In project management, the critical path is the longest sequence of tasks that must be completed to successfully conclude a project, from start to finish. The tasks on the critical path are known as critical activities because if they’re delayed, the whole project will be delayed. By identifying the critical path, you can determine the total duration of a project, and you’ll have a clear picture of the project’s actual schedule.

The critical path method (CPM) is used in project management to create project schedules and helps project managers create a timeline on a Gantt chart for the project. The critical path method includes: 

• Identifying every task necessary to complete the project and the dependencies between them

• Estimating the duration of the project tasks

• Calculating the critical path based on the tasks’ duration and dependencies to identify the critical activities

• Focusing on planning, scheduling and controlling critical activities

• Setting project milestones and deliverables

• Setting stakeholder expectations related to deadlines

After making these considerations, you gain an insight into which activities must be prioritized. This allows for the allocation of the necessary resources to get these important tasks done. Tasks you discover that aren’t on the critical path are of a lesser priority in your project plan, and may be delayed if they’re causing the project team to become overallocated.

Before understanding the calculation of the critical path, we need to understand the various terms used in this method.

Forward Pass

The first step in the calculation process is known as the Forward Pass. In the forward pass, the Early Start and Early Finish values for each activity, along with the overall Project Duration, are calculated. To facilitate schedule calculations, an “end of day” notation is used for both the Early Start and the Late Start values.

Backward Pass

The second step in the calculation is the Backward Pass. Through this pass, the Late Start and Late Finish values are calculated.

Float Calculations

The importance of calculating Total Float and Free Float are found in the definitions of these terms. As stated earlier, float values indicate how much each individual activity can be delayed before affecting successor activities or the planned project completion date.

Total Float vs. Free Float

Total float: This is the amount of time that an activity can be delayed from the early start date without delaying the project finish date or violating a schedule constraint.

Free float: This refers to how long an activity can be delayed without impacting the following activity. There can only be a free float when two or more activities share a common successor.

Critical Path

Once the float values are calculated, the string of critical activities will be identified. This continuous string of critical activities is called the Critical Path. Critical activities are those that do not possess any float. In the diagram shown it would be Activities A, B, D and F.