Slack is the amount of time in a task plan where a task can be delayed without causing another task (or the task plan itself) to be delayed. Slack is also referred to as float. There are two types of slack: free slack and total slack. Free slack is the amount of time that a single task can be delayed before that delay causes another task in the task plan (or the task plan itself) to be delayed. Total slack is the amount of time that one or more tasks in a task plan can be delayed before that delay causes the task plan to be delayed.
In general, it is preferable to have a task plan with some (but not too much) slack present. This allows room in the timeline to adjust to changes in your overall business that can affect the outcome of the task plan. For example, a change in resource availability may cause a delay to one or more tasks (if a resource is removed) or may allow one or more tasks to be completed sooner (if a resource is added). These kinds of changes will be visible in the task plan as slack (available or negative, free or total).
When a task's slack equals zero, there is no room in the schedule for that task to be delayed without causing a delay to another task in the task plan (or to the task plan itself). A task with zero slack should be considered to be a critical task; any delay to the completion of this single task will cause a similar delay to another task (or to the entire task plan). The following example shows three critical tasks with zero slack:
Available slack occurs when the total duration of one or more tasks in the task plan is less than the total duration of the task plan itself. The following example shows three tasks with available slack. The first task has two days of free slack, the middle task has four days of free slack, and the final task has one day of free slack. The task plan has seven days of total slack:
Negative slack can occur when a task is scheduled to start before another (predecessor) task is scheduled to finish, when a task is scheduled to finsh after another (successor) task is finished, or when a task must be finished sooner than the schedule currently allows. The following example shows three tasks, each having available slack and one having negative slack. The final task is currently scheduled to be started before the middle task is scheduled to be finished:
Negative slack can be removed by using the available slack in the project. In this example, the middle task could be started sooner, the final task could be finished later, or a constraint on the final task (if one is present) could be adjusted.