Calculate total tolerance stack-up for assemblies using worst case and root sum of squares (RSS) methods.
Last updated April 2026
Worst case assumes every dimension hits its extreme limit simultaneously. This is the most conservative method and guarantees 100% of assemblies will be within the calculated range.
RSS (Root Sum of Squares) assumes tolerances follow a normal distribution and are statistically independent. This predicts that 99.73% of assemblies (3 sigma) will fall within the calculated range. RSS gives tighter predicted tolerances and is appropriate when manufacturing processes are in statistical control.
Tolerance stack-up is the cumulative effect of individual part tolerances in an assembly. When multiple parts are assembled, their dimensional variations combine and can cause the assembly to exceed its functional requirement. Analyzing stack-up before manufacturing prevents costly rework by ensuring all parts fit together even at worst-case dimensional extremes.
Use worst case analysis when 100% interchangeability is required, such as safety-critical aerospace or medical assemblies. Use RSS (root sum of squares) when statistical probability is acceptable, typically for high-volume production where it is unlikely all parts land at their worst limits simultaneously. RSS gives roughly 30-40% tighter stack-up than worst case for the same input tolerances.