Why printed parts do not fit first time
A printed part rarely matches the CAD model exactly. Plastic shrinks as it cools, extrusion lines bulge a little past the model outline, holes come out small, and the first layers spread wider than the rest. When two parts have to fit together, those small errors decide whether a pin slides in, jams, or rattles. This calculator handles the two numbers you can control in the model: shrink compensation and fit clearance.
How the calculation works
To correct for shrinkage, scale the dimension up so it shrinks to the size you want:
model dimension = target / (1 − shrink % / 100)
For a hole that receives a shaft or pin, add a clearance based on the fit you need:
hole diameter = shaft diameter + clearance
Apply shrink compensation to the result if your material shrinks noticeably.
Typical clearances and shrinkage
| Fit type | Clearance (diameter) | Use |
|---|---|---|
| Press fit | 0.0–0.1 mm | Bearings, pins that should stay put |
| Snug / slip fit | 0.15–0.25 mm | Parts that slide together by hand and hold position |
| Loose / running fit | 0.3–0.5 mm | Hinges, rotating parts, print-in-place joints |
| Material | Typical shrinkage |
|---|---|
| PLA | 0.2–0.5% |
| PETG | 0.3–0.6% |
| ABS / ASA | 0.5–0.8% |
| Nylon | 1–2% |
These are starting points. Your printer, slicer, flow setting and part geometry move the real numbers, so test before committing to a batch.
Worked example
You need a PLA block 20 mm wide with a hole for an 8 mm steel shaft that should slide in by hand. You use 0.3% shrinkage and 0.2 mm clearance.
- Outer dimension: 20 / (1 − 0.003) = 20.06 mm
- Hole before shrink: 8 + 0.2 = 8.2 mm
- Hole after compensation: 8.2 / 0.997 = 8.22 mm
With PLA the compensation is small, often smaller than your printer's own variation. With nylon at 1.5%, the same 20 mm part would need 20.30 mm, which matters.
Things that change the result
Horizontal and vertical holes
Holes printed vertically (axis pointing up) are made from polygon segments and slightly squeezed by the extrusion lines pressing inward, so they usually come out 0.1 to 0.3 mm undersized. Holes printed horizontally (axis lying flat) sag at the top where the layers bridge over, so they are both undersized and slightly oval. For horizontal holes, add extra clearance, or model a teardrop or flat top so the bridge does not droop into the hole.
Elephant foot
The first few layers get squished against the bed and spread outward. That widens the bottom edge of a part and narrows holes near the bed. Use the slicer's elephant foot compensation, often 0.1 to 0.2 mm, or add a small chamfer to the bottom edges of the model.
Flow rate and line width
Over-extrusion makes every outer surface bigger and every hole smaller. Calibrate flow before tuning fits, otherwise your clearances only work for one spool.
Test with a tolerance gauge
The fastest way to find your real clearances is a test print: a plate with holes at 0.05 or 0.1 mm steps and a matching peg, or a set of pins in slots. Print it in the same material, orientation and settings as the real part. Note which gap gives a press, slip and running fit, and use those numbers for future designs.
Common mistakes
- Using one clearance for every orientation. Horizontal holes need more.
- Scaling the whole model for shrinkage and also adding clearance by guess. Do one controlled change at a time.
- Forgetting the slicer's own compensation settings. XY hole compensation and elephant foot settings stack with your model changes.
- Testing in a different material. PLA results do not transfer to PETG or nylon.
Practical tips
- Design critical holes slightly small and drill or ream them to size for precise fits.
- Add a 0.5 mm chamfer at hole entrances so shafts start straight.
- For press-fit bearings, start at 0.05 mm and test, as printed walls vary.
- Keep a notes file of the fits that worked for each printer and material.