Calculate Cpk Using Excel

Evaluate your process capability and verify your Excel formulas instantly.

Calculation Results

Cp (Potential Capability):

CpK (Process Capability):

Cpu (Upper):

Cpl (Lower):

Understanding CpK and How to Calculate it in Excel

CpK, or the Process Capability Index, is a statistical tool used in manufacturing and quality control to measure how well a process can produce output within specified limits. Unlike Cp, which only looks at the spread of the process, CpK takes the centering of the process into account.

The Mathematical Formula

To calculate CpK manually or in a spreadsheet, you must first determine the capability of the upper and lower limits separately:

• Cpu = (USL – Mean) / (3 * Standard Deviation)
• Cpl = (Mean – LSL) / (3 * Standard Deviation)
• CpK = Min(Cpu, Cpl)

Step-by-Step: Calculate CpK Using Excel

If you have a dataset in column A (A2:A101), follow these steps to calculate CpK in Excel:

1. Calculate the Mean: Use the formula =AVERAGE(A2:A101). Let's say this is in cell C1.
2. Calculate Standard Deviation: Use =STDEV.S(A2:A101) for a sample. Let's say this is in cell C2.
3. Define Limits: Enter your USL in C3 and LSL in C4.
4. Calculate Cpu: In cell C5, enter =(C3-C1)/(3*C2).
5. Calculate Cpl: In cell C6, enter =(C1-C4)/(3*C2).
6. Final CpK: In cell C7, enter =MIN(C5, C6).

What do the numbers mean?

A "capable" process typically requires a CpK value of at least 1.33. In high-precision industries like aerospace or pharmaceuticals, a CpK of 1.67 or higher is often required to ensure Six Sigma quality levels.

CpK Value	Interpretation
< 1.0	Process is not capable. Significant defects expected.
1.0 – 1.33	Marginally capable; requires close monitoring.
1.33 – 1.67	Good capability; process meets industry standards.
> 1.67	Excellent capability (Six Sigma level).

function calculateCpK() { var usl = parseFloat(document.getElementById('usl_input').value); var lsl = parseFloat(document.getElementById('lsl_input').value); var mean = parseFloat(document.getElementById('mean_input').value); var sd = parseFloat(document.getElementById('sd_input').value); if (isNaN(usl) || isNaN(lsl) || isNaN(mean) || isNaN(sd) || sd <= 0) { alert("Please enter valid numeric values. Standard deviation must be greater than zero."); return; } if (usl = 1.67) { interpretationBox.innerText = "Status: Excellent Capability. Your process is highly centered and well within limits."; interpretationBox.style.color = "#27ae60"; } else if (cpk >= 1.33) { interpretationBox.innerText = "Status: Capable. Your process meets standard quality requirements."; interpretationBox.style.color = "#2980b9"; } else if (cpk >= 1.0) { interpretationBox.innerText = "Status: Marginally Capable. The process is prone to producing defects if it shifts."; interpretationBox.style.color = "#f39c12"; } else { interpretationBox.innerText = "Status: Not Capable. Immediate process improvement or adjustment is required."; interpretationBox.style.color = "#c0392b"; } document.getElementById('results_area').style.display = 'block'; }