Project Slack is amount of time which project tasks can be delayed before affecting next tasks. You may need to define a project slack (also called a project float) in order to identify the Critical Path of your project – a sequence of tasks that have certain leeway for being started earlier or to be delayed. Tasks on a project activity diagram can be characterized by availability of total slack (which is how much a task can slip before it delays the whole project) and free slack (the amount of time that a task can be delayed without postponing its successor tasks).
“Slack” analogy appears because of the project task diagrams: tasks are interconnected there with a help of lines that may look like ropes – tasks with float time can be imaginably “moved” closer to earlier or later tasks – they are not tightly fixed. Calculating project slack is based upon studying all tasks and determining their start & finish dates, durations, predecessor dates, task dependencies, and constraints. Sometimes slack can be a negative number – this indicates that this amount of time must be saved earlier to prevent delay (in other words not enough time was allocated for the task).