How to Calculate Total Slack
If you're managing a project, you may have to allow for the prospect of your team falling behind schedule. The total slack for a project task measures how late your team can finish the task without affecting the project's completion date. If the slack is zero, the task is critical, and any delay will make the project miss deadline.
To calculate total slack, you first identify the earliest point at which you can start and finish the task, assuming nothing happens to delay you. Then identify the latest dates at which you can start and finish the task without making the project overdue. Next, do two calculations -- late start minus early start and late finish minus early finish. Whichever figure is smaller gives you the total slack in the project. If early start is, for example, June 10 and late start is June 25, you have 15 days of slack.