Tolerance Stack-Up Calculator

A tolerance stack-up is the cumulative effect of individual part tolerances in an assembly. When multiple parts fit together, each part's tolerance adds up. The total variation determines whether the assembly will function correctly or if parts may interfere or have excessive gaps.

Worst-case analysis assumes every dimension is at its maximum or minimum tolerance simultaneously. This gives the largest possible variation but is very conservative. RSS (root sum square) analysis uses statistics and assumes tolerances follow a normal distribution. RSS gives a more realistic estimate because it is unlikely all dimensions will be at their extremes at the same time.

Use worst-case analysis for safety-critical assemblies where every unit must function, such as aerospace and medical devices. Use RSS analysis for high-volume production where a small percentage of out-of-spec assemblies is acceptable and can be caught by inspection. RSS analysis typically results in looser individual tolerances, which reduces manufacturing cost.

Cp measures process capability by comparing the specification width to the process spread (6 sigma). A Cp of 1.0 means the process just fills the spec range. Cpk also accounts for how centered the process is within the spec. A Cpk of 1.33 or higher is generally considered capable. Higher values mean fewer defects.

Include every dimension in the tolerance chain that contributes to the critical assembly gap or fit. Start at a reference surface and trace a path through each part and gap to the feature you are analyzing. Each dimension along that path is one entry in the stack-up. Typically 3 to 10 dimensions are common, but complex assemblies can have more.