A CAD model is dimensionally perfect. A printed part never is. The gap between the two is tolerance, and it’s the most common reason a first-run 3D printed part doesn’t fit the way it did on screen. That gap moves depending on which additive manufacturing process printed the part, and by how much depends on which one you picked.
Why tolerance varies this much by process
Tolerance in 3D printing comes from how the process builds material, so there’s no single accuracy spec you can look up once. FDM tolerance is limited by nozzle diameter and layer adhesion between extruded lines. SLA tolerance comes from laser or projector resolution curing a liquid resin in very thin layers, which is why it holds tighter features than an extrusion-based process. SLS and MJF sinter powder without support structures, which removes support-related distortion but introduces its own shrinkage and thermal behavior. Each process is accurate in its own way, for its own reasons, and design decisions need to account for which one is running the job.
What each process actually holds
Independent process testing, including Formlabs’ published accuracy and tolerance guidance, puts general-purpose FDM in the ±0.2 to 0.5 mm range depending on part size and printer quality. SLA holds meaningfully tighter, commonly ±0.05 to 0.15 mm on small to mid-size features, with tolerance widening slightly as feature size increases. SLS and MJF typically land around ±0.3 mm or a percentage of part dimension, whichever is larger, with no support-induced warping since the powder bed supports the part during the build. DMLS for metal holds tighter dimensional control than the polymer processes but usually needs a post-machining pass on critical features to hit final tolerance. If you’re weighing metal printing in-house, our breakdown of metal 3D printer vs. printing service runs the numbers.
Tolerance and minimum feature size are separate limits
Designers sometimes conflate the smallest wall a process can print with how accurately it holds a dimension. A process can print a thin wall and still miss the target dimension on that wall by a meaningful margin. Both numbers matter for a working design, and they don’t move together. Checking one without the other is how a feature prints successfully and still doesn’t function.
Design for fit, beyond the CAD number
Mating parts need clearance built in, beyond accurate dimensions on each half. A shaft and bore that are each within tolerance can still bind or rattle if the clearance between them wasn’t designed around the process’s tolerance range. As a starting point, snap fits and sliding fits in FDM generally need more built-in clearance than the same features in SLA or SLS, because FDM’s tolerance band is wider. Press fits are workable in the tighter-tolerance processes but a poor match for FDM, where dimensional variation makes a reliable interference fit hard to hit. Treat these as starting points to verify against the process, material, and printer running the job. Our post on living hinge design covers another feature where printed tolerance matters.
How the industry tests this
The additive manufacturing industry has a standardized way to measure tolerance: ISO/ASTM 52902 defines benchmark test geometries used to assess the geometric capability of an AM system under controlled conditions. When a shop’s tolerance numbers trace back to this kind of standardized testing rather than a best-case sample, that’s a more reliable number to design around.
When printed tolerance isn’t tight enough
Some features need tighter control than any additive process delivers as printed. That’s a design decision point, and forcing a printed part into a role it can’t hold only moves the problem downstream. A secondary machining operation on critical features, moving the tight feature to a mating part made another way, or switching to CNC machining or injection molding are all legitimate answers. Which one is right depends on the part and the quantity, and our guide to choosing a prototype process for 1, 10, or 100 parts walks through that call.
Send us the drawings and our team will walk through which process holds the tolerance your design needs, and where a print alone won’t get you there.
Frequently asked questions
For polymers, SLA typically holds the tightest tolerances, commonly ±0.05 to 0.15 mm on small to mid-size features. DMLS offers good dimensional control in metal, though critical features usually get a finish-machining pass.
General-purpose FDM typically lands in the ±0.2 to 0.5 mm range, depending on part size, material, and printer quality.
It depends on the process. FDM fits need more clearance than SLA or SLS because its tolerance band is wider. Start from the process’s published range, then verify with a test print before committing.
Yes. Secondary machining on critical features like bores, sealing faces, and threads is common, especially on metal DMLS parts.
