FDM dimensional tolerance describes how much a 3D printed part deviates from its designed dimensions due to material shrinkage, thermal contraction, and printer calibration. Understanding expected tolerances lets you design parts that fit correctly on the first print.
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Tolerance Results
FDM Shrinkage Reference by Material
| Material | Shrinkage Range | Typical Tolerance | Design Compensation |
|---|---|---|---|
| PLA | 0.1–0.3% | ±0.2mm | Usually not needed; +0.1mm for precise fits |
| PETG | 0.2–0.4% | ±0.3mm | +0.1–0.2mm for clearance fits |
| ABS | 0.4–0.8% | ±0.5mm | +0.3–0.5mm oversize for functional parts |
| ASA | 0.4–0.7% | ±0.45mm | Similar to ABS; +0.3mm for fits |
| Nylon | 0.4–0.6% | ±0.5mm | Hygroscopic expansion post-print may change fit |
| TPU (flexible) | 0.5–1.0% | ±0.8mm | Flexibility compensates; measure and adjust |
How to Use FDM Tolerance Guide for Accurate Parts
FDM printing is not a precision machining process, but with proper understanding of tolerances and design compensation, you can achieve functional fits for mechanical assemblies.
Design for Your Tolerance Stack
When mating two printed parts, both have tolerances. A 50mm PLA shaft into a 50mm PLA bore: shaft may be 49.8mm (0.2mm under), bore may be 49.8mm (0.2mm under). If you print both to nominal, the shaft won't fit. Design bore 0.4mm oversized (50.4mm) to get a 0.4mm clearance that accounts for both part tolerances.
Print Calibration Before Critical Parts
Print a 20mm calibration cube and measure with calipers. If it's 19.8mm (1% undersized), apply a global XY compensation factor of +0.2mm in your slicer. Most slicers have a "XY size compensation" or "outer contour offset" setting for this. Calibrate once per material — different materials need different compensation values.
FAQ
Is this guide free?
Yes, completely free with no signup required.
How much does PLA shrink when 3D printing?
PLA typically shrinks 0.1-0.3% linearly. For a 100mm dimension, expect 99.7-99.9mm printed (0.1-0.3mm smaller). This is relatively minor and often doesn't affect fit unless you're designing precise mechanical interfaces. Calibrating your printer's flow rate and XY compensation can reduce this further.
Why does ABS warp and shrink so much?
ABS has a large temperature coefficient of thermal expansion — it expands significantly when hot and contracts when cooling. The coefficient is about 7-9×10⁻⁵/°C, much higher than PLA at 6-8×10⁻⁵/°C. Rapid cooling creates internal stresses that cause warping and layer separation. ABS shrinkage (0.4-0.8%) is 2-4× greater than PLA, requiring careful enclosure management and design oversize compensation.
What tolerance should I design for press-fit connections in PETG?
For a snug press-fit in PETG, design the male part to be 0.1-0.2mm larger in diameter than the female socket. PETG has moderate flexibility that allows press-fit without cracking. For clearance fit (rotating or sliding), allow 0.3-0.5mm clearance. Print orientation matters — features perpendicular to layer lines are more accurate than features parallel to layers.
How can I improve dimensional accuracy in FDM printing?
Key accuracy improvements: (1) Calibrate e-steps (extruder steps per mm). (2) Calibrate flow rate with a flow cube test. (3) Print a calibration cube and measure with calipers, then adjust XY scaling factor. (4) Allow proper cooling — fast cooling improves dimensional accuracy but may reduce layer adhesion. (5) Use slower print speeds for critical dimensional features.