In project management and scheduling, “float” (or “slack”) refers to the amount of time a task can be delayed without affecting the overall project timeline. It helps project managers identify the flexibility they have in scheduling and task completion.
Types of Float
- Total Float
- Free Float
- Negative Float
1. Total Float
Definition:
Total float is the amount of time a task can be delayed without delaying the overall project’s end date or affecting the project’s critical path.
Formula:
Total Float = Late Start (LS) – Early Start (ES)
Or, Total Float = Late Finish (LF) – Early Finish (EF)
Practical Example:
Let’s say you have two tasks in your project:
- Task A: Duration of 5 days (from Day 1 to Day 5)
- Task B: Duration of 4 days (from Day 6 to Day 9) and it starts immediately after Task A.
If Task A is delayed by 2 days (finishing on Day 7 instead of Day 5), Task B can still start on Day 8. Since Task B’s total float is 2 days, the overall project end date won’t be affected.
2. Free Float
Definition:
Free float is the amount of time a task can be delayed without affecting the start date of its successor task.
Formula:
Free Float = Early Start (ES) of the next task – Early Finish (EF) of the current task
Practical Example:
- Task X: Duration of 3 days (from Day 1 to Day 3)
- Task Y: Duration of 5 days (from Day 5 to Day 9) starts immediately after Task X.
If Task X finishes on Day 4 instead of Day 3, Task Y can still start as planned on Day 5, so the free float for Task X is 1 day. However, if Task X is delayed by 2 days, Task Y will have to start late, consuming its free float.
3. Negative Float
Definition:
Negative float occurs when the project or task is behind schedule, meaning there is not enough time to complete the remaining activities without impacting the overall project completion date.
Formula:
Negative Float = Total Float < 0
Practical Example:
- Task C: Duration of 4 days, but due to delays, it needs 2 additional days to complete.
If Task C was initially scheduled to finish by Day 5, but now will finish by Day 7, and the next task must start on Day 6, Task C now has a negative float of -2 days, indicating a delay that will affect subsequent activities.
Bonus
The terms total float can be sometimes confusing. Let’s break it down further.
Total Float Explained
Total float is the amount of time a task can be delayed without delaying the overall project completion date. This means that even if a task is delayed, it does not affect the project’s critical path or end date as long as the delay stays within the total float time.
If delaying a task would push back the project’s end date, that means the task has zero total float and is part of the critical path.
Practical Example
Let’s break it down step-by-step with a clearer scenario:
Project Setup:
- Task A: Duration of 5 days
- Starts on Day 1
- Finishes on Day 5
- Task B: Duration of 4 days
- Planned to start on Day 6
- Finishes on Day 9
- Project End Date: Day 10
Scenario 1: No Delay
- Task A finishes on Day 5.
- Task B starts on Day 6 and ends on Day 9.
- Project finishes on Day 10 as planned.
In this scenario, total float for Task B is 1 day, because:
- If Task B were delayed by 1 day and started on Day 7 instead of Day 6, it would finish on Day 10.
- The overall project would still finish on Day 10, so the project end date is not affected.
Total Float Calculation for Task B:
- Late Start (LS) for Task B = Day 7
- Early Start (ES) for Task B = Day 6
- Total Float = LS – ES = 7 – 6 = 1 day
Scenario 2: Delayed Task B
- Task A finishes on Day 5.
- Task B starts on Day 8 (delayed by 2 days) and ends on Day 11.
Now, because Task B’s delay extends beyond Day 10 (project’s end date), the overall project completion date is pushed to Day 11.
Result:
- Task B exceeded its total float of 1 day, so now it’s causing a delay in the project’s end date.
Summary
- If Task B only used 1 day of delay (total float), the project’s end date (Day 10) would remain unchanged.
- If Task B’s delay exceeded its total float of 1 day, the project end date is pushed beyond Day 10.
Key Point:
Total float tells you how much time you can delay a task before it starts affecting the overall project’s timeline. If Task B starts on Day 8 (exceeding its float), it does indeed delay the project’s completion.
Practical Application
Understanding float is crucial for identifying critical and non-critical tasks, prioritizing activities, and making informed decisions on where delays can occur without impacting the project’s overall delivery.
