How do you calculate Pp?

Divide the tolerance width (USL minus LSL) by six long-term standard deviations. With a 0.3 tolerance band and a 0.27 six-sigma spread, Pp = 0.3 / 0.27 = 1.11.

What is a good Pp value?

A Pp of 1.33 is the common minimum for an established process and 1.67 is expected for safety- or PPAP-critical characteristics. The example value of 1.11 is marginal and would typically require improvement before approval.

What is the difference between Pp and Cp?

Both compare tolerance to six sigma, but Pp uses the long-term overall standard deviation (all variation) while Cp uses the short-term within-subgroup sigma. Pp is almost always the lower, more honest number.

What is the difference between Pp and Ppk?

Pp assumes the process is perfectly centered and only measures spread; Ppk penalizes the process for being off-center. If Pp is much higher than Ppk, your process is capable but mis-centered.

Why is my Pp lower than my Cp?

Long-term (Pp) sigma includes shifts, drift, and between-batch variation that within-subgroup (Cp) sigma excludes, so Pp is expected to be equal to or lower than Cp on a real process.

No. Tolerance width and process spread are both positive, so Pp is always positive. A low but positive value like 1.11 simply signals limited capability.

Quality & Metrology calculator

Pp Calculator

Process performance index (Pp) measures how well a process would fit inside its specification limits if it were perfectly centered, using the actual long-term standard deviation rather than the within-subgroup estimate used for Cp. Quality engineers, PPAP submitters, and Six Sigma practitioners report Pp during production part approval and long-term capability studies because it captures every source of variation the process experienced over the reporting window. A Pp of 1.33 means the tolerance is 33% wider than the spread; anything below 1.0 means the process cannot physically hold the tolerance even when centered. Because Pp ignores centering, it is always read alongside Ppk to separate a spread problem from an off-target problem.

What this calculator does

Estimate process performance Pp by comparing the tolerance width to the long-term process spread for a feature.
Use it when you have long-term variation data and want overall process performance, not just short-term capability.
It divides the total specification tolerance width by the long-term six-sigma process spread to produce the Pp performance ratio.

Formula used

Pp = tolerance width ÷ long-term process spread
Converted Pp = Pp × reporting conversion factor

Inputs explained

Tolerance width (USL minus LSL):
Long-term process spread (6 sigma):
Reporting conversion factor:

How to use the result

Use it for PPAP capability studies, long-term audits, and any time you have enough data to estimate the true overall standard deviation rather than a short-term within-subgroup one.
Pp assumes the data are approximately normal and says nothing about centering — a process can post a high Pp while producing scrap because the mean is shifted toward one limit, which only Ppk reveals.

Current U.S. benchmarks

U.S. manufacturing runs at 75.6% of capacity (Federal Reserve, May 2026). New factory orders are up 2.3% year over year (Census).

Common questions

How do you calculate Pp? Divide the tolerance width (USL minus LSL) by six long-term standard deviations. With a 0.3 tolerance band and a 0.27 six-sigma spread, Pp = 0.3 / 0.27 = 1.11.
What is a good Pp value? A Pp of 1.33 is the common minimum for an established process and 1.67 is expected for safety- or PPAP-critical characteristics. The example value of 1.11 is marginal and would typically require improvement before approval.
What is the difference between Pp and Cp? Both compare tolerance to six sigma, but Pp uses the long-term overall standard deviation (all variation) while Cp uses the short-term within-subgroup sigma. Pp is almost always the lower, more honest number.
What is the difference between Pp and Ppk? Pp assumes the process is perfectly centered and only measures spread; Ppk penalizes the process for being off-center. If Pp is much higher than Ppk, your process is capable but mis-centered.
Why is my Pp lower than my Cp? Long-term (Pp) sigma includes shifts, drift, and between-batch variation that within-subgroup (Cp) sigma excludes, so Pp is expected to be equal to or lower than Cp on a real process.

Last reviewed 2026-05-12.