3D Printing Tolerances: How Accurate Are FDM, SLA, SLS and MJF?

“How accurate will my part be?” is one of the first questions engineers ask before ordering a 3D printed part. The honest answer depends on the process, the material, the size of the part and the feature you are measuring. This guide explains the typical tolerances of FDM, SLA, SLS and MJF, why parts move away from their CAD size, and how to design so your parts fit the first time.

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Typical tolerances by process

Print services usually express tolerance as a percentage of the dimension with a minimum value. For example, “±0.3% (minimum ±0.3 mm)” means a 50 mm feature can vary by about ±0.3 mm, while a 300 mm feature can vary by about ±0.9 mm. The table shows common industry values for well-designed parts.

Process	Typical tolerance	Best for
FDM (desktop)	±0.5% (minimum ±0.5 mm)	Low-cost prototypes, jigs, large simple parts
FDM (industrial)	±0.15% (minimum ±0.2 mm)	Functional parts in engineering plastics
SLA	±0.15–0.5% (minimum about ±0.1 mm)	Fine detail, smooth surfaces, small precise parts
SLS	±0.3% (minimum ±0.3 mm)	Strong nylon parts with complex shapes
MJF	±0.3% (minimum about ±0.2–0.3 mm)	Repeatable nylon parts and small production runs

These are general guidelines, not guarantees. If a dimension is critical, mark it on a drawing and ask your print service to confirm before production.

Why printed parts are not exactly the CAD size

Shrinkage and warping

Every process heats or cures material, and materials shrink as they cool or cure. Printers compensate with scaling, but the shrink is never perfectly even. Large flat parts and thick sections next to thin ones move the most.

Layer lines and the stair-step effect

Parts are built in layers, typically 0.1–0.3 mm thick in FDM and 0.025–0.1 mm in SLA. Curved and sloped surfaces become small steps, which slightly change their measured size, especially along the vertical (Z) axis.

Holes come out small

In FDM, the extruded plastic is squeezed slightly outward on each layer, so internal features like holes and slots usually come out undersized. Outer dimensions tend to be slightly oversized for the same reason.

Post-processing

Support removal, sanding, bead blasting and dyeing all remove or add a little material. Smooth-finish options are great for appearance but can affect tight fits.

How to design for accurate fits

• Add clearance to mating parts. As a starting point, leave 0.3–0.5 mm per side in FDM, about 0.2 mm in SLA and 0.4–0.5 mm in SLS and MJF.
• Design critical holes undersize and finish them. Print a hole 0.2–0.3 mm small, then drill or ream it to the exact size. This is the most reliable way to get a press fit or bearing seat.
• Use inserts for threads. Heat-set brass inserts give accurate, reusable threads in plastic parts.
• Keep important faces flat on the build plate in FDM, where accuracy is best in the X and Y directions.
• Avoid long, thin, flat parts when flatness matters. Add ribs to stiffen them instead.
• Print a test piece. For a new snap-fit or press-fit, print a small test coupon with a few clearance options before ordering the full batch.

Which process should you choose for tight tolerances?

For small parts with fine detail, SLA usually gives the best accuracy and surface finish. For functional nylon parts where repeatability across many copies matters, MJF is a strong choice. SLS offers similar accuracy with more material options. FDM is the most affordable, and with good design and a little post-machining it handles many engineering fits well.

When a part needs tolerances tighter than about ±0.1 mm across the whole part, consider printing it close to size and machining the critical features, or switching to CNC machining for that part.

Next steps

Check your design against our 3D printing design guidelines, compare processes in FDM vs SLS vs SLA vs MJF, and upload your CAD file for an instant quote. Add a note about any critical dimensions when you order.