Schedule Slack (or float) is a measure of flexibility in the project schedule: there is total float that means the amount of time which a task can be protracted without delaying the overall project’s completion date, and free float – a period of time that a task can be procrastinated without affecting any other tasks in the project’s schedule. In other words schedule slack is used by project managers to know how many of appropriate time units (days, hours, etc) an activity can be delayed before the project completion date will be delayed as well. Tasks on the project’s schedule, which do not have any slack time are considered as critical ones (cannot be delayed).
To calculate schedule slack you need to:
- Calculate the earliest start (ES), the earliest finish (EF), the latest finish (LF) and the latest start (LS) for every activity;
- Slack time is the amount of time between its earliest and latest start time (ES-LS), or between its earliest and latest finish time (EF-LF). Slack time is the time that an activity can be delayed past its earliest start (ES) or earliest finish (EF) without making the whole project late;