Enter the upper specification limit, lower specification limit, standard deviation, and mean to determine the Ppk.

Ppk Formula

The following formula is used to calculate a process performance index:

Ppk = min ( (USL – X)/ 3*STD , (X – LSL)/3*STD )

  • Where Ppk is the process performance index (it is equal to the value of whichever value is smaller)
  • USL is the upper specification limit
  • LSL is the lower specification limit
  • X is the mean
  • STD is the standard deviation

Ppk Definition

What is Ppk?

Ppk, short for process performance index, is a metric that describes how well a system is meeting the specifications that it was designed to hit.

Example Problem

How to calculate Ppk?

First, determine the upper specification limit. For this example, we will be looking at the manufacturing of a product with specific dimensions. The critical dimension has an upper limit of 5 inches.

Next, determine the lower specification limit. In this example, the lower limit is 4.5 inches.

Next, determine the mean of the dimension in a lot of products. In this case, the mean is 4.78 inches.

Next, determine the standard deviation. The products have a standard deviation of .1 inches.

Finally, calculate the Ppk using the formula above:

Ppk = min ( (USL – X)/ 3*STD , (X – LSL)/3*STD )

Ppk = min ( (5 – 4.78)/ (3*.1) , (4.78 – 4.5)/3*.1 )

Ppk = .73