Critical path
The critical path comes out of the CPM (Critical Path Method), a scheduling technique that orders a project’s activities by their dependencies and computes the shortest possible duration. Of all the chains of activities running from start to finish, the critical one is the longest: not the most important in cost, but the one that sets the timeline. Shortening the job requires compressing activities that sit on this path.
To trace it you calculate, for each activity, its earliest start and finish dates (forward pass) and its latest start and finish dates (backward pass). The difference between the two is the float (or slack): the amount of time an activity can slip without affecting the final date. Activities with zero float form the critical path; those with float can be rescheduled within that margin without moving delivery.
On site, the critical path is the central tool of schedule control: it tells the project manager and superintendent where to watch closely and where the schedule can absorb setbacks. A project can have more than one critical path at the same time. The path can also shift during execution: if a non-critical activity slips more than its float, it becomes critical and reroutes the path.
It is used on both private and public projects. In public works, tender documents commonly require a construction schedule, and it is standard for bids to ask for the activity network or the Gantt chart with the critical path identified. It has traditionally been calculated in MS Project, Primavera P6 or Excel; today it is also handled on cloud platforms such as Matterial.
Formula
Example
A job with two chains from the start: A→B→D takes 5+8+4 = 17 days; A→C→D takes 5+3+4 = 12 days. The critical path is A-B-D (17 days) and sets the total timeline; activity C has 5 days of float (17 − 12), so it can slip up to 5 days without moving delivery.
Frequently asked questions
How is the critical path calculated?
You list the activities with their durations and dependencies, run a forward pass to get the earliest starts and finishes and a backward pass for the latest ones. Each activity’s float is the difference between the two; the zero-float activities form the critical path, which is the longest chain of activities in the project.
What is the difference between the critical path and float?
The critical path is the sequence of activities with no margin: a delay on them pushes back the whole job. Float (or slack) is the time a non-critical activity can slip without affecting the delivery date. By definition, the activities on the critical path have zero float.
Can a project have more than one critical path?
Yes. When two or more chains of activities share the same maximum duration, all of them are critical at once. The path can also change during execution: if an activity with float slips more than its margin, it becomes critical and reshapes the path.