FDM Tolerances — What to Expect When You're Printing for Fit

The first time you design a part meant to fit onto something else and print it, there's a strong chance it won't fit. Not because your CAD is wrong and not because the printer is broken — but because FDM has dimensional deviations that are normal, predictable, and completely unaccounted for in the default workflow.

Most introductory 3D printing guides skip this topic or mention it briefly. "Account for tolerance" is the advice, with no real explanation of how much or why. So beginners design to nominal dimensions, print, discover the part doesn't fit, iterate two or three times, and eventually develop an intuition they couldn't articulate.

This post is about articulating that intuition.

Why FDM parts deviate from design

Three main sources of dimensional deviation in FDM:

*Filament extrusion*: The extruder pushes a nominal amount of filament, but the actual extrusion width varies slightly. Over-extrusion makes walls slightly wider than intended. Under-extrusion makes them slightly thinner. Most printers are calibrated reasonably well but not perfectly, and calibration drifts.

*Thermal shrinkage*: PLA, PETG, and most FDM materials shrink as they cool from the extrusion temperature. This is a small effect — typically 0.1–0.5% depending on material and geometry — but it's enough to matter for close-tolerance parts. The shrinkage is non-uniform: the outside of a part cools faster than the inside, so there's sometimes slight warping or contraction that's asymmetric.

*Path-to-surface translation*: When the slicer generates the outer wall path, the center of the extrusion bead runs along the theoretical surface. But the bead has width — typically 0.4mm at standard settings — and the effective surface is the outer edge of that bead. Depending on how your slicer handles this, the effective outer dimension may be slightly larger or smaller than nominal, and the effective inner dimension of holes and channels may be smaller than nominal.

Practical numbers

For a well-calibrated printer printing PLA at standard settings:

Outer dimensions (length, width, height of a solid part): typically accurate to within ±0.2mm. Some printers are better. The XY plane is usually more accurate than Z.

Holes and channels: typically 0.1–0.3mm smaller than designed. A 5mm hole will usually measure 4.7–4.9mm. This matters a lot for press-fit applications and clearance fits.

Pins and shafts: typically 0.0–0.1mm larger than designed. A 5mm shaft will usually measure 5.0–5.1mm.

The implication: for a shaft-in-hole clearance fit, design the hole 0.3–0.4mm larger than the shaft. For a press fit, design the hole 0.1mm smaller than the shaft and test.

These are starting points, not rules. Every printer, every filament brand, and every slicer combination behaves slightly differently. The practical approach is to print a calibration test, measure it, and adjust.

What to do

For any functional part where fit matters: design your clearances and tolerances with these effects in mind. Don't use nominal dimensions and hope. Add at least 0.2–0.3mm clearance for sliding fits, more if the geometry is complex.

Print a test piece before committing to a full print. A small test with just the critical fit features — a 10×10mm block with the hole you're concerned about, a pin cross-section — costs almost nothing in material and time and tells you exactly how your specific printer is behaving with your specific filament that day.

Keep notes. Dimensional behaviour is consistent enough across prints on the same printer with the same filament that calibration information from previous prints is useful for future designs. The first time you figure out "my printer's 5mm holes actually come out at 4.75mm," you know what to design in the future.

If you're outsourcing to a print service, ask about their typical dimensional accuracy and account for it the same way. Consistent printers at a print service will have consistent deviations.