Cp Index Calculator

Calculator

Lower Spec Limit (LSL)

Minimum acceptable value.

Upper Spec Limit (USL)

Maximum acceptable value.

Sigma source

Pick the best method for your shop floor data.

Sigma (σ)

Use within-process σ when available.

Process mean (x̄)

If blank, only Cp is shown.

Measurements

Example: 10.01, 9.99, 10.02 ...

Subgroup size (n)

Matches how ranges were collected.

Subgroup ranges

Each range = max − min of one subgroup.

Target Cp

Used for pass/fail labeling.

Decimal places

Controls display rounding.

Tip: Cp measures potential capability. If the process is off-center, Cpk will be lower than Cp.

Example data table

Scenario	LSL	USL	Measurements (10)	Mean	Std dev (s)	Cp
Machined shaft diameter	9.90	10.10	10.01, 9.99, 10.02, 10.00, 9.98, 10.03, 10.01, 10.00, 9.97, 10.02	10.0030	0.0189	1.7650

Values are illustrative. Your Cp depends on the sigma method you choose.

Formula used

Cp = (USL − LSL) ÷ (6σ)
CPU = (USL − x̄) ÷ (3σ)
CPL = (x̄ − LSL) ÷ (3σ)
Cpk = min(CPU, CPL)
Range estimate: σ ≈ R̄ ÷ d2 (with subgroup size n)
PPM estimate (optional): assume normal distribution using x̄ and σ

How to use this calculator

Enter your LSL and USL from the drawing or control plan.
Select how you will provide sigma: measurements, direct sigma, or ranges.
If you want Cpk and PPM, provide a process mean or use measurements.
Click Calculate Cp to see results above the form.
Use CSV/PDF export to share results with your team.

FAQs

1) What does Cp measure?

Cp measures potential capability by comparing specification width to process variation (6σ). It assumes the process is stable and uses an appropriate estimate of within-process sigma.

2) What is a good Cp target?

Common targets are 1.33 for capable and 1.67 for critical characteristics. Targets vary by industry, customer, and risk. Always follow your quality plan or customer requirements.

3) Why can Cp be high but Cpk low?

Cp ignores centering. If the mean is closer to one spec limit, Cpk drops because one side has less margin. Centering the process often improves Cpk without changing variation.

4) Which sigma should I use?

Use a within-process estimate when evaluating short-term capability. For longer horizons, variation may grow due to shifts or drift. Ensure your method matches the purpose of the study.

5) What does “ranges” mode estimate?

Ranges mode estimates sigma from subgroup ranges using σ ≈ R̄/d2. It is useful when you collect subgroups on the shop floor and want a quick within-process variation estimate.

6) How many measurements should I enter?

More is better if the process is stable. As a starting point, 25–30 observations often provide a more reliable sigma estimate. Use a rational sampling plan that reflects process behavior.

7) Are the PPM numbers always accurate?

PPM is an estimate assuming a normal distribution and stable mean and sigma. If data are skewed, non-normal, or unstable, the estimate can be misleading. Validate with actual defect data.

8) What should I do if Cp is below target?

First verify measurement system and process stability. Then reduce variation by addressing key sources, tooling wear, and setup practices. If needed, review specs and process design with engineering.