What is a tolerance stack?

A tolerance stack is the cumulative effect of individual part tolerances in an assembly. When multiple parts fit together, each part's dimensional variation adds up, potentially causing the assembly to be too tight or too loose.

What is the difference between worst case and statistical analysis?

Worst case assumes every part is at its maximum tolerance simultaneously, which is extremely unlikely. Statistical (RSS) analysis uses root sum of squares to calculate a more realistic expected variation, typically at a 3-sigma confidence level covering 99.7% of assemblies.

What tolerances should I use for 3D printed parts?

A well-calibrated FDM printer typically achieves plus or minus 0.1-0.2mm on XY dimensions and plus or minus 0.05-0.1mm on Z. SLA printers achieve plus or minus 0.05-0.1mm. Use your measured printer accuracy for best results.

How do I reduce tolerance stack-up?

Design datum surfaces so parts reference from common features. Reduce the number of parts in the tolerance chain. Use locating pins or alignment features. Print critical mating features in the same orientation for consistent shrinkage.

3DPrintCalcs

Tolerance Stack Calculator — Multi-Part Assembly

Calculate cumulative tolerance for multi-part 3D printed assemblies using worst case and statistical (RSS) methods.

Number of Parts

Parts in the tolerance chain (2-5)

Nominal Gap

Design gap between mating parts

Part 1 Tolerance (+/-)

Dimensional tolerance for part 1

Part 2 Tolerance (+/-)

Dimensional tolerance for part 2

Part 3 Tolerance (+/-)

Dimensional tolerance for part 3

Analysis Method

Choose which analysis methods to display

How We Calculate This

Formulas

Worst Case = T1 + T2 + T3 + ... + Tn
Statistical (RSS) = k/3 x sqrt(T1 squared + T2 squared + ... + Tn squared)
Min gap = Nominal gap - Total tolerance
Max gap = Nominal gap + Total tolerance

The RSS method assumes each tolerance follows a normal distribution. The confidence multiplier (k) determines the percentage of assemblies that will fit: k=3 covers 99.7%.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Always verify settings with test prints before committing to final prints.