PERT diagram: how to visualize project schedule using the network method?

A project manager should be able to present complex processes in a simple, understandable way. Visual management is an invaluable technique that allows for quick and accurate assessment of the current status of a project. Discover the PERT chart – a proven tool for visual management.

In this article, you will learn:

What a PERT diagram is and when to use it
How tasks and dependencies are represented in a network
How three-time estimates improve duration accuracy
How to calculate expected task time
How to identify the critical path
Why critical tasks determine the project timeline

What is a PERT diagram and where did this technique come from?

Our brain processes images instantly. What takes you half a minute to read, you can absorb in a fraction of a second through graphics. Visual representation of processes not only provides information, but also allows the brain to understand it immediately and focus on what is most important. That is why visual project management drastically facilitates work and speeds up decision-making by the project manager.

While Gantt chart in planning are mainly used for simple planning, PERT charts are used for effective visual management, especially in the case of complex and new projects. It is a network diagram consisting of nodes (circles) representing tasks or milestones, which are connected by arrows indicating the sequence and dependency of tasks. It was created in the 1950s by the United States Navy, which needed a tool to coordinate hundreds of subcontractors working on an extremely complex project to arm nuclear submarines.

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Key concepts in a PERT diagram

What is its greatest strength? It helps estimate the duration of tasks, even if they are completely new and unpredictable. Traditional planning is often based on a single, optimistic estimate, which increases the risk of delays. To avoid this, the PERT diagram introduces a method of three time estimates. What exactly does it involve? Instead of providing a single, often uncertain task duration, the PERT diagram requires the team to provide three different time scenarios. This allows the expected duration (Te) to be calculated with a much higher probability:

Optimistic time (To): the shortest possible time when everything goes perfectly.
Most likely time (Tm): realistic time under normal conditions (this has the greatest weight).
Pessimistic time (Tp): the longest possible time, taking into account most problems and delays.

The PERT diagram assumes that the Tm scenario is four times more important than the others, hence a weighted average based on the Beta distribution is used: Te= (To + 4Tm + Tp)/ 6

Consider the following task: implementing a new payment method. The team provides estimates:

To (optimistic timeframe) = 4 days
Tm (most likely) = 6 days
Tp (pessimistic) = 14 days

The calculations for the expected time (Te) are as follows: Te = (4+4*6 +1)/ 6 = 4+24+14/6 = 42/6 = 7

So, the expected duration of the task is 7 days. This result is more reliable for planning purposes because it takes into account both the chance of early completion (4 days) and the risk of significant delay (14 days).

PERT CPM chart – how to use citical path analysis in project planning?

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How to draw a PERT diagram step by step?

Drawing a PERT diagram involves visualizing all tasks in a project and their interdependencies in the form of a network. What steps should be taken?

Step 1: Preparation and time calculations.

Identify tasks and dependencies: list all tasks and determine which ones must be completed before others can begin.
Three time estimates: for each task, collect the optimistic, most likely, and pessimistic times.
Calculate the expected time: use the formula Te= (To + 4Tm + Tp)/ 6 to calculate the weighted average for each task.

Step 2: Creating a network.

Nodes and arrows: the diagram is a network consisting of nodes (circles marking the start and end of tasks) and arrows (lines marking the tasks themselves and their duration Te
Draw the network by connecting the tasks with arrows so that no task begins before all of its preceding tasks have been completed. The network must have one start node and one end node.

Step 3: Analysis and critical path

Calculate path times: add up the times of these tasks for each path (i.e., sequence of tasks) leading from the beginning to the end of the project.
Find the critical path: the critical path is the path with the longest total duration. The duration of this path determines the minimum time required to complete the entire project.

PERT Diagram Example

PERT diagram - practical example

To understand the potential of PERT diagram, it is worth looking at a practical example – a project to implement a new CRM system. We start by compiling a list of tasks and using the three-point estimation method to calculate the expected duration for each of them:

Task (A)	Predecessor (B)	To (days)	Tm(days)	Tp days)	Te= (To + 4Tm + Tp)/ 6
A: Analysis of needs	None	4	5	12	6
B: System selection	A	3	4	5	4
C: Setup	B	8	10	18	11
D: Training	B	2	3	4	3
E: Testing and corrections	C, D	4	6	8	6
F: Launch	E	1	2	3	2

Next, we identify all possible paths from the beginning to the end of the project and add up the times Te:

Path P1:
- Task order: A → B → C → E → F
- Te = 6+4+11+6+2=29 days
Path P2:
- Task order:: A → B → D → E → F
- Te = 6+4+3+6+2=21 days

The PERT diagram analysis shows us the minimum duration of the entire project. It is always necessary to look at the critical path, i.e., the longest one. In our example, it is path P1, lasting 29 days. What other conclusions can be drawn?

Delaying any task on the critical path automatically delays the entire project. These tasks should be assigned the highest priority.
Path P2 has 8 days of slack time. This means that the training task can be delayed by up to 8 days and the deadline will remain safe. This gives flexibility in project resources management.
If you need to speed things up, shorten the tasks on the critical path. Focusing your efforts on tasks with slack time will not speed up the completion of the project.

FlexiProject and PERT diagram

If you rely on solutions such as PERT diagrams, project management tools are essential for the work of a project manager. FlexiProject takes over all the most difficult and time-consuming work associated with this method, enabling effective schedule management. Instead of manual drawing and complex calculations, FlexiProject acts as an intelligent assistant:

Automatic visualization: all you need to do is enter a list of tasks and their interdependencies. The program immediately generates the entire diagram automatically – you get a clear, visual map of the project connections instead of a long list. This eliminates errors and saves a lot of time.
Critical path “in real time”: FlexiProject, a project planning system, not only calculates times, but also continuously monitors and indicates the Critical Path in real time.
Proactive control: if a delay causes other tasks to become critical, the program immediately recalculates the project schedule and highlights the new critical path. This way, you always know what to focus on, which minimizes risk and allows you to actively control the project completion date and resources in the project.

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Summary: When is a PERT diagram essential in project management?

Although project managers have many tools at their disposal, the PERT diagram stands out as an extremely effective method for assessing timelines, necessary resources, and, most importantly, the longest sequence of tasks in a project. This diagram complements earlier activities such as the project charter and planning. It allows for precise modeling of task time uncertainty and is a great choice when you need to create a simple but precise action plan to share with your team and stakeholders, bridging the gap between initial definition and detailed implementation.

Włodzimierz Makowski

CEO FlexiProject

Włodzimierz is a board member at FlexiProject and an expert in project management. Over the past 20 years, he has gained extensive experience working with international companies on the delivery of dozens of large-scale projects - today, he passionately applies this expertise in developing the FlexiProject system. He leads the team responsible for its development, implementation, and promotion, helping modern businesses achieve their goals.