Calculate slack (float) time in project schedules using task durations, dependencies, earliest/latest start and finish times.
The Slack Time Calculator determines how much schedule flexibility exists for tasks in a project. Slack time is the amount a task can be delayed without pushing back project completion. Tasks with zero slack are on the critical path, so any delay to them delays the entire project.
In project management, slack helps with resource allocation, risk management, and schedule planning. Total float shows how much a task can slip without affecting the project end date. Free float shows how much a task can slip without affecting any successor task.
This calculator lets you enter tasks with durations, earliest start (ES), earliest finish (EF), latest start (LS), and latest finish (LF) times. It computes total float, free float, and critical-path status for schedule analysis.
Use this calculator to see where a project schedule has flexibility and where it is tight. It helps with risk management, resource planning, and identifying tasks that need close attention.
Total Float = LS - ES = LF - EF. Free Float = ES(successor) - EF(current). Critical Path: Tasks where Total Float = 0. Project Buffer = Minimum non-zero total float across near-critical tasks.
Result: Task A: 0 float (critical), B: 2 days float, C: 0 float (critical)
Tasks A and C have zero float — they're on the critical path. Task B has 2 days of float (can slip 2 days without affecting the project end date).
The Critical Path Method identifies the longest sequence of dependent tasks. Tasks on this path have zero total float — they determine the minimum project duration. Non-critical tasks have positive float, providing scheduling flexibility. CPM was developed in the 1950s and remains the foundation of project scheduling.
Tasks with minimal slack are high-risk: any delay propagates. Project managers monitor near-critical paths (1-5 days of float) because they can become critical if delays occur. By tracking float trends over time, managers can detect schedule erosion early and take corrective action before delays become critical.
Waterfall projects use CPM slack extensively. Agile projects use sprint velocity variance as an analog. Critical Chain Method replaces individual task slack with shared project and feeding buffers. Lean construction uses the Last Planner System. Regardless of methodology, understanding where flexibility exists is always valuable.
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Total float is delay without affecting the project end date. Free float is delay without affecting any successor task. Free float ≤ Total float.
Tasks with zero slack form the critical path — the longest chain through the project. Any delay to these tasks directly delays the project.
Negative slack means the project is behind schedule. To finish on time, tasks with negative slack must be accelerated (crashed).
Forward pass: ES = max(EF of all predecessors). EF = ES + Duration. Backward pass: LF = min(LS of all successors). LS = LF - Duration.
Slack is inherent in the schedule based on path analysis. Buffer is intentionally added time. Critical Chain Method adds explicit buffers; CPM reveals natural slack.
Tasks with high slack can be delayed if their resources are needed on critical path tasks. This flexibility enables resource leveling.