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