Critical path and slack
The critical path is the sequence of work that currently controls a project’s calculated finish date. A delay to that sequence delays the project unless another scheduling input changes enough to compensate. It is not necessarily a long chain: in some plans, one task can be the controlling sequence. Microsoft Project uses the path to focus attention on the work that puts the finish date at risk.1
Critical does not mean permanently important
Section titled “Critical does not mean permanently important”By default, desktop Project marks a task critical when it has zero slack. Certain constraint types or a task scheduled to finish on or beyond its deadline can also make Project mark it critical. The critical path can move as work completes or another linked sequence is delayed, so it is a current result of the schedule rather than a permanent property of a task.1
Desktop Project can expose the result in several ways. Critical tasks can appear as red bars in a Gantt Chart, be highlighted in other task views, or be filtered and grouped by their critical status. The Network Diagram also shows critical tasks in red. These colors and commands are interface conventions; the useful question is whether a task currently has the scheduling flexibility to slip.
You can also raise the configured slack threshold used to identify critical tasks. For example, treating tasks with one day of slack as critical is a way to watch work that is close to consuming its buffer. This changes the warning boundary, not the amount of time available in the plan.2
Slack measures schedule flexibility
Section titled “Slack measures schedule flexibility”Slack, also called float, is calculated time that can absorb movement in the schedule. The two measures answer different questions:
| Measure | What a delay can avoid affecting |
|---|---|
| Total slack | The project’s finish date |
| Free slack | A successor task |
Total slack is the delay available before a task delays the project finish.
Free slack is the delay available before a successor is delayed. When a task has no successors, Project sets its free slack equal to its total slack. A task can therefore have total slack while having little or no free slack when a successor must start on time even though the overall project still has room.
Negative total slack is not extra buffer in the opposite direction. It signals that the schedule lacks enough time to meet its dates, often because of constraints. A deadline set before the end of a task’s available slack can also recalculate total slack; when that value reaches zero, the task becomes critical.2
In Project Plan 365, the critical path and slack calculation features provide a product-specific way to review the schedule’s controlling tasks and available flexibility.
Use the path to improve a finish date
Section titled “Use the path to improve a finish date”If a required finish date is at risk, changing noncritical work usually will not move the finish date. Examine the critical path instead, then evaluate which schedule driver can realistically change. Options include reducing task duration or work, relaxing a constraint, splitting work so it can proceed in parallel, revising dependencies, adding lead time, scheduling overtime, and changing resource assignments. A different sequence can become critical after an intervention, so reassess the path after each meaningful change.1
This is a planning decision, not merely a date-editing exercise. Critical tasks are a useful starting point for evaluating schedule changes.
One overall path, and sometimes several diagnostic paths
Section titled “One overall path, and sometimes several diagnostic paths”Desktop Project normally shows one overall critical path: the sequence that controls the project’s finish date. Its multiple-critical-path setting can additionally show a critical path for each independent task network. This can help when tracking phases, milestones, or subprojects, but these secondary paths do not replace the overall path that determines the project finish.
For a master project, Project can also calculate an overall path across linked projects or subprojects when inserted projects are calculated like summary tasks.1
Project Plan 365’s Erix AI can help investigate a critical path by identifying driving or late tasks and suggesting ways to address them.