Project Management guide · Scheduling and time
Project Network Diagram: Types, Examples and How to Draw One
A project network diagram is a picture of a project's activities and the dependencies between them, drawn as boxes or circles joined by arrows. It shows the logical flow of work from start to finish and is the basis for the critical path method.
Key takeaways
- A network diagram shows activities and dependencies as a flow.
- Activity-on-node (PDM) is the modern standard and supports all dependency types.
- Merge points combine risk; burst points spread delays.
- Use it to check logic and find the critical path.
Types of network diagram
| Activity-on-node (AON) | Activity-on-arrow (AOA) | |
|---|---|---|
| Activities shown as | Boxes (nodes) | Arrows |
| Dependencies shown as | Arrows between boxes | Nodes (events) where arrows meet |
| Dependency types | FS, SS, FF and SF | Finish-to-start only |
| Dummy activities | Not needed | Sometimes needed to show logic correctly |
| Also called | Precedence diagramming method (PDM) | Arrow diagramming method (ADM) |
| Use today | Standard in modern tools | Mostly in older texts and some PERT teaching |
How to draw an activity-on-node diagram
- List the activities, durations and predecessors; see predecessors and successors.
- Add a Start node on the left.
- Place activities with no predecessors after Start, then add each successor to the right of its predecessors.
- Draw an arrow for every dependency, labelled with the type and any lead or lag if it is not a plain FS.
- Join all activities without successors to a Finish node.
- Write ES, EF, LS, LF and float in each node once you have done the forward and backward pass.
- Highlight the critical path.
Worked example
For activities A (3 days), B (4, after A), C (2, after A), D (5, after B), E (3, after C) and F (2, after D and E), the diagram looks like this in text form:
- Start -> A
- A -> B -> D -> F (critical, 3 + 4 + 5 + 2 = 14 days)
- A -> C -> E -> F (3 + 2 + 3 + 2 = 10 days)
- F -> Finish
A is a burst point (one activity feeding two) and F is a merge point (two activities feeding one). Merge points are where delays combine, so they deserve close monitoring.
| Node box layout | Top row | Middle | Bottom row |
|---|---|---|---|
| Example for C | ES 3 | Dur 2 | EF 5 | C Book removals | LS 7 | TF 4 | LF 9 |
Network diagram vs Gantt chart
| Network diagram | Gantt chart | |
|---|---|---|
| Shows | Logic and flow | Timing against a calendar |
| Best for | Checking dependencies and the critical path | Communicating dates and tracking progress |
| Scale | Hard to read at large sizes | Easier to collapse and summarise |
Tips
- Draw a small diagram on a whiteboard with the team before entering data into a tool.
- Check for loops (A depends on B, which depends on A) and dangling activities.
- Keep a single start and a single finish node.
- For large projects, view the network in your tool filtered to critical and near-critical paths.
Free templates for this topic
Frequently asked questions
What is a project network diagram?
A diagram of activities and their dependencies, showing the logical sequence of work from start to finish.
What is the precedence diagramming method for a network diagram?
Another name for activity-on-node diagramming, where activities are boxes and arrows show dependencies.
What is the difference between AON and AOA network diagrams?
In AON activities are nodes; in AOA activities are arrows and nodes are events. AON supports all four dependency types.
What is a dummy activity in a network diagram?
A zero-duration arrow used in activity-on-arrow diagrams to show a dependency without adding work.
What is a merge activity in a network diagram?
An activity with two or more predecessors, such as F in our example.
What is a burst activity in a network diagram?
An activity with two or more successors, such as A in our example.
How do you find the critical path on a network diagram?
Add up the durations along each path, or do a forward and backward pass. The longest path is critical.
Is a PERT chart a network diagram?
Yes. A PERT chart is a network diagram that uses three-point duration estimates.
Which software draws network diagrams?
Many scheduling tools generate them automatically from dependencies; diagramming tools can draw them manually.
Do I need a network diagram if I have a Gantt chart?
Most tools build both from the same data. The network view is mainly useful for checking logic.