A Beginner’s Guide to Network Diagrams

12 min readOct 9, 2025

What is Project Planning?

Project Planning is the process of defining the project’s objectives, scope, deliverables, tasks, schedule, resources, and risks in order to create a clear road map for successful project execution. It involves determining what needs to be done, who will do it, how long it will take, and how progress will be tracked.

Project planning involves breaking down the work into parts and assign these to project team members, anticipate problems that might arise and prepare tentative solutions to those problems. The project plan, which is created at the start of the project, is used to communicate how the work will be done to the project team and customers and to help assess progress on the project.

In this phase, tools such as Gnatt Charts, Network Diagrams, and Work Breakdown Structures (WBS) are used to organise and visualise the workflow.

What is Network Diagram Means ?

A Network Diagram is a visual map that illustrates a project’s scope by displaying activities as boxes or nodes, and the dependencies between these activities as arrows.

This structured layout helps clarify the connection between the various tasks required to complete the project.

There are two main formats,

Activity on Arrow (A-o-A)
Activity on Node (A-o-N)

Activity on Arrow (A-o-A):

An Activity-on-Arrow (A-o-A) diagram is a type of network diagram where are represented by arrows, and nodes (circles) represent the start and events of those activities. It is also know as an Arrow Diagram and is mainly used in the Program Evaluation and Review Technique (PERT)

Each arrow shows the direction of workflow, indicating the order in which activities must be performed. Dummy arrows may also be used to show logical relationships between tasks without representing actual work.

In A-o-A, arrows = tasks, and circles = points in time (events). It focuses more on events and dependencies between activities.

Activity on Node (A-o-N):

An Activity-on-Node (A-o-N) diagram is a type of network diagram where activities are represented by nodes (boxes), and arrows show the logical relationships or dependencies between them. It is the standard format used in the Critical Path Method (CPM) and most modern project management tools (like MS Project or Primavera).

In A-o-N, boxes = tasks, and arrows = task connections. It focuses more on activities and is easier to read and update.

Importance of the Network Diagram:

A Network Diagram plays a vital role in project planning and scheduling because it helps project managers clearly understand how different activities are connected and in what order they must be completed. It acts as a visual road map that guides the entire project project form start to finish.

1. Visualises the Entire Project Structure:

A network diagram gives a clear picture of all the project activities and how they depend on one another. This helps team members and stakeholders easily understand the workflow without getting lost in complex documentation.

2. Helps in Scheduling and Time Management:

By showing the sequence of tasks, it allows project managers to estimate the total project duration and identify which activities can be done simultaneously. This ensures efficient time allocation and realistic deadlines.

3. Identifies the Critical Path:

One of the most important uses of a network diagram is to identify the critical path-the longest path of dependent tasks that determines the minimum time required to complete the project. Understanding the critical path helps in focusing efforts on the most time-sensitive activities.

4. Detects Slack or Float Time:

The network diagram helps calculate slack time, which shows how much an activity can be delayed without affecting the overall project completion. This is useful for balancing workloads and managing limited resources.

5. Helps Identify Dependencies and Risks:

By showing how activities are connected, it becomes easier to identify dependencies, such as which tasks must finish before others can begin. This helps anticipate potential bottlenecks and manage risks before they occur.

6. Assists in Progress Tracking and Control:

During project execution, the network diagram can be updated to show completed and pending tasks. This allows managers to monitor project progress, compare it with the plan, and tasks corrective actions if delays occurs.

7. Improves Communication and Coordination:

Because it visually presents the project flow, everyone on the team — from developers to clients-can understand the plan clearly. This transparency improves communication and reduces confusion or misinterpretation.

8. Enhance Decision-making:

With a network diagram, managers can make better decision when facing changes in schedule, scope or resources. It helps visualise the impact of changes before taking action.

How to draw a Network Diagram (A-o-N Method)?

When drawing an Activity-on-Node (A-o-N) network diagram, the first step is to prepare a data table that includes the activity name or ID, its predecessor(s), and the duration of each activity. This table acts as the foundation for the diagram, showing the sequence in which tasks must be completed.

Predecessor is an activity that must be completed before another activity can start. It shows the dependency relationship between tasks in a project.

If an activity has no predecessor, it can start immediately at the beginning of the project.
If an activity has one or more predecessors, it cannot start until all its predecessors are finished.

After preparing the table, identify which activities have no predecessors — these will be the starting points of your network and will be connected from the Start node (Start Box). Then, continue linking each activity to its predecessor using arrows, showing the logical flow of work. Activities that depend on others should be connected accordingly, moving step by step toward the End node (Finish Box), which represents the completion of the final tasks.

In the Critical Path Method (CPM) network diagram, each activity is represented by a node (box). This box is usually divided into nine smaller squares arranged in a 3×3 grid. Each section of the box displays specific scheduling information about that activity. After the Start Node, every activity in the project is drawn as one of these CPM boxes. It helps visualise all the time-related details of each task — such as when it starts, when it finishes, and how much flexibility it has. The below picture shows the names of each part of the box.

Early Start (ES):

The earliest time an activity can begin without delaying any of its predecessor activities.

For the first activity, ES = 0 (Since there is no predecessor, this is the starting point of the network. Then calculate EF = ES + Duration)
If the CPM node is connected to only one previous activity, then the ES (Early Start) of the current activity = EF (Early Finish) of the previous activity. Activity that, find EF = ES + Duration.
If the CPM node is connected to two or more previous activities, then check all their EF values and select the maximum EF among them. That Maximum EF becomes the ES of the current activity. Then calculate EF = ES + Duration.
Continue doing this from left to right through the network diagram until you reach the final activity.

Forward Passing (Calculating ES and EF):

When drawing a Network Diagram, the Forward Pass is the first step in determining the time schedule for each activity. This process helps to find the Earliest Start (ES) and Earliest Finish (EF) times of all activities.

When doing the Forward Pass, we always start from the beginning of the project and move from left to right through the network diagram.

Early Finish (EF):

The earliest time an activity can finish, calculated using:

EF = ES + Duration

This means if an activity starts early, it can also finish early.

Late Finish (LF):

The Latest time an activity can finish without delaying the entire project.

For the last activity, Late Finish (LF) become the last Early Finish. It can shows as EF = LF for the last activity.

If one CPM node is connected to only one next CPM node, then the Late Finish (LF) of the current activity = The Late Start (LS) of that next activity. Because the next activity must start only after the previous one finishes.

Let’s look at this kinda Nodes.

If a CPM node is connected to two or more next CPM nodes, then we take the minimum LS value among all those next activities. That minimum LS becomes the LF of the current activity.

Backward Passing:

Backward Passing is the process of calculating the latest possible times each activity in a project can start and finish without delaying the overall project completion date. In this method, we move from the end of the network diagram to the diagram to the beginning (right to left), working backward through all activities.

Late Start (LS):

The latest time an activity can start without delaying the project’s finish date. It’s calculated using:

LS = LF — Duration

Slack:

Slack, also known as Float, is the amount of free time that an activity can be delayed without affecting the project’s overall completion time. It shows how flexible a task is in the project schedule. Some activities can be delayed a little and still not delay the final project — those have positive slack. But if an activity has zero slack, it means it is critical — any delay in that activity will delay the whole project.

Critical Path:

The Critical Path is the longest sequence of activities in a project that determines the shortest possible time required to complete the entire project. It represents the set of tasks that directly affect the project’s completion date, meaning that any delay in one of these activities will delay the whole project. Activities on the critical path have zero slack, indicating that there is no flexibility in their scheduling. In other words, the critical path defines the minimum duration of the project and highlights the activities that must be closely monitored to ensure timely completion.

Critical Path Duration:

The Critical Path Duration refers to the total time required to complete all the activities along the critical path of a project. It represents the minimum possible time in which the entire project can be completed, assuming there are no delays in any of the critical activities.

Key Considerations When Constructing a Network Diagram:

When drawing a network diagram, we need to follow certain rules and guidelines to ensure the diagram is clear, accurate, and easy to understand. These rules help to maintain a proper flow of activities, avoid confusion, and correctly represent dependencies between tasks. The below points highlight the key things to consider when preparing a network diagram.

Lines should not overlap or cross each other.
The diagram should flow from left to right in a logical order.
The first activity should have a slack time of 0.
Each activity (node) must be clearly labelled with ES, EF, LS, LF, and Slack values.
Arrows should clearly show the dependency between activities.
Every activity should connect properly — no disconnected paths.
The network should represent all activities included in the project.
Keep the diagram neat and easy to read for better understanding.

Exercises:

Draw Network Diagram for the followings:

Question 01:

Critical Path = A →B →C →E →F →G

Get Bihansith Mandhive Kamburugamuwa’s stories in your inbox

Join Medium for free to get updates from this writer.

Remember me for faster sign in

Critical Path Duration

= A →B →C →E →F →G

= 2 + 4 + 2 + 8 + 6 + 7

= 29 Days

Question No 2:

Critical Path = A →B →D →F

Critical Path Duration

= 3 + 5 + 10 + 4

= 22 Days.

Question No 03:

Critical Path:

T1 → T3 → T9 → T11 → T12
T4 → T8 → T10 → T12

Critical Path Duration:

T1 → T3 → T9 → T11 → T12

= 10 + 15 + 15 + 10 + 10

= 60 days

T4 → T8 → T10 →T12

= 10 + 25 + 15 + 10

= 60 days.

Activity T5 receives its Late Finish (LF) value of 60 days because it has no successor activities in the project network. In critical path analysis, when an activity does not lead to any other task, it is considered an end activity or a terminal node. During the backward pass, the Late Finish for such an activity is assumed to be equal to the total project duration, since there is no later task that depends on it. In this case, the total project duration is 60 days (determined from the critical paths ending at T12). Therefore, the Late Finish of T5 is taken as 60 days, meaning it can finish anytime before day 60 without delaying the overall project completion.

Question No 04:

Critical Path:

= A →C →D →F →G

Critical Path Duration:

A →C →D →F →G

= 2 + 4 + 5 + 3 + 4

= 18 Days

Question No 05:

Critical Path:

b →d →f →h →i

b →c →e →h →i

Critical Path Duration:

b →d →f →h →i

= 5 + 5 + 2 + 2 + 2

= 16 Days

b →c →e →h →i

=5 + 4 + 3 + 2 + 2

= 16 Days

Question No 06:

Critical Path:

T2 →T3 →T5 →T6 →T7 →T8 →T9

Critical Path Duration:

T2 →T3 →T5 →T6 →T7 →T8 →T9

= 2 + 4 + 5 + 4 + 4 + 4 + 5

=28 days

Question No 07:

Critical Path:

A2 → A3 → A4 →A7 →A9 →A10

Critical Path Duration:

A2 → A3 → A4 →A7 →A9 →A10

= 2 + 4 + 3 + 4 + 5 + 4

= 22 Days

Question No 08:

Critical Path:

A3 → A4 → A6 → A7 → A8 → A9 → A10
A1 → A2 → A4 → A6 → A7 → A8 → A9 → A10

Critical Path Duration:

A3 → A4 → A6 → A7 → A8 → A9 → A10

= 4 + 4 + 4 + 5 + 4 + 5 + 4

= 30 Days

A1 → A2 → A4 → A6 → A7 → A8 → A9 → A10

= 2 + 2 + 4 + 4 + 5 + 4 + 5 + 4

= 30 Days

Keep in mind that when identifying critical paths, it’s essential to follow every dependency carefully without skipping any intermediate activities. Even if two tasks seem related, one cannot begin until all its required predecessors are complete. In this case, A7 depended on A6, which in turn depended on A4, and A4 came after A2. So, although it looked like A2 and A7 were directly connected, there were crucial steps in between. Always trace the full sequence of dependencies to ensure that every connecting activity is included in the path this helps avoid missing hidden links and ensures your critical path analysis is accurate.