3D Printing Tolerancing Guide

How it works

Printed parts rarely come out at exactly the size you modeled. The printer’s accuracy scatters each dimension, shrinkage makes material parts smaller as they cool, and small holes tend to print undersized. To get a fit that works, you add a clearance between the two parts on top of the nominal size.

Here c is the total clearance for the type of fit and process, and share is the part of it applied to the hole (1 means the shaft stays at nominal). The scatter is the process’s usual size tolerance, taken as the larger of a fixed floor and a percentage of the size. Two parts each scatter by t, so the clearance between them varies by twice that.

Worked example

A 20 mm pin in a sliding fit, printed in FDM with PLA. The base clearance for an FDM sliding fit is 0.25 mm. The scale for PLA at 0.3% shrinkage is 1 ÷ (1 − 0.003) = 1.0030. With the clearance on the hole, the hole is modeled at (20 + 0.25) × 1.0030 = 20.31 mm and the shaft at 20 × 1.0030 = 20.06 mm. FDM scatter at 20 mm is the larger of 0.2 mm and 0.5% of 20 mm (0.1 mm), so ±0.2 mm per part. The actual clearance can therefore range from 0.25 − 0.4 = -0.15 mm (binding) to 0.25 + 0.4 = 0.65 mm (sloppy). That wide spread is why a test print and a measured correction are worth it for a good fit.

Things to know

These are starting values, not guarantees. Every printer, slicer profile, nozzle and filament batch is a little different. Print a small test piece with several clearances (0.1, 0.2, 0.3 mm …) and pick the one that works on your machine.

Holes print small. Round holes in FDM usually come out 0.1–0.3 mm under size because the nozzle path cuts the corners. The clearances above already assume that, but very small holes (under about 3 mm) often need extra and are best drilled or reamed after printing.

Orientation matters. Z (layer) dimensions and X/Y dimensions have different accuracy. Put critical fits in the XY plane where you can, and avoid circular holes that lie along the build direction without support.

Shrinkage is not the same on every axis, and it depends on temperature, enclosure and part size. Scale only if your own test prints show a consistent error, and avoid applying shrinkage twice if your slicer already compensates.

Resin parts need post-curing, which can change size slightly. Fit-check after the final cure. Wash well so uncured resin does not stay in small gaps.

SLS powder can get trapped in small clearances, so moving parts need larger gaps (or drain holes) to print as working assemblies. Many suppliers compensate shrinkage for you.

Press fits and threads. Printed threads are rough; use heat-set inserts or tap the holes. For press fits, test with the real parts, because plastic creeps and loses grip over time.

Common questions

What clearance should I use between two printed parts?

For FDM, about 0.2 to 0.3 mm total for a sliding fit and 0.4 to 0.5 mm for parts that must turn freely is a common starting point. Resin can go tighter. Always confirm with a test print.

Do I need to apply shrinkage compensation?

Often not for PLA, where shrinkage is small. For ABS, ASA and nylon it matters on large parts. Check the printed result against the model first, and only scale if the error is consistent.

Why does the real clearance have such a wide range?

Each printed part can be off by the process’s tolerance in either direction. With two parts, the errors add up. A tighter process (resin) or a calibrated printer narrows the spread.

What is the difference between a press fit and a snug fit?

A press fit has no clearance, or even interference, so the parts are forced together and held by friction. A snug (locating) fit slides together by hand but keeps the parts aligned without much play.

Sources

• Typical accuracy and wall thickness figures from printer and service-bureau design guides (Formlabs, Protolabs, Xometry).
• ISO/ASTM 52900, Additive manufacturing: general principles and terminology.

Last reviewed: October 22, 2026