Upper and lower specification limits (USL and LSL) are important measurements in process control. They measure the low and high limits acceptable within a process, particularly to customers. Fortunately, these are both easy to work with in spreadsheets. Let’s learn how to calculate USL and LSL in Microsoft Excel.
How to Calculate USL and LSL in Excel
LSL values are typically defined by a customer. They represent the lowest acceptable level in the range of values within specification for a given process. USL values, then, represent the highest acceptable level.
With LSL as a given value, you can utilize an Excel formula to derive the USL for a process. The following inputs are needed:
- The LSL (already given)
- The Capability Index for the process
- The Standard Deviation for the process
The capability index and standard deviation values can be found in two ways: either through the use of Excel formulas, or they can already be given. For this tutorial, we’ll assume the values have already been calculated.
To calculate the USL value, type each of the three required inputs into separate cells. For example, we’ll place the LSL (60) in cell B1, the capability index (2.25) in B2, and the standard deviation (1.5) in B3.
The formula for USL is as follows:
(Capability Index * Standard Deviation * 6) + Lower Specification Limit
This isn’t a built-in Excel function, but it’s easy to build as a standalone formula. Click into an empty cell and enter the following formula:
Hit Enter, and Excel will automatically calculate the USL of your process: 80.25.
As you can see, it’s easy to calculate USL and LSL values in Excel, using a simple formula just like this. By doing so, you can help set the parameters of a process to ensure that it always remains in control.