3D Printing Tolerances Explained

Understanding the dimensional accuracy of FDM 3D printing prevents disappointment and helps you design parts that actually fit and function as intended. This guide covers what tolerances to expect, what affects them, and how to design around them.

What tolerance can I expect?

On our Bambu Lab machines printing PETG, you can typically expect:

• XY plane (parallel to build plate): ±0.1–0.2 mm for most features
• Z axis (layer height direction): ±0.1–0.2 mm for external dimensions; internal depths may vary more
• Holes: Printed holes typically come out 0.1–0.3 mm undersize due to material flow and cooling
• Fine detail layer height (0.12 mm): Improves Z accuracy and surface finish; XY accuracy is similar

These are typical values for good-condition printers well-calibrated for the material. They're adequate for most functional applications — brackets, housings, jigs, fixtures — but require design consideration for tight-fitting components.

What affects tolerance?

Part geometry

Large flat parts are more prone to warping, particularly in PETG without a heated enclosure. This causes dimensional variation at the edges of flat parts more than at the centre. Avoid very flat, large parts without some structural geometry (ribs, chamfers) to resist warping.

Feature size

Very small features (under 1 mm) are affected more by nozzle diameter and layer height than large features. Holes smaller than about 3 mm diameter may need drilling to final size. Pins or bosses under 2 mm diameter may not print accurately enough to engage reliably with mating parts.

Print orientation

Features perpendicular to the build plate have slightly different tolerance behaviour than features parallel to it. Holes printed with their axis vertical (parallel to Z) are typically more accurate than holes printed horizontally, because each layer is a clean circle rather than a stairstepped approximation.

Designing for tolerance

Clearance fits (parts that slide together)

For parts that need to fit inside each other with sliding clearance, add at least 0.3–0.5 mm total clearance (0.15–0.25 mm per side). More if you need easy hand-fitting; less if you want a snug fit that stays in place.

Interference fits (press fits)

Press fits are difficult in PETG — the material is somewhat elastic and print tolerances make consistent interference dimensions hard to achieve. Use fasteners or heat-set inserts for connections that need retention rather than relying on interference.

Holes for fasteners

Design holes 0.3–0.5 mm larger than the nominal bolt diameter for clearance holes. For tapped holes, print slightly undersized and tap with a metal tap for best thread quality. For through-bolted connections, drill the printed hole to final size after printing for accuracy.

Heat-set inserts

Heat-set brass inserts solve most fastener tolerance problems. Design the hole to match the insert manufacturer's specification (usually a few tenths of a mm undersize), heat the insert with a soldering iron, and press it in. The result is a metal thread with consistent dimensions that doesn't strip under repeated assembly and disassembly.

When to test first

Test a single print before committing to a batch when:

• Your part has close-tolerance mating features (holes, bosses, slots)
• The part needs to fit precisely to an existing component
• You're designing press-fit or snap-fit features
• You have more than about 10 units in your batch (the economics of testing become compelling)

Our quote tool makes single-unit test prints easy and cost-effective. It's almost always worth it for precision applications.