Total Float: Late and Early Date Difference in CPM
PMBOK v8 Definition
Total float is the difference between late and early dates, which indicates how much an activity can be delayed without affecting the project’s finish date. It is calculated using the critical path method (CPM) by performing a forward pass (calculating earliest start and finish dates) and a backward pass (calculating latest start and finish dates). Activities with zero total float form the critical path.
Why It Matters for the Exam
Total float is a core concept tested in PMI exam questions related to schedule management, critical path analysis, and network diagram interpretation. Expect questions that ask you to calculate total float from given early/late dates, identify critical path activities, or determine schedule flexibility. These appear in both calculation-based questions and scenario-based situational questions.
Key Points to Remember (for the exam)
- Definition: Total float = Late Finish (LF) – Early Finish (EF) OR Late Start (LS) – Early Start (ES)
- Critical Path: Activities with zero total float form the critical path—the longest path in the schedule network
- Free Float vs. Total Float: Free float is the time an activity can be delayed without affecting the early start of its successor; total float is the time an activity can be delayed without affecting the project finish date
- Forward Pass: Calculates earliest possible start (ES) and finish (EF) dates, beginning at project start
- Backward Pass: Calculates latest allowable start (LS) and finish (LF) dates without delaying the project, beginning at project end
- Calculation Convention: When using Day 1 as start, formulas are: EF = ES + Duration – 1 and LS = LF – Duration + 1
- Negative Float: Occurs when constraints force dates earlier than logic and duration allow; used to explore acceleration strategies
Typical PMI Exam Example
A project has Activity B with an early start of Day 3, early finish of Day 8, late start of Day 6, and late finish of Day 11. What is the total float?
Answer: Total float = LS – ES = 6 – 3 = 3 days (or LF – EF = 11 – 8 = 3 days). This means Activity B can be delayed up to 3 days without affecting the project finish date.
PMI Exam Traps
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Trap: Confusing total float with free float
Reality: Total float affects the project finish date; free float only affects the successor’s early start -
Trap: Assuming all activities with positive float are not critical
Reality: The critical path is defined by zero total float, but positive float may occur if a constraint is placed later than the calculated early finish -
Trap: Using incorrect calculation formulas
Reality: When using Day 1 start convention, use EF = ES + Duration – 1 and LS = LF – Duration + 1 -
Trap: Thinking only one critical path exists
Reality: Some schedule networks may have multiple near-critical paths; software tools allow defining how criticality is measured
Important PMI Connections
| Related Concept | Relationship Type | Exam Attention Point |
|---|---|---|
| Critical Path Method (CPM) | Core technique | CPM uses total float to identify the critical path |
| Free Float | Complementary concept | Free float is a subset of total float; tested for distinction |
| Schedule Compression | Application | Negative float analysis can be used to explore acceleration strategies |
| Leads and Lags | Schedule logic | Duration estimates do not include leads or lags; they affect float calculations |
Quick Review Questions
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If an activity has an early start of Day 4, early finish of Day 9, late start of Day 7, and late finish of Day 12, what is the total float?
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What is the difference between total float and free float in terms of what each affects?
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An activity on the critical path will have what total float value?
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If a constraint forces a date earlier than logic and duration allow, what type of float occurs?
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When using the Day 1 start convention, what is the formula to calculate late start (LS) from late finish (LF) and duration?
PMBOK v8 Reference
Section 5.4 - Critical Path Method (CPM)