How to add a calculated column to show the ‘Average’ in a SharePoint List?

I am using SharePoint online. I know that there are formulas for calculating [column 1], [column 2], etc. type stuff but I have one column “turnaround” that is calculated off of 2 dates. For instance 5/1/2018 and 5/10/2018 would spit out a turnaround of “9” days. Each item in my list will produce it’s own “turnaround” based on a start date and finish date. Because I only have one column of numbers, how can I calculate an average? For instance, lets say the “turnaround” column has 4 different quantities listed, one for each item in the list, (12, 9, 1, and 5). The average of these numbers is 6.75. I personally can do the math easily, but this list is something that will expand to 100+ entries/items. So, how can I take all of the numbers in one column (“turnaround”) and come up with an average? 

To give a little more background, I have a specific task where I work and we tell our clients that there’s a 10 day turn around. There are times, like now, where it is completely dead, and then when there’s a new product release, I’ll get hammered. We want to see at any given time what  the current average turnaround period is so that we can relay that to our clients, mostly when we’re busy and getting behind. So if the turnaround period is, lets say 15 days rather than the standard 10, we can inform the clients to expect a 15 day turnaround based on the current workload.

Thank you for the help.