In project management, “slack” or “float” refers to the amount of time that you can delay a task without causing a delay to:
- Other subsequent tasks (this is known as “free slack”), or
- The project completion date (this is known as “total slack”).
Slack is a measure of the project schedule flexibility and can be used to manage the delays and resource allocation. It’s important in both project planning and execution.
In a project, every task can have its own slack, and understanding this can help project managers to see where there’s room for flexibility and where there isn’t. If a task has zero slack, it means that any delay in that task can cause a delay in the project. If a task has a slack time of, say, 3 days, it means that the task could be delayed by up to three days without impacting the overall project timeline.
Slack time can be calculated after performing a critical path analysis on a project network diagram. The critical path is the longest path through the network diagram and identifies those tasks that have zero slack or float. These tasks are the most risk to the project schedule and any delay in these tasks will cause a project delay.
It’s important to note that while having slack in your project can be a good thing as it gives you flexibility, too much slack can mean that resources are being underutilized and the project could be completed more quickly or at a lower cost.
Example of Project Slack
Let’s imagine a project that involves three tasks:
- Task A is the first step and takes 5 days to complete.
- Task B follows Task A and takes 3 days to complete.
- Task C can be done concurrently with Task B but takes 5 days to complete.
- Task D can only start once both Tasks B and C are finished and takes 2 days to complete.
Here, Tasks A, C, and D form the critical path, because it takes a total of 12 days (5 for A, 5 for C, and 2 for D), which is the longest duration to complete the project.
Task B, however, only takes 8 days total (5 for A, 3 for B). This means that Task B has 4 days of slack (12 – 8 = 4). This means that the start of Task B could be delayed by up to 4 days without delaying the overall project, or that during its execution, it could afford some delays without affecting the project end date.
Therefore, if there’s a resource shortage or an unexpected problem, the project manager knows that Task B has some flexibility and can be pushed back up to 4 days if needed. Meanwhile, Tasks A, C, and D have no slack and need to stay on schedule to prevent the project from being delayed.