Float (Slack) Calculation

Float Calculation in Project Management

Understanding Float Calculation in Project Management

Float (also called slack) is a critical concept in project schedule management that represents the amount of time an activity can be delayed without delaying the project completion date or violating a schedule constraint. Mastering float calculation is essential for the PMI-SP certification and effective project schedule management.

Why Float Calculation is Important

Float calculation is crucial because it:

• Identifies schedule flexibility
• Helps prioritize activities
• Enables resource optimization
• Identifies the critical path
• Supports proactive schedule management
• Allows for contingency planning

Types of Float

1. Total Float (TF): The amount of time an activity can be delayed from its early start date before it delays the project end date.

Formula: TF = Late Finish (LF) - Early Finish (EF)
or TF = Late Start (LS) - Early Start (ES)

2. Free Float (FF): The amount of time an activity can be delayed before it delays the early start of any successor activity.

Formula: FF = Early Start of successor - Early Finish of activity

3. Project Float: The amount of time a project can be delayed beyond its planned completion date before it exceeds constraints like contract dates.

How Float Calculation Works

Step 1: Create the Network Diagram
• Identify all activities
• Establish dependencies
• Determine durations
• Draw the network diagram

Step 2: Forward Pass (Calculate Early Times)
• Start with ES = 0 for initial activities
• Calculate EF = ES + Duration - 1
• For subsequent activities, ES = largest EF of predecessors + 1
• Continue until project end

Step 3: Backward Pass (Calculate Late Times)
• For the last activity, LS = ES and LF = EF
• For preceding activities, LF = smallest LS of successors - 1
• Calculate LS = LF - Duration + 1
• Continue until project start

Step 4: Calculate Float
• Total Float (TF) = LF - EF or LS - ES
• Free Float (FF) = ES of successor - EF of activity

Example Calculation

Consider a simple network with 3 activities:

Activity A: Duration = 5 days
Activity B: Duration = 3 days (depends on A)
Activity C: Duration = 4 days (depends on A)

Forward Pass:
A: ES = 1, EF = 5
B: ES = 6, EF = 8
C: ES = 6, EF = 9

Backward Pass:
C: LF = 9, LS = 6
B: LF = 9, LS = 7
A: LF = 5, LS = 1

Total Float:
A: TF = 0 (critical)
B: TF = 1
C: TF = 0 (critical)

Identifying the Critical Path

The critical path consists of activities with zero float. These activities determine the project duration, and any delay in a critical activity will delay the entire project.

In our example, Activities A and C form the critical path.

Exam Tips: Answering Questions on Float Calculation

• Read carefully: Pay attention to whether the question asks for total float, free float, or project float.

• Draw it out: For complex networks, always draw the diagram to visualize dependencies.

• Check your calculations twice: Common errors occur during the forward and backward pass calculations.

• Remember the formulas: Memorize the key formulas for total float and free float.

• Watch for calendar adjustments: Some exam questions may involve non-working days or calendar constraints.

• Consider resource constraints: Some questions may factor in resource limitations affecting float.

• Start with zero: Remember that in PDM (Precedence Diagramming Method), day one is actually day zero in calculations.

• Practice with examples: Work through sample problems to build speed and accuracy.

• Keep track of units: Make sure you're using consistent time units (days, hours, etc.).

• Look for trick questions: Sometimes the exam will test your understanding by presenting unusual network constraints.

Mastering float calculation requires practice. Work through multiple examples until the process becomes second nature. Understanding float is key to becoming an effective project schedule manager and passing the PMI-SP certification exam.