Before the modern Gantt chart, the US Navy invented a method to plan projects with uncertain durations: the PERT chart. Still taught today, it stays useful whenever a task is hard to estimate precisely.
PERT (Program Evaluation and Review Technique) is a scheduling method developed in the 1950s for projects where task durations are uncertain. Unlike the Critical Path Method (CPM), which uses a fixed duration per task, PERT computes an estimated duration from three scenarios: optimistic, pessimistic, and most likely.
Visually, a PERT chart represents tasks as nodes connected by dependency arrows — a network, rather than a timeline like a Gantt chart. This network structure highlights the relationships between tasks rather than their position in time.
Optimistic duration (O), most likely duration (M), pessimistic duration (P) — asked from the person who will actually do the task.
Estimated duration = (O + 4×M + P) ÷ 6. The likely scenario weighs 4 times more than the extremes.
Place each task with its predecessors and calculate the project's total duration from the estimated durations.
Just like CPM, the longest sequence in the network sets the project's minimum duration.
Example: a task estimated at 3 days (optimistic), 5 days (likely), and 11 days (pessimistic) gives a PERT duration of (3 + 4×5 + 11) / 6 = 5.7 days — more realistic than a simple average (6.3 days) or a gut-feel estimate.
| Criteria | PERT | Gantt |
|---|---|---|
| Visualization | Node network | Timeline |
| Handling uncertainty | ✅ Native (3 scenarios) | Limited |
| Day-to-day readability | Harder | ✅ Easy |
| Typical use | R&D, innovative projects | Everyday operational tracking |
In practice, most teams use PERT durations to estimate tasks, then visualize them on a Gantt chart for day-to-day tracking — the two methods complement each other more than they compete. The PMI teaches both techniques as complementary in its PMBOK framework.
Estimate your tasks with whichever method fits, then track them on a clear Gantt chart. Free.