What is the Critical Path Method?
The Critical Path Method (CPM) is a project scheduling algorithm that identifies the longest sequence of dependent tasks in a project network — the critical path. Any delay on a critical path activity directly delays the project completion date.
CPM was developed in the late 1950s by DuPont and Remington Rand for managing complex chemical plant projects. Today it is the foundation of virtually every project scheduling tool, including Primavera P6 and Microsoft Project.
Why the Critical Path Matters
Understanding the critical path tells you three essential things:
- What will delay the project — any slip on a critical activity flows directly to completion
- Where you have float — non-critical activities have total float, meaning they can be delayed without affecting the end date
- Where to focus resources — crashing or fast-tracking only makes sense on critical path activities
The Forward Pass
The forward pass calculates the earliest start (ES) and earliest finish (EF) for each activity by working from the start of the project to the end. The formula is:
ES = max(EF of all predecessors) | EF = ES + Duration
The Backward Pass
The backward pass calculates the latest finish (LF) and latest start (LS) by working backwards from the project end date:
LF = min(LS of all successors) | LS = LF − Duration
Total Float
Total Float (TF) = LS − ES = LF − EF. An activity with zero total float is on the critical path. Activities with positive total float have scheduling flexibility — they can be delayed or extended within their float without delaying the project.
Near-Critical Activities
Near-critical activities have small amounts of total float (commonly defined as ≤5 working days or ≤10% of remaining duration). They deserve almost as much management attention as critical path activities, because they can easily become critical through small delays.
Schedule Compression Techniques
Crashing adds resources to critical path activities to shorten their duration. It always costs money and has diminishing returns. Fast-tracking overlaps activities that were originally sequenced — adding schedule risk. Both techniques should only be applied to critical path activities.
CPM in Primavera P6
P6 calculates the critical path using either Total Float ≤ X or Longest Path. The Longest Path method is more accurate in open-ended networks (those without hard constraints) and is recommended for major infrastructure projects.
Module 2 of the free course covers CPM in full — forward pass, backward pass, float, near-critical activities, schedule compression, and a practical P6 walkthrough.
Go to Module 2 →