PERT Chart vs Gantt Chart Differences
PERT charts are generally used before a project begins to plan and determine the duration of each task. Gantt charts are used while a project is happening to break projects into smaller tasks and highlight scheduling constraints.
When you and your team need to complete a project, you’ll realize that there are a ton of diagrams you could use for project management. Work breakdown structures help you split your project into manageable pieces. Gantt charts help you see the progress your team has made. But only one diagram can do it all.
A PERT chart allows you to break a project into smaller tasks, estimate the time necessary to complete each task, and keep your project on track. Learn more about PERT charts, including the differences between Gantt charts and PERT charts and the steps to build an effective PERT chart.
Want to skip ahead to a PERT chart template?
What is a PERT chart?
PERT stands for Program Evaluation and Review Technique. A PERT chart illustrates a project as a network diagram. The U.S. Navy created this tool in the 1950s as they developed the Polaris missile (and time was of the essence—this was during the Cold War, after all). Project managers create PERT charts to analyze the tasks and minimum time necessary to complete a project.
When should I use a Gantt chart instead?
Many people who ask “What is a PERT chart?” will immediately follow up with a second question: “How is it different from a Gantt chart?” Both visualizations deal with project management, but you should consider some key differences between Gantt charts and PERT charts before you decide which will work better for your project.
Like PERT charts, Gantt charts break projects into smaller tasks and highlight scheduling constraints. However, project managers use Gantt charts while a project is happening—they schedule tasks by date and show how much work has been completed. Every activity is represented with a bar that stretches from the start date to the end date of that activity.
PERT charts are generally used before a project begins to plan and determine the duration of each task—so they don’t have to show the actual dates of your project. They also do a better job of showing whether certain tasks need to be completed in order or whether they can be completed simultaneously. Use a PERT chart if you need to:
- Show the interdependency of certain tasks.
- Anticipate the amount of time it’ll take to complete a project.
- Determine the critical path to meet your deadlines.
- Plan for large or more complex projects.
If you decide that a Gantt chart will best fit your needs, see how you can create a simple Gantt chart right in Lucidchart. If you decide that you need a PERT chart, continue onward! This article will show you how to create one.
How to draw a PERT chart
You can design your PERT chart in a few different ways. The traditional PERT chart (also known as an activity-on-arrow diagram) contains two different elements:
- Nodes represent events or milestones in your project. You can use either numbered circles or numbered boxes.
- Arrows represent tasks. The direction of the arrows shows the sequence of tasks. Diverging arrows indicate that you can complete those tasks concurrently. In the example below, tasks 1, 2, 4, and 6 have to be completed in order.
Now the activity-on-node diagram, a variation of the original PERT chart, has become more common. The nodes represent activities, and the arrows simply show dependencies and relationships between tasks. In this version, the nodes are usually small charts (the ERD shapes in Lucidchart work well for this purpose). These charts show the task, the date you anticipate starting the task, the time it’ll take to complete the task, and finally the date you anticipate finishing the task.
Regardless of which structure you follow, go through these steps to gather information and build your PERT chart:
1. List the activities involved in the project. Break your project down into more manageable tasks. Limit your list to only high-level activities; otherwise, you may have trouble managing the resulting PERT chart.
Note: You may want to start with a work breakdown structure to determine all the tasks required to complete a project and then create a PERT chart to arrange those tasks and predict milestone and completion dates. Read our blog post about work breakdown structures to learn what they are and how to build one.
2. Consider dependencies. For every activity, ask yourself whether the task depends on the completion of another task. This step will determine the order of nodes on your PERT chart.
3. Place nodes and arrows based on the information you have gathered. For activity-on-arrow diagrams, number a node for each milestone in the project. These milestones are generally based on the tasks you wrote down, such as “Project start,” “Hardware designed,” “Tutorials drafted,” etc. If you’d like to keep track of these milestones, you can create a numbered list to accompany the diagram. For activity-on-node diagrams, create a node for each task and write the task directly on the node.
Organize these nodes based on the dependencies you found. If activities need to be completed in sequence, place them in the right order and connect with arrows. You can use a diverging arrow to show tasks that do not depend on other tasks. For activity-on-arrow diagrams, once you connect your nodes, you’ll write the activity name on the arrow.
3. Add the completion time for each activity. Estimate the amount of time you and your team need to complete each task. Write that amount under the task name. If you’re not confident in your estimation, experts recommend a three-point method. For each task, write down:
- An optimistic time estimate (o), or the shortest possible time the task will take
- A most-likely time estimate (m),
- A pessimistic time estimate (p), or the longest possible time if the task takes longer than expected
Then plug those numbers into the following equation to get the best estimate: (o + 4m +p)/6
Depending on the style of your PERT chart, you can add expected start and finish dates for each tasks. And that’s it!
Been wondering how to create a PERT chart in Excel? Lucidchart makes it easier to visualize tasks and move them around until you find the critical path for project completion. And, with our free add-in, you can insert your PERT chart into Microsoft Excel in just a few clicks.
How to interpret your PERT chart
Pretty simple, right? Once you draw your PERT chart, you’ll want to use that information to determine a realistic timeframe for your project.
Many people combine PERT with the critical path method (CPM). Look at your diagram and find the longest path based on the time estimations you have entered. Remember that you’re not looking for the path with the most activities—you’re looking for the path with the activities that will take the longest. Once you add the times from each activity together, you’ll know how long the entire project should take.
After you find the right deadline and start your project, your PERT chart doesn’t just become useless. Refer to this diagram to ensure that you finish your project by the deadline or to adjust the times as circumstances change.
Ready to get started? PERT charts make it simple to see the critical path and determine how long a project will take. Give stakeholders a realistic timeline for your projects and communicate expectations to your team with Lucidchart.