PERT chart 101: a step-by-step guide for project managers

PERT is one of the most effective tools for handling complex projects with uncertain times. With some math and a detailed diagram, you’ll immediately know when you’ll complete your project if the worst or the best were to happen.

In this guide, you’ll learn everything you need to plan projects with PERT. Follow five simple steps to create a PERT chart, visualize your activities and estimate project completion (even in unclear scenarios.)

What is PERT in project management?

PERT stands for Program Evaluation and Review Technique. It’s a project management tool that estimates how long it would take to complete a project when activities have uncertain time frames.

A crucial aspect of PERT is how activities appear in a visual sequence of nodes representing activities and arrows indicating dependencies (or what activity goes next).

Here’s an example of a very simple PERT chart template.

It’s a visual representation that helps project managers understand what tasks they must complete to keep the project on track.

Often, PERT charts are more complex, including critical time estimates in boxes, as in the image below.

PERT has several key characteristics that make it a unique tool for complex projects, including the following:

• Use of three-time estimates (optimistic, most likely and pessimistic)

• Designed for projects with undetermined timelines

• Helps visualize task dependencies

• Identifies the critical path of a project

• Useful for complex, non-routine projects

PERT, by definition, evaluates and reviews your project through the lens of probability, not possibility. It doesn’t just ask, “What could happen?” but “What is likely to happen?”

For beginners in project management, understanding the PERT’s meaning can help you shift your mindset from hoping for the best-case scenario to preparing for the most likely one.

PERT vs. CPM

CPM (critical path method) is another project management tool that complements PERT. While PERT provides a framework for scheduling tasks, you can use CPM to find the shortest time to finish your project or critical path.

The main difference between CPM and PERT is how they handle time estimates. PERT uses three-time estimates, while CPM uses one.

Pros and cons of PERT

PERT’s origins date back to the late 1950s when the U.S. Navy developed it for the Polaris submarine missile program. While project management books focus on industry trends like PMBOK, agile or SCRUM, PERT remains a powerful tool for complex projects with “what if” scenarios.

Abraham Zavala-Quinones, a project program manager for Baxter International Inc., states:

Like any tool, PERT has its challenges if you use it as a one-size-fits-all solution rather than a strategic component of your project plan.

Advantages of PERT

Using PERT has several benefits besides handling complex projects and managing unknown timelines.

According to a 2023 project management survey of 217 respondents, 31% of project managers indicated unrealistic deadlines as one of their top challenges. PERT can be a key component in defining your project baseline.

Here are the advantages of using PERT:

• Stakeholders’ expectations. PERT charts set realistic expectations. They help to visually communicate to stakeholders why optimistic timelines aren’t always feasible.

• Dependency visualization. PERT charts show your project’s task relationships. If one task takes more time than expected, you can quickly see which other tasks it will affect.

• Identification of time-sensitive activities. By determining the critical path and time-sensitive activities, you’ll know where to focus contingency plans.

• Team motivation. Showing your team PERT diagrams and how their tasks interconnect can boost morale and foster a sense of fellowship. Everyone sees how their work contributes to and affects the project timeline.

• “What if” scenario planning. When the unexpected happens, PERT gives you room for time adjustments.

While the PERT methodology has many advantages, it’s important to be aware of and prepare for its potential challenges if you plan to use this project management tool.

Disadvantages of PERT

PERT offers many opportunities in project management, but there are also limitations to its use:

• Time-consuming. PERT can be difficult to set up at the beginning. Using three-point estimates for each task requires input from multiple team members and stakeholders, which consumes a lot of time in meetings.

• Requires maintenance. Whenever there’s a change, you need to update your PERT chart by recalculating project durations and even redrawing parts of the diagram.

• Overly complex for simpler projects. The effort you put into creating and maintaining a PERT chart might outweigh the benefits for simpler, more predictable projects.

• Can overlook non-critical tasks. Identifying the project’s critical path in PERT can lead to focusing too much on time-sensitive tasks. Non-critical tasks might receive less attention, impacting the overall project quality or timeline.

• Doesn’t account for resource allocation. A PERT chart might show that certain tasks can happen in parallel, but timelines become unrealistic if they require the same limited resources.

While PERT’s strength is uncertainty, estimates might not be helpful if an inaccuracy is large enough.

PERT vs. Gantt charts: which should you use?

Both PERT and Gantt are popular tools in project management and complement each other in many cases.

Generally, project managers use PERT charts to plan projects and map out dependencies. On the other hand, Gantt charts help with execution by helping monitor day-to-day activities and resource allocation.

A Gantt chart is a useful project management tool to visualize projects with well-defined tasks and timelines.

It provides a clear representation of tasks over time, which allows project managers to see:

• When each task starts and ends

• How long each task is expected to take

• How tasks overlap or run in parallel

• The project timeline from start to finish

A Gantt chart uses horizontal bars on a timeline to display the duration of a task and lines to connect them and indicate dependencies.

At first glance, Gantt charts are more intuitive to read than PERT charts. The length of each bar directly corresponds to the task’s duration and the project’s completion date is visible at the end.

In contrast, PERT charts help visualize complex task dependencies and interdependencies. They’re helpful when task durations are hard to estimate or when you need to identify critical paths in your project.

While you can identify the critical path in both PERT and Gantt charts, it’s more straightforward in PERT. The trick to choosing one or the other is to understand the nature and needs of your project.

Here are some key differences to help you decide:

How a PERT chart works

To understand how a PERT chart works, you should start with its structure, terminology and how to interpret it. Here are the basics:

Structure and components of a PERT chart

PERT diagrams can come in two forms:

• Activity-on-arrow (AOA). when you want to emphasize the flow of time in your project and show dummy activities (activities that don’t consume time or resources but show dependencies)

• Activity-on-node (AON). when your project emphasizes the activities and relationships (like start-to-start or finish-to-finish dependencies)

In this guide, we’ll focus on AON diagrams because they’re easier to create and modify. However, you can choose one according to industry standards or your project’s and stakeholders’ specific needs.

Initially, you’ll sketch your chart using circles to represent activities. Once you need more information, your node will be a 2x3 grid that includes:

• Activity name

• Expected Time (ET)

• Early Start (ES)

• Early Finish (EF)

• Late Start (LS)

• Late Finish (LF)

• Slack or float

The rectangular grid is more useful for communicating time constraints and helping you plan your project. Plus, it gives you more information in a snapshot.

Terminology

The following terms are the building blocks of PERT charts and calculations. Familiarizing yourself with them can help you interpret and communicate about PERT charts with confidence.

The three-point estimates

The reason behind PERT’s ability to handle uncertainty in project timelines:

Expected Time (ET)

The weighted average of the three-point estimates you can calculate using the PERT formula:

Time analysis components

Crucial for project managers to analyze the timing and flexibility of activities in a PERT chart.

These components help you understand when activities can start and finish and how much extra room you have for them in your schedule.

The critical path

The critical path is the sequence of dependent activities that determines the shortest duration of your project. You’ll often see the critical path’s arrows highlighted with a different color in PERT charts.

Here are some main characteristics of the critical path:

• Any lag time in an activity on the critical path will directly delay the project

• All activities in the critical path have zero slack

• Not all activities with zero slack are in the critical path

• The critical path can change if there are project bottlenecks

The critical path is the backbone of your project schedule. If you can see your critical path, you can find opportunities to shorten the project timeline.

How to interpret a PERT chart

After learning the terminology, check the PERT chart below to see how it all comes together.

Here are some questions you might get asked when analyzing a PERT chart and their answers:

PERT chart FAQs

How to create a PERT chart in 5 simple steps

Creating a PERT chart can be tricky as each component you include in the node has a specific calculation. However, knowing how to build one is a valuable skill to keep in your project management toolkit.

Here are five simple steps to create an effective PERT chart:

1. Define the project activities

Make a list of all the activities necessary for your project. You can begin by asking yourself:

• “How does the project start?”

• “How does the project end?”

• “Are there any specific steps I should take from start to finish?”

Remember to consider both internal and external factors that can affect your project. For guidance, you can check past projects or use the waterfall methodology to structure your activities. Its sequential phases can help define activities.

If you’re unsure about including an activity, consider whether it contributes to the project goals. If not, it might be unnecessary.

2. Identify dependencies between activities

Identifying dependencies is an iterative process. When you start building your PERT chart, you might find dependencies you didn’t consider initially.

For example, let’s say your team has to build a mobile app for a client. Your table of activities, along with its dependencies, might look like this:

The predecessors are the activities you must complete before the current activity can begin. In this case:

• Activity B can’t start until A is complete, neither can C or D

• E can only start when C is done

• F depends on B

• G needs both D and E to reach completion

• H requires finishing F and G first

Discuss the project activities with your team to know if there are activities they need to perform in a specific order.

3. Include time estimates and calculate the expected time

Start by getting the optimistic, most likely and pessimistic times for each activity. You can get the time estimates using multiple sources, such as:

• Subject matter experts (SMEs): experts are a reliable source of information. They have experience and knowledge from performing similar tasks before.

• Historical data: check how long similar activities took in previous projects at your company. Consider both successful completions and instances where things didn’t go to plan.

• Brainstorming: do a brainstorming session with your team to combine knowledge from different fields.

Once you have your three-point estimates for each activity, add them to your table and use the PERT formula to calculate the expected time.

While expected time is a more realistic estimate than most likely, it’s still an estimate. Make sure to adapt your estimates as the project progresses.

4. Calculate other node components

Here’s how to calculate the remaining node components:

Early Start (ES) and Early Finish (EF)

Calculate these moving forward from project start to finish (start from A.)

ES is the earliest an activity can begin, meaning it can only start when its predecessors finish. For the first activity, ES is always 0. If an activity has multiple predecessors, use the largest EF of all predecessors as the ES.

To calculate EF use the formula:

Late Start (LS) and Late Finish (LF)

Calculate these moving backward from project finish to start (start from H.)

LF is the latest time an activity can end without delaying the project. For the last activity, LF equals its EF. Use the smallest LS of all successors as the LF for activities with multiple successors.

To calculate LS use the formula:

Slack

Slack is the amount of time an activity can be delayed without affecting the project’s end date.

You can calculate it using either of these formulas:

Here’s what the table in your hypothetical example would look like:

You can draw an initial PERT chart to help you visualize dependencies and complete it as you make calculations.

5. Create a network diagram and highlight the critical path

Draw arrows between nodes to show dependencies. Arrows go from predecessor to successor activities. You can use Excel or Lucidchart to draw your PERT chart.

Include the calculations for ET, ES, EF, LS, LF and Slack in each of your nodes. You can identify the critical path in your diagram by finding the path in which Slack is zero. Highlight it with a different color for easy visualization.

Now, you have a PERT chart with all the information you need to make decisions about your project schedule.

How to follow the PERT process in Pipedrive

Once you’ve created the PERT chart, you can set up your project in Pipedrive’s CRM system using its project management software.

There are two ways to create a project in Pipedrive.

Use native Pipedrive features

Create a new pipeline and set the different phases for your project (e.g., planning, implementation, review, done, etc.)

Add tasks as deals to their corresponding phase. You can set time estimates and add dependencies within each deal using custom fields.

Setting reminders for when tasks reach their completion time can help you keep track of these dependencies.

Use Projects by Pipedrive (for more complex projects)

Go to the Projects tab and click the “+ Project” green button or the “Quick add” button next to the search bar.

Then, define the phases of your project and milestones.

Use your PERT chart to complete the information in your project with Projects’ additional features:

• Include tasks for each project phase (and subtasks, if necessary)

• Set the start and end date of each task

• Use custom fields for the three-point estimates and expected time

• Track your entire project with a progress bar (it updates whenever you complete a task)

In your project details tab, you can store all your project files, leave notes for your team and check emails related to your project.

Final thoughts

PERT charts take time to develop but are worth the effort. Being able to quickly glance at your chart and know that if activity D is delayed three days your project is still on track allows for more confident decision-making.

Start by deciding if PERT is the right tool for your project, create your chart and achieve flexibility in project scheduling – even when time estimates aren’t entirely predictable.