Tolerance is something you specify, not something a printer has. That single distinction explains most of the frustration buyers hit when a functional part comes back and the holes are tight, the shaft won't slide, or the assembly won't close. The machine did not "fail" — nobody told it what mattered.
This guide covers what tolerance, accuracy, precision, and repeatability actually mean, why the numbers differ by process, and how to specify a part so it comes back usable. When your part has real dimensional requirements, the fastest path is to describe them to a shop that runs the right process — browse the provider directory and put the requirement in writing.
Four Words That Are Not Synonyms
Getting these straight is most of the battle:
- Tolerance — the deviation you allow. A design requirement you write down: this bore is 12 mm, +0.1/-0.0. It is a property of your drawing, not of any machine.
- Accuracy — how close the finished part is to the nominal dimension in your CAD model. A machine that prints every 12 mm bore at 11.8 mm is inaccurate.
- Precision — how tightly repeated results cluster together, regardless of whether they are correct. Tight spread, not correct value.
- Repeatability — the same machine, same setup, producing the same result part after part. Reproducibility extends that across machines, operators, and providers — which is what matters when you reorder next year or move to a second supplier.
The practical consequence: a process can be precise and inaccurate at the same time. Every part identical, every part undersized by the same amount. That is a good problem — a known, consistent offset can be compensated with a scale factor. Random scatter cannot. When you evaluate a provider, consistency is worth more than a single lucky part.
Why Tolerance in 3D Printing Isn't One Number
Ask "what tolerance can you hold" and a good provider will answer with a question, because the honest answer depends on the process, the specific machine and its calibration, the material's shrinkage behavior, the part's size and geometry, and the build orientation. The last two are the biggest variables the buyer never sees.
This is why bureaus typically publish tolerance as a percentage of the nominal dimension with a minimum floor — something like "±0.3% with a lower limit." That model tells you something important: absolute tolerance grows with part size. A 200 mm dimension does not hold what a 20 mm dimension holds. Buyers who assume a flat ± across a large part are usually the ones who get surprised.
Typical Tolerance by Process — General Guidance Only
The table below is general industry guidance for comparing processes, not a specification and not a promise. Published capability varies widely between machines, materials, and shops, and any given provider may do better or worse on your specific geometry. Use it to shape the conversation; get the real number from the provider quoting your part.
| Process | Commonly quoted range | Character |
|---|---|---|
| FDM (extrusion) | Loosest of the common processes | Layer lines, anisotropy, warp on large parts; wide variation between desktop and industrial machines |
| SLA / MSLA (resin) | Often quoted around ±0.05–0.13 mm (±0.002–0.005 in) on small parts | Fine feature resolution; accuracy degrades with part size, supports, and cure shrinkage |
| SLS / MJF (nylon powder) | Commonly quoted around ±0.3% of nominal with a floor | Dimensionally stable and near-isotropic; shrinkage compensated by the bureau |
| Metal (DMLS/SLM) | Tighter capability, but critical features are usually post-machined | Thermal stress and support removal dominate; as-printed rarely holds critical fits |
| CNC machining (for reference) | ±0.025–0.125 mm (±0.001–0.005 in) routinely | Cuts solid stock; the benchmark printing is measured against |
Across most industrial 3D printing, ±0.1–0.3 mm on typical small-to-medium parts is a reasonable planning assumption — and an assumption is all it is. If your part depends on the number, ask. A provider who will not commit to a tolerance on your geometry is telling you something useful.
For the process trade-offs behind these numbers, see SLA vs FDM printing explained and SLA vs SLS 3D printing.
Anisotropy: XY and Z Are Not the Same Axis
A printed part is not a solid block of uniform material. It is layers, and that makes it anisotropic — its behavior depends on direction, both mechanically and dimensionally.
- XY (in-plane) dimensions are defined by the machine's motion or optics — nozzle path, laser spot, or LCD pixel. This is generally the finer, better-controlled axis.
- Z (vertical) dimensions are built by stacking discrete layers, so they are quantized to the layer height. A feature that does not land on a layer boundary gets rounded to the nearest one. First-layer compression, thermal history, and support contact all land here too.
The same asymmetry shows up in strength: a printed part is generally weakest across its layer lines. So orientation is doing two jobs at once — setting which dimensions are accurate and which direction the part can carry load. Those two goals frequently conflict, and the provider cannot resolve the conflict without knowing how the part is used. That is your job to tell them. For more on the load side, see functional and end-use 3D printed parts.
Shrinkage and Warp
Most processes involve a phase change — melting and solidifying, or curing — and material dimensions change as that happens.
- Shrinkage is largely predictable. Semicrystalline materials (nylon, ABS, polypropylene) shrink more than amorphous ones, and powder-bed parts shrink as the build cake cools. Experienced bureaus apply scale compensation factors per material and machine — precisely the institutional knowledge you are buying when you outsource. A shop running a material for the first time has no compensation history for it.
- Warp is the dangerous one, because it is differential. Uneven cooling pulls a part out of shape — corners lift, long flat faces bow. A machine can be perfectly calibrated and still hand you an out-of-spec part because the geometry fought the thermal gradient. Large flat areas, thick-to-thin transitions, and long unsupported spans are the usual suspects.
Shrinkage is a math problem the provider solves. Warp is a design problem you solve together — one of the best reasons to send geometry to a shop early rather than at the end.
Why Orientation Changes the Answer
Build orientation is the highest-leverage decision made after you send the file — and it is usually made by someone who does not know what your part does.
Orientation determines which dimensions land on the coarse Z axis, where supports touch (support contact scars the surface it touches, degrading its accuracy and finish), and which direction the layers run relative to load. Vertically printed holes on FDM commonly come out undersized — a well-known effect shops routinely design around, if they know the hole matters.
The fix is not technical. It is communication: tell the provider which features are critical and how the part is used. A shop that asks you about orientation and load before quoting is the one to trust.
How to Specify Tolerance — and Why You Should
Sending a bare STL and hoping is the most common failure mode in this entire category. An STL is tessellated geometry with no dimensional intent — no datums, no callouts, nothing that says which feature matters. Send a STEP file plus a 2D drawing (or a model with embedded PMI). See STL vs STEP files for why the format matters.
On that drawing:
- Call out only the critical few. Mating faces, bores, bearing seats, sealing surfaces, hole positions. A tight blanket tolerance on every dimension inflates the quote, may trigger post-machining you did not need, and buries the features that actually matter in noise.
- Use datums. "±0.1 mm" means little without a reference the measurement is taken from.
- State the fit, not just the number. "This shaft must slide freely in this bore" gives a provider more to work with than a lone tolerance value.
- State material, environment, and load. These drive shrinkage compensation and orientation.
- Say what happens next. A part destined for post-machining should be quoted with stock left on the critical faces — say so up front.
How to prepare your files for a 3D printing quote covers the rest of the package.
When to Ask for a First Article or Inspection
Ask when a dimension is genuinely critical, when the part enters a production run, or when a failure is expensive. A first article inspection means the shop produces one part and measures your called-out dimensions against the drawing before committing to the rest of the run.
Practical asks:
- A dimensional or CMM report on your critical dimensions — not the whole part.
- Material lot traceability if the application requires it.
- An explicit answer to "what happens if a part is out of tolerance?" A reputable shop reprints at their cost when the failure is theirs. A vague answer is a warning sign.
For a one-off visual prototype this is overhead you do not need. For a hundred end-use brackets, skipping it is how you find out too late. Industrial 3D printing services: a buyer's guide goes deeper on certifications and QA.
Designing Clearances for Fits
The honest guidance here is uncomfortable but correct: the right clearance depends on your process, machine, material, and part size, and anyone who gives you one universal number is guessing on your behalf.
What holds generally:
- Design in clearance deliberately. Two mating parts modeled at identical nominal dimensions will not fit. Ever. In any process.
- Looser processes need more clearance. FDM sliding fits typically need a few tenths of a millimetre; SLA and SLS commonly need less. These are starting points to test, not values to trust.
- Print a test coupon. The only method that actually works: one small part carrying the same feature at several clearance steps, printed on the machine that will make your part, in your material. It costs one cheap print and converts guesswork into a measurement.
- Ask the provider. They already know their machines' numbers. Use that instead of rediscovering it.
- Design the precision out where you can. Heat-set inserts, pressed bearings, and dowel pins bring their own precision surfaces. Let hardware hold the tolerance the print cannot.
When Printing Can't Hold It
Sometimes the requirement is simply outside what the process does. Three real options:
- Print and post-machine the critical features only. Ream the bore, face the sealing surface, tap the thread — usually cheaper than machining the entire part, and standard practice on metal 3D printed parts.
- Machine the part instead. If tight fits and finish drive the design, CNC machining vs 3D printing lays out the crossover.
- Redesign the fit. Adjustment slots, compliant features, or hardware that supplies the precision surface.
Describe the requirement to a provider and most will tell you which is cheapest — including when the answer is "don't print this."
Get a Quote With Your Tolerances Specified
Accuracy is not something you hope for after the fact. It is something you specify before the part is made, to a shop that runs the right process and will commit to a number.
Send a STEP file, a drawing with the critical few dimensions called out, the material, the quantity, and one sentence on how the part is used. Then ask what tolerance they will hold on your geometry — and what happens if they miss it.
Get a quote from vetted 3D printing providers →
Every quote request is captured on-platform and routed to shops that fit your part — you submit once and we match it. Run a shop that holds tolerance and issues inspection reports? Claim your listing to start receiving quote requests.
Related Resources
- CNC Machining vs 3D Printing — when the tolerance requirement decides the process
- Functional and End-Use 3D Printed Parts — designing parts that survive real loads
- Industrial 3D Printing Services: A Buyer's Guide — certifications, QA, and writing an RFQ
- STL vs STEP Files for 3D Printing — why format carries dimensional intent
- How to Prepare Your Files for a 3D Printing Quote — the complete quote package
- SLA vs SLS 3D Printing — detail versus durability
- Browse 3D Printing Providers — get quotes from shops that hold tolerance
3D Prototyping Hub connects buyers with vetted 3D printing and manufacturing providers — submit a quote request and we route it to shops that fit your part. Tolerance figures in this article are general industry guidance for comparison only, not specifications; confirm achievable tolerances with your provider for your specific geometry and material.