The critical path method (CPM) is a scheduling technique that identifies the longest chain of dependent activities in a project. That chain — the critical path — determines the shortest possible project duration, so any delay on it delays the entire project.
How the critical path method works
You map every activity, its duration, and its dependencies (often as an activity-on-node diagram), then work out which sequence takes the longest from start to finish. CPM uses two passes over the network:
- Forward pass — move from the first activity to the last, adding durations to find the earliest each activity can start and finish.
- Backward pass — move from the end back to the start to find the latest each activity can start and finish without delaying the project.
The float (or slack) of an activity is its late start minus its early start. Any activity with zero float lies on the critical path.
A worked example
Consider five activities: A (3 days) is followed by B (6 days); A is also followed by C (2 days), which is followed by D (5 days); B and D both feed a final activity E (4 days).
| Activity | Depends on | Duration |
|---|---|---|
| A — Requirements | — | 3 days |
| B — Backend build | A | 6 days |
| C — Design | A | 2 days |
| D — Frontend build | C | 5 days |
| E — Integration and test | B, D | 4 days |
There are two paths: A → B → E = 13 days and A → C → D → E = 14 days. The longer path (14 days) is the critical path, so the project cannot finish sooner. Activity B has one day of float and can slip a day without affecting the deadline. This is the practical value of CPM: it shows where to focus effort.
Common mistakes in practice
- Confusing “critical” with “important”. A critical-path activity is simply the one with no schedule buffer; high-value work often sits off the critical path.
- Ignoring near-critical paths. A path with only a day of float is one bad estimate away from becoming critical.
- Trusting the software blindly. Tools calculate the critical path automatically, but if the dependencies are wrong the answer is confidently wrong.
- Forgetting to recalculate. As activities finish or slip, the critical path can move.
The critical path method in the PMBOK Guide
The critical path method is documented in the current PMBOK Guide, 8th edition (2025), in the Tools and Techniques section (Figure 5-4), where it is described as a method used to estimate the minimum project duration. It calculates early and late start and finish dates through a forward and backward pass and derives the total and free float for each path; activities with zero total float form the critical path. The guide also notes that a network can contain several near-critical paths. Earlier, process-based editions used the technique within the Develop Schedule process.
Frequently asked questions
What is the difference between the critical path and the critical path method?
The critical path is the longest sequence of dependent activities and sets the minimum project duration. The critical path method is the technique used to identify that path and the float of every activity.
Can a project have more than one critical path?
Yes. If two or more paths share the same longest duration, they are all critical, which makes the schedule more fragile.
What is the difference between CPM and PERT?
CPM uses a single, deterministic duration per activity. PERT uses three estimates (optimistic, most likely, pessimistic) to account for uncertainty.
Last updated: 4 July 2026. Written by Tom, PMP-certified since 2004.