3D printing tolerances: how much clearance a fit needs
FDM printers print holes slightly smaller and pins slightly larger than designed. How much depends on the printer, nozzle, filament, temperature and even the orientation of the hole. That is why a single “add 0.2 mm” rule fails so often. This guide gives practical starting values and shows how to measure the right value for your own printer in ten minutes.
- Typical fit gap
- 0.2–0.4 mm on the diameter (well-tuned printers)
- Press fit
- nominal + 0–0.1 mm
- Sliding / rotating fit
- nominal + 1 × fit gap
- Loose fit
- nominal + ~1.8 × fit gap
- Best practice
- Print a calibration coupon once per printer/material
Starting values for common fits
| Fit | Use | Hole / slot size | With 0.3 mm fit gap |
|---|---|---|---|
| Press fit | Bearings, pins that must not move | nominal + 0.4 × fit gap | +0.12 mm |
| Snug fit | Knobs on shafts, lids that stay on | nominal + 0.4–0.6 × fit gap | +0.12–0.18 mm |
| Sliding fit | Drawers, sliding lids, hinges | nominal + 1 × fit gap | +0.30 mm |
| Rotating fit | Axles, hinge pins | nominal + 1–1.3 × fit gap | +0.30–0.40 mm |
| Loose / drop-in | Holders, cradles, cases | nominal + 1.8–3 × fit gap | +0.55–0.9 mm |
Why holes print undersized
- Curves are approximated by straight segments in the STL (low $fn makes holes smaller).
- The nozzle's extrusion squishes outward; inner perimeters bulge into the hole.
- Horizontal holes sag at the top (use teardrop holes over ~8 mm).
- First-layer squish (“elephant foot”) shrinks holes near the bed — chamfer the bottom edge.
Measure your printer's fit gap
- 01Print a coupon with holes of increasing clearance (e.g. +0.10 to +0.50 mm) and a matching pin.
- 02Try the pin in each hole and pick the first hole where it slides in smoothly without wobbling.
- 03That clearance is your fit gap. Re-test when you change nozzle size or filament type.
- 04Use it everywhere: FitPrint's free calibration page generates the coupon and stores the value for every future design.
Frequently asked questions
What tolerance should I use for 3D printed parts?
For most well-tuned FDM printers a sliding fit needs about 0.2–0.4 mm of extra diameter, a press fit 0–0.1 mm, and a loose drop-in fit 0.5–1 mm. Calibrate with a test coupon for your exact printer and material.
Is tolerance per side or on the diameter?
The values above are on the diameter (total). Per side it is half.
Do PETG and PLA need different tolerances?
Often yes: PETG tends to string and bulge slightly more, so it usually needs a little more clearance than PLA. Calibrate per material.
Related
Updated · Generated parts are a starting point — always test-fit before relying on a part.