Why 3D printed holes come out small
Updated 2026-10-19
A hole modelled at 5.0 mm rarely prints at 5.0 mm. It almost always comes out smaller, and the amount is consistent for a given printer, material and slicer profile. Measure it once and you can correct every hole after that.
Why holes shrink
Facets. An STL stores a circle as straight segments whose corners touch the true circle, so the flats sit inside it. For a 5 mm hole exported with 32 segments the flats are 0.02 mm smaller across; with a coarse 16-segment export it is 0.10 mm. Export curved surfaces at a fine resolution and this mostly goes away.
Extrusion. The bigger cause is plastic: too much flow or too wide a line pushes material into the hole, and tight arcs shrink as they cool. Calibrating flow helps but rarely removes it completely (3D Printerly).
Measure once
Print a small test plate with holes at the sizes you use, say the tap drill and clearance hole for your common screws. Measure each with calipers. The difference between modelled and measured diameter is your offset. Every tap drill size page has a box where you enter it and get the diameters to model.
Compensate in the slicer
- Cura: Hole Horizontal Expansion grows holes only.
- PrusaSlicer: XY Size Compensation changes the whole part’s outline, not just holes, so it is no substitute for hole-only compensation (Prusa forum).
- OrcaSlicer and Bambu Studio: both inherited a hole-only X-Y compensation setting from SuperSlicer.
Modelling the offset into the CAD file works in every slicer and travels with the part.
Tapping into a print
Model the hole at the tap drill diameter plus your offset, then run a hand tap through slowly, backing off to clear chips. Printed threads wear quickly if the screw goes in and out often; for those joints use a brass heat-set insert instead.
Heat-set insert holes
Brands size their knurls differently, so model the hole their listing gives. Side by side with the tap drill:
| Thread | Tap drill | CNC Kitchen | ruthex |
|---|---|---|---|
| M2 | 1.60 mm | 3.2 mm | 3.2 mm |
| M2.5 | 2.05 mm | 4.0 mm | – |
| M3 | 2.50 mm | 4.0 mm | 4.0 mm |
| M4 | 3.30 mm | 5.6 mm | 5.6 mm |
| M5 | 4.20 mm | 6.4 mm | 6.4 mm |
| M6 | 5.00 mm | 8.0 mm | – |
| M8 | 6.80 mm | 9.7 mm | – |
Other brands, inch inserts and minimum wall thickness are on each size page.