What Is Slack Time?
Slack time (also called float) is the amount of time an activity in a project schedule can be delayed without delaying the overall project completion date. It is a core concept in the Critical Path Method (CPM) used in project management. Activities with zero slack lie on the critical path — any delay to them pushes back the whole project.
How to Use This Calculator
Enter the four scheduling values for one activity: Earliest Start (ES), Latest Start (LS), Earliest Finish (EF) and Latest Finish (LF). These typically come from a forward and backward pass through your network diagram. The calculator returns the slack and flags whether the activity is critical.
The Formula Explained
Total float is computed two equivalent ways: \(\text{Slack} = \text{LS} - \text{ES}\) and \(\text{Slack} = \text{LF} - \text{EF}\). Both expressions give the same value for a well-formed schedule. If slack is zero, the task has no flexibility and is on the critical path.
$$\text{Slack} = \text{LS} - \text{ES} = \text{LF} - \text{EF}$$
Worked Example
Suppose an activity has \(\text{ES} = 3\), \(\text{LS} = 5\), \(\text{EF} = 8\) and \(\text{LF} = 10\). Then
$$\text{Slack} = \text{LS} - \text{ES} = 5 - 3 = 2,$$$$\text{LF} - \text{EF} = 10 - 8 = 2.$$The activity has 2 time units of float, so it is not critical and could be delayed up to 2 units.
FAQ
What does zero slack mean? It means the activity is on the critical path and cannot be delayed without delaying the project.
Can slack be negative? Yes — negative float indicates the schedule is behind and the activity must be accelerated to meet a target date.
Is float the same as slack? Yes, total float and slack are used interchangeably in CPM scheduling.
