, or split the difference. Then test — print fits aren't machine fits, so plan on one iteration.
- Press / interference fit: reverse it — make the hole slightly smaller than the shaft, and don't overdo it or you'll crack the part going together.
- Bolt clearance holes: add ~0.3 mm to the nominal bolt diameter. An M5 bolt wants a hole modeled at ~5.3 mm to drop through cleanly.
- Threads: for anything cycled more than a few times, design for a brass heat-set insert rather than printing the thread directly. Printed threads are fine for low-stress, low-cycle uses.
- Bearing seats, dowel references, anything that must be precise: print it undersized and ream or drill to final size. It's cheap and dead-accurate — just flag the dimension.
Our design-for-3D-printing tips cover hole and fit design (and a lot more) in practical detail.
Orientation matters too
A dimension that runs along the layer plane (X/Y) is more accurate than one that runs in the build direction (Z), because Z is built up layer by layer and each layer adds a tiny amount of variability. If a part has one dimension that has to be tight, we'll orient the print so that dimension lies in the accurate plane when we can. Conversely, a flat surface that has to be truly flat prints best face-down on the build plate. Tell us which surface or dimension is the critical one and we'll orient around it — it's free and it matters.
Where layer lines change the geometry
Layer height (usually 0.1–0.3 mm for FDM, finer for SLA) shows up as stair-stepping on angled and curved surfaces. A 45° ramp printed at 0.2 mm layers has a consistent 0.2 mm "tread" on each step; a shallow dome shows visible terracing. This doesn't change the nominal dimension — the part is still the right size on average — but it changes how the surface feels, how light catches it, and critically how it seals against a mating part. If a printed face has to mate flush against an O-ring, a gasket, or another flat part, either orient it so that face prints flat (no stair-stepping), specify a finer layer height, or plan a light sanding pass. SLA's much finer layers make this mostly a non-issue, which is why fluid-handling and sealing parts often go to resin.
Thermal effects on larger parts
Plastic shrinks as it cools, and on a big part that shrinkage adds up. A 200 mm ABS or ASA part can come off the bed 0.5–1.0 mm shorter than designed — that's a ±0.2% effect, which is exactly why FDM tolerances are usually quoted as "±0.2 mm or ±0.2%, whichever is greater." It's also why a part that fits perfectly at 30 mm can be noticeably off at 250 mm. PLA and PETG shrink far less than ABS/ASA, so for anything dimension-critical over ~100 mm we default to those materials. We can also compensate in CAD — scale a known-shrinky dimension up by the expected shrinkage — but that only works if we know which dimension matters, so tell us. Powder-bed processes (SLS, MJF) handle large parts best of all because the whole bed cools uniformly; if you have a big part that has to hold tight dimensions, that's worth a conversation.
Material-specific behavior at a glance
- PLA — low shrinkage, dimensionally stable, easy to hold tolerance on. Weak point is heat: a PLA part softens and can creep out of spec in a hot car or near a heat source.
- PETG — low-to-moderate shrinkage, stable, our default for dimension-critical functional parts. Slightly more "squish" than PLA on first-layer-sensitive features.
- ABS / ASA — the highest shrinkage of the common filaments; great heat and (for ASA) UV resistance, but plan for warpage and shrink compensation on big parts.
- Nylon — absorbs moisture, which changes its dimensions slightly over time; excellent toughness, but not the material for a part that has to stay dimensionally perfect in a humid environment.
- Polycarbonate — strong and heat-resistant, moderate shrinkage; needs careful printing to stay flat.
- SLA resin — very good small-feature accuracy; large parts can warp during the post-cure, so it shines below ~100 mm.
A worked example: a part that has to mate with a bearing
Say you need a printed housing with a press-fit seat for a 22 mm OD ball bearing. If you model the bore at exactly 22.0 mm and print it in FDM, you'll get something around 21.7–21.9 mm — too tight to press the bearing in without cracking the housing, or it'll go in crooked. The right approach: model the bore at ~21.9 mm (a deliberate slight interference for the press fit) and tell us it's a bearing seat so we orient the print to put the bore in the accurate X/Y plane and, if needed, ream it to a measured 21.85 mm after printing. Or, if the part is small enough, print the whole thing in SLA, where the bore will come out close to nominal without compensation. Either way the part fits the first time — but only because the critical dimension was flagged. Model it blind and you're on iteration two.
What to ask for when tolerance matters
- Flag critical dimensions on your drawing. Not everything needs ±0.1 mm — but the fit to a bearing, a bolt, a shaft, or another part probably does. A part with three flagged dimensions and the rest "general" is easy to quote accurately; a part with no callouts gets the general process tolerance everywhere.
- Share the mating part, if there is one. Send us the bolt, the shaft, the bracket it attaches to, or its dimensions. We can tune the printed feature to match your actual hardware rather than a nominal spec.
- Ask about secondary operations. Reaming, drilling, tapping, facing, and light machining after printing can hit ±0.05 mm on specific features cheaply — far cheaper than chasing that accuracy with the printer alone. Heat-set inserts give you durable, accurately-located threads.
- Consider SLA — or industrial SLS — for small, high-precision parts. Below ~30 mm, the cost of an SLA print is often less than the cost of post-machining an FDM part to the same accuracy. See our FDM vs SLA guide.
- Pick the right material. If a part has to hold tight dimensions and also take heat, that's a real constraint — tell us both and we'll find the material that does it (or tell you honestly if printing isn't the right answer).
FAQ
What tolerance can you actually hold? Roughly ±0.2 mm or ±0.2% on FDM, ±0.1 mm or ±0.1% (under ~100 mm) on SLA, as a default across general dimensions. Specific flagged features can be held tighter — sometimes much tighter — with orientation choices, shrink compensation, and post-machining. Tell us the dimension and the target and we'll tell you what's realistic and what it costs.
My part measured fine but doesn't fit — why? Almost always a hole/shaft issue (printed holes run undersized), a stair-stepping issue on a mating face, or a thermal-shrinkage issue on a large dimension. All three are predictable and designable-around — see the sections above, or just send us the part and the thing it has to fit.
Can I get a 3D-printed part to machined-part tolerances? On specific features, yes — by post-machining them (reaming, facing, tapping). Across the whole part, no — that's what CNC machining is for. A common pattern is to print the bulk geometry and machine the two or three faces that need to be precise.
Does a finer layer height make the part more accurate? It improves surface finish and reduces stair-stepping on curves, but it doesn't dramatically change in-plane dimensional accuracy — and it adds a lot of print time. We'll suggest a finer layer height when the geometry benefits from it.
Do you inspect parts before they ship? Yes — we dimensionally check parts, and for parts with flagged critical dimensions we measure those specifically. If part-to-part consistency across a batch is mission-critical, tell us and we'll add tighter QC.
Tolerances aren't magic — they're a contract between how a part is designed, how it's made, and how it'll be used. When you send us a quote, tell us what matters and we'll tell you exactly what we can guarantee.