Cp/Cpk Process Capability Calculator
Calculate Cp, Cpk, CPU and CPL from specification limits, process mean and standard deviation with stability/normality cautions.
Calculate Cp and Cpk process capability
Compare process spread and centering with lower and upper specification limits.
Cp and Cpk formulas
NIST gives the common normal-process capability relationships Cp = (USL − LSL) ÷ 6s and Cpk = min[(USL − mean) ÷ 3s, (mean − LSL) ÷ 3s]. The one-sided components are often called CPU and CPL. NIST process capability guidance.
Cp versus Cpk
Cp compares specification width with six estimated standard deviations and therefore describes potential capability if the process is centered. Cpk also accounts for where the mean sits between the limits. If Cpk is materially lower than Cp, centering is reducing capability.
Do not calculate capability on an unstable process
A single Cp/Cpk number cannot repair poor sampling or special-cause variation. Check process stability using appropriate control methods and inspect the distribution. NIST notes that capability estimates need adequate independent data and that normal-based indices are not automatically valid for non-normal processes.
No universal acceptance cutoff
Values such as 1.00, 1.33 or 1.67 are used in different industries and contracts, but the required capability is a business/customer/quality-system decision. Do not label a process “acceptable” from a generic internet threshold; compare the result with the requirement governing that characteristic.