PERT Chart: Definition, Example, Formula, and PERT vs Gantt | AFFiNE
PERT Chart: Definition, Example, Formula, and PERT vs Gantt
Content
What is a PERT chart and when should you use it?
A PERT chart is a network diagram that shows a project's activities as nodes, the dependencies between them as arrows, and an expected duration for each activity calculated from three estimates: optimistic, most likely, and pessimistic. Teams use it when durations are uncertain, such as a first-time launch, a research effort, or a project with unfamiliar vendors, and when they need to know which chain of activities sets the earliest realistic finish. PERT stands for Program Evaluation and Review Technique. It was developed in the late 1950s for the U.S. Navy's Polaris program, and its core idea has not changed: collect three estimates per activity, combine them into one expected time, then add expected times along every path through the network and find the longest.
PERT Chart Anatomy: Nodes, Arrows, Milestones, Paths, and Slack
Nodes (activities). Each box is one unit of work with a clear finish condition. In the activity-on-node convention used throughout this article, the box carries the activity's name, its expected time (TE), and the three estimates it came from.
Arrows (dependencies). An arrow from A to B means B cannot start until A finishes. This is a finish-to-start dependency, the only type used in this article. If two activities have no arrow between them, directly or through other nodes, they can run in parallel.
Milestones. A milestone is a zero-duration event, such as "site live" or "content approved."
Paths. A path is any route from start to finish following the arrows.
Slack (float). Slack is how much an activity or path can slip before it delays the finish.
The PERT Formula: Expected Time from Three Estimates
Every activity in a PERT chart gets three estimates from the people who will do the work:
| Symbol | Name | What it means |
|---|---|---|
| O | Optimistic time | The shortest realistic duration if nearly everything goes right. |
| M | Most likely time | The duration under normal working conditions, the value the estimator would bet on. |
| P | Pessimistic time | The longest realistic duration if known problems occur. |
The expected time is the weighted average:
TE = (O + 4M + P) / 6
Worked Calculation 1: Activity A, "Confirm scope and sitemap."
TE = (3 + 4 × 5 + 9) / 6 = 5.33 days
Worked Calculation 2: Activity D, "Build pages."
TE = (8 + 4 × 12 + 22) / 6 = 13 days
PERT Chart Example: A Fictional Nonprofit Website Relaunch
Assumptions. One designer, one developer, one content lead, and a volunteer coordinator.
Activity Register
| ID | Activity | Predecessors | O | M | P | TE = (O + 4M + P) / 6 | σ = (P − O) / 6 |
|---|---|---|---|---|---|---|---|
| A | Confirm scope and sitemap | — | 3 | 5 | 9 | 5.33 | 1.00 |
| B | Design page templates | A | 5 | 8 | 15 | 8.67 | 1.67 |
| C | Write and migrate content | A | 6 | 10 | 18 | 10.67 | 2.00 |
| D | Build pages | B | 8 | 12 | 22 | 13.00 | 2.33 |
| E | Integrate donation form | B | 4 | 6 | 14 | 7.00 | 1.67 |
| F | Accessibility review and fixes | C, D | 3 | 4 | 8 | 4.50 | 0.83 |
| G | Train volunteer editors | C | 2 | 3 | 6 | 3.33 | 0.67 |
| H | Launch and post-launch checks | E, F, G | 1 | 2 | 4 | 2.17 | 0.50 |
Path Register
| Path | Activities | Expected length (working days) | Slack vs. longest path |
|---|---|---|---|
| 1 | A → B → D → F → H | 33.67 | 0 |
| 2 | A → B → E → H | 23.17 | 10.50 |
| 3 | A → C → F → H | 22.67 | 11.00 |
| 4 | A → C → G → H | 21.50 | 12.17 |
Conclusion
A PERT chart earns its place when durations are uncertain and dependencies decide the finish. The method is small: three estimates per activity, TE = (O + 4M + P) / 6, a network of finish-to-start arrows, and a sum along every path to find the longest. Keep the registers next to the diagram, write down the basis for every pessimistic number, and total the paths again whenever anything moves.