The critical path method (CPM) helps estimate the shortest possible project duration when activity durations and dependencies are known. The critical path is the longest-duration path through the dependency network; activities on it have zero total float under the standard calculation. A delay to a critical activity can delay the project unless the schedule is changed or recovered.
List activities, durations and dependencies
Begin by splitting the project into activities with estimated durations. For each task, record which preceding tasks must finish before it can start. Use one consistent time unit, such as working days.
Consider a small example: A takes 3 days, B takes 4 days after A, C takes 2 days after A, and D takes 5 days after both B and C. This creates two paths: A–B–D and A–C–D.
Activity | Duration | Predecessor
A | 3 days | None
B | 4 days | A
C | 2 days | A
D | 5 days | B and CCalculate the early start and early finish
The early start (ES) is the earliest an activity can begin based on its predecessors. The early finish (EF) is ES plus duration. When a task has multiple predecessors, its early start is the latest early finish among those predecessors.
In the example, A starts at day 0 and finishes at day 3. B starts at day 3 and finishes at day 7. C starts at day 3 and finishes at day 5. D cannot begin until both B and C finish, so it starts at day 7 and finishes at day 12.
- A: ES 0, EF 3.
- B: ES 3, EF 7.
- C: ES 3, EF 5.
- D: ES 7, EF 12.
Find the critical path
Add the durations along each complete path. A–B–D takes 3 + 4 + 5 = 12 days, while A–C–D takes 3 + 2 + 5 = 10 days. The longest path is A–B–D, so the estimated project duration is 12 days and those activities form the critical path in this simplified example.
This does not mean C is unimportant. C still must finish before D can begin. It simply has some schedule flexibility under the stated assumptions.
Path A-B-D = 3 + 4 + 5 = 12 days
Path A-C-D = 3 + 2 + 5 = 10 days
Critical path: A-B-D
Estimated duration: 12 daysUnderstand float and schedule flexibility
Float is the amount of time an activity can be delayed without delaying a specified milestone or the project. A backward pass calculates late start and late finish values. Total float is commonly computed as LS − ES or LF − EF. In the example, C can be delayed by 2 days before it pushes the finish past day 12, assuming no other constraints apply.
Project software may use day-counting conventions that differ from this zero-based teaching example. Keep the same convention throughout the forward and backward passes.
How to manage a critical-path project
Review the critical path whenever a duration or dependency changes. If a critical activity slips, possible responses include reducing scope, removing an unnecessary dependency, adding an appropriately skilled resource, changing the sequence or accepting a revised finish date. Each option has trade-offs and should be assessed rather than assumed to work.
CPM relies on estimates and dependency logic. It does not automatically account for resource conflicts, uncertainty, approval delays or changing requirements unless those are explicitly modeled.
- Validate activity durations with the people doing the work.
- Confirm dependencies are real and necessary.
- Recalculate after schedule changes.
- Monitor near-critical paths as well as the longest path.
- Document any approved change to the finish date or scope.
Understanding the concept is the first step.
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