What is a slack in MS Project?

Total slack

The Total slack field contains the amount of time a task's finish date can be delayed without delaying the project's finish date and is calculated as the difference between the ‘early finish’ and the ‘late finish'.

Free slack

The Free slack field contains the amount of time that a task can be delayed without delaying any successor tasks and is the difference between the dates in the early finish and late finish against the successor. Tasks that cannot be delayed without affecting the project finish date are the critical tasks. In a typical project, many tasks have some slack and can therefore be delayed a bit without delaying other tasks or affecting the project finish date.

When is a task critical?

A task is critical if it meets any one of the following conditions:

• It has no slack (or float) - delaying it will delay the project
• It has a Must Start On (MSO) or Must Finish On (MFO) date constraint - because you have indicted it is 'critical' that this task is done on a certain date, which means that delaying is not an option
• It has an As Late As Possible (ALAP) constraint in a project scheduled from a start date (project options in project information window)
• It has an As Soon As Possible (ASAP) constraint in a project scheduled from a finish date (project options in project information window)
• It has a finish date that is the same as or beyond its deadline date

STOP being critical

A task stops being critical when it’s completed, because it can no longer affect the completion of successor tasks or the project finish date. A critical path can be of negative Total Slack value. This happens when using Task Constraints or by adding deadlines to the schedule.