Backward Pass is a scheduling technique in the Critical Path Method (CPM) used to calculate the late start (LS) and late finish (LF) dates for activities by working backward from the project’s end date. It helps determine the latest possible time each task can start and finish without delaying the overall schedule.
Key Aspects of Backward Pass
- Works in Reverse – Starts from the project’s last activity and moves backward.
- Determines Late Start & Finish – Identifies the latest an activity can begin and end without affecting the project deadline.
- Calculates Total Float – Measures scheduling flexibility by comparing early and late start times.
- Essential for Critical Path Analysis – Ensures activities align with the overall project timeline.
Backward Pass Formula
- Late Finish (LF) = Minimum Late Start (LS) of Successor Activities
- Late Start (LS) = LF – Duration
- If an activity has multiple successors, the smallest LS among them determines its LF.
Example Scenario
A project consists of three tasks with the following durations:
- Task A: 3 days
- Task B: 5 days
- Task C: 4 days (dependent on Task A and Task B)
- Project Completion Deadline: Day 12
The backward pass starts from the project’s completion (LF = 12) and calculates backward.
Mermaid Diagram: Backward Pass Calculation
graph LR; End["Project Completion (LF 12)"] -->|LF 12| C["Task C (4 days) LF 12, LS 8"] C -->|LF 8| A["Task A (3 days) LF 8, LS 5"] C -->|LF 8| B["Task B (5 days) LF 8, LS 3"] A -->|LF 5| StartA["Task A Start (LS 5)"] B -->|LF 3| StartB["Task B Start (LS 3)"]
Calculated Late Start (LS) & Late Finish (LF)
| Task | Duration | Late Finish (LF) | Late Start (LS) |
|---|---|---|---|
| C | 4 days | 12 | 8 |
| A | 3 days | 8 | 5 |
| B | 5 days | 8 | 3 |
Why Backward Pass Matters
- Identifies Critical Path – Highlights tasks that directly impact the project deadline.
- Optimizes Scheduling – Ensures tasks start as late as possible without affecting successors.
- Reveals Float – Determines which tasks have flexibility and which are critical.
See also: Forward Pass, Critical Path Method, Schedule Network Analysis, Total Float.