Float (Slack)
DE: Pufferzeit (Slack)
The time an activity can be delayed without delaying the project.
Detailed Explanation
Float (also called slack) is the amount of time a schedule activity can be delayed from its early start date without delaying the project finish date or violating a schedule constraint. Total float is the more commonly used measure.
There are two types: Total Float — time an activity can slip without delaying the project end; and Free Float — time an activity can slip without delaying any successor activity. Critical path activities always have zero total float.
Float is calculated by CPM: Total Float = Late Start - Early Start (or Late Finish - Early Finish). Float is a shared resource along a path — if one activity uses it, less remains for others on the same path.
Key Points
- Total Float: delay without affecting project end
- Free Float: delay without affecting any successor
- Calculated via CPM: TF = LS - ES
- Critical path activities have zero total float
- Float is shared along a path — not owned by individual activities
- Near-zero float paths are high-risk areas
Practical Example
In a schedule with Path A (critical, 50 days) and Path B (45 days), Path B has 5 days of total float. Activity B3 on Path B has 5 days TF. If B3 uses 3 days, only 2 days remain for other B-path activities. The PM monitors Path B because its 5-day buffer is slim enough to become critical if problems arise.
Tips for Learning and Applying
Never assume float belongs to one activity — it belongs to the path
Monitor near-critical paths (low float) as closely as the critical path
Do not use float as a free extension — protect it as a buffer
Recalculate float regularly as the schedule evolves
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