You may be wondering why you can have individual items with slack in your schedule but then have total slack for the whole project show zero. Is there a way to show on your schedule how many days of slack you have for the entire project? The answer is complicated.

First, some basics. Slack is the available time that a task can be delayed (or extended in Duration) without changing the project Finish date.

Now, here’s how to work with slack in Project 2013. First, we’ll create a brand new Project and look at it in the Detailed Gantt View:

As you can see, the project Start date is Tuesday, November 25, 2014 and the project Finish date is Wednesday, December 10, 2014. The total duration is 12 days. Why? Because Tasks 1, 2 and 3 are on the critical path! Critical path is the path on which if any task is delayed, the whole project will be delayed.

Now let’s extend Duration for Task 3 from three days to eight days:

That changes the Finish date to Wednesday, December 17, 2014. The total duration is 17 days.

Notice that Tasks 4, 5 and 6 have 11 days of Total Slack! So if we extend, for example, duration for Task 4 from one day to seven days, we’ll get:

You can see that Finish date is the same, but Total Slack time for tasks 4 through 6 is now five days.

Task 7 still has 16 days of Total Slack.

So why does the Project Summary Task (Task #0) show Total Slack as zero? Because Total Slack is not the sum of all slacks; it must be zero because on every project there is a critical path, and on the critical path, there is no slack! Because there is no slack, the Total Slack is zero.

Now for the complication. In this example, what is Total Slack? Is it 16 days (because that’s the longest slack on Task 7)? Is it five days (because that’s what it is for Tasks 4 through 6?? Or is it 21 (the sum of those two slacks)? Or is it 31 (the sum of all slacks)? It doesn’t make sense at all to have Total Slack for the entire project to be different than zero.