Cp
Also known as: Process Capability Ratio
Definition
Cp is a process capability ratio that measures the spread of a process relative to its specification width, without accounting for centring; values above 1.33 indicate the process is narrow enough to fit within spec.
Formula
Cp = (USL − LSL) / (6σ̂_within)In depth
Cp tells you whether the process is precise enough in principle; it ignores where the mean sits. A Cp of 2.00 means the spec is twice as wide as the process spread, but the process could still produce defects if it's not centred.
The difference between Cp and Cpk reveals centring: if Cp ≫ Cpk, the process is precise but offset from the spec midpoint — tooling adjustment, fixturing, or set-up procedure are common culprits.
Report Cp alongside Cpk: Cp is the ceiling Cpk could reach if the process were perfectly centred.