Slack Time Calculator
Enter earliest start, latest start, and activity duration to discover how much scheduling flexibility each task has.
Use the same time unit for all fields (hours, days, weeks).
Slack Analysis
Slack / float
4 days
Slack as % of duration
133.3%
Task critical?
No
Some slack—task can slip without affecting finish.
How to Use This Calculator
Gather network diagram data
Use CPM (Critical Path Method) outputs or project scheduling software to obtain earliest and latest start times.
Input consistent time units
Enter start times and durations using the same time unit (e.g., days). Adjust the unit label for clarity.
Interpret slack to spot risk
Zero slack indicates critical tasks. Use positive slack to balance resources or absorb schedule changes.
Formula
Slack = Latest Start − Earliest Start
Slack % = Slack ÷ Activity Duration
Example: Earliest start 6 days, latest start 10 days, duration 3 days → Slack = 4 days. Slack % = 4 ÷ 3 ≈ 133%. The task can slip up to 4 days without delaying the project.
Slack Time Basics
Slack (or float) measures scheduling flexibility. Positive slack allows tasks to start later than the earliest start without affecting completion. Zero slack tasks form the critical path.
- Free slack: Delay allowed before impacting successor tasks.
- Total slack: Delay allowed before impacting project finish.
- Negative slack: Schedule already behind—add resources or rebaseline.
- Use slack to prioritize resources toward critical activities.
Frequently Asked Questions
What if I only know finish times?
Slack can also be computed as Latest Finish − Earliest Finish. Just ensure you use consistent start or finish values.
Can slack be negative?
Yes. Negative slack indicates the schedule is impossible under current constraints—you must fast-track or extend the deadline.
How do I handle resource leveling?
After applying resource leveling, recalculate earliest and latest times; slack can change as tasks shift to resolve conflicts.
Do agile teams use slack?
Agile teams still benefit from buffer awareness. Slack helps gauge capacity within sprints or release trains.