Worst-case tolerance analysis is the traditional type of tolerance stackup calculation. The individual dimensions are set at their tolerance limits to make the stackup measurement as large or as small as possible.

The worst-case model does not consider the statistical distribution of the individual dimensions, but rather that those dimensions do not exceed their respective specified tolerance limits. Mathematically, the model assumes that all the tolerance dimensions will equal one of their limit values, which yields an extreme stackup condition. In other words, this model predicts the maximum expected variation of the measurement.

Designing to worst-case tolerance requirements guarantees 100 percent of the parts will assemble and function properly, regardless of the actual component dimensional variation and stackup combination. The major drawback is that the worst-case model often requires very tight individual component tolerances. The obvious result is expensive manufacturing and inspection processes and/or high scrap rates.

Worst-case tolerancing is often required for critical mechanical interfaces and spare part replacement interfaces. When worst-case tolerancing is not a contract requirement, properly applied statistical tolerancing can ensure acceptable assembly yields with increased component tolerances and lower fabrication costs.