How To Calculate Critical Path in Project Management

Mike Kulakov, August 14, 2020
Critical Path in Project Management

At first, understanding how to calculate Critical Path, and how to use Critical Path method in project management can seem daunting. But when you move past the concept of using a lot of data and algorithms to calculate it, you realize it’s pretty common sense!

In this article, we’ll take a dip into the Critical Path concept and its components, talk about why it’s a useful tool, and give you some Critical Path method examples, solutions for using project management software, and tools to aid you in calculating Critical Path.

What Is Critical Path

In short, the definition of Critical Path is as follows:

Critical path is the longest sequence of activities in a project plan which must be completed on time for the project to complete on its due date.

Most projects are broken down into tasks, or activities (whatever you want to call the smaller bits that need to be completed in order to get the project done).  The critical path is the series of those tasks that, when followed in sequential order and accounting for all the above variables, will take the longest to complete the project. 

Simply put, critical path calculates the SHORTEST project duration possible by lining up the LONGEST sequence of dependent tasks necessary to complete the project.

Importance of the Critical Path Method

A project without a critical path is like a ship without a rudder.

D. Meyer

The beginnings of using sophisticated project management systems such as the critical path method can be traced back to monumental projects such as the building of the Great Pyramid of Giza, the Great Wall of China, the Panama Canal, and the Trans-Siberia Railroad, just to name a few.

Imagine what these history-altering construction projects would look like if it weren’t for project management systems like the Critical Path Method. Chaos, right?

The same is true for current projects in today’s organizations. Without the ability to calculate potential obstacles within the timeline of a project, analyze the components of the project required for completion, and develop solutions resolving those obstacles, many projects would never leave the ground.

Critical Path is called “critical” because it’s the “red alert” of paths. If your project doesn’t follow the sequence and timing of the critical path, it won’t be finished before the deadline successfully.

Note the word “successfully” there because the worst-case scenario is, the project will be late and fail to achieve its goal(s). 

While the Critical Path Method may not be the only project management method out there, it’s the go-to methodology for many project managers.

project management methodology
Most Popular Project Managers Methodologies, Compared

See this post for a comparison of this and other popular project management methodologies!

How to Calculate Critical Path

Before we dive in, for the visual learners among you, here’s a great, highly detailed video showing how to calculate Critical Path:

Finding the Critical Path for a project rests first on six steps completed in order. Let’s break down those steps!

STEP 1. Divide the Project into Tasks

  • Make a list of your tasks
  • Assign each task with a name or a shortcode

STEP 2. Order and Identify Dependencies 

  • Put your tasks in a logical line-up
  • Identify dependencies

EDITOR’S NOTE: There are usually three main types of anticipated dependencies in a project.
Mandatory dependencies – dependencies due to the physical limitations of the task. Often tasks in research and development projects are labeled as compulsory.
Discretionary dependencies – dependencies due to decisions made by teams along the course of a project. 
External dependencies – dependencies due to a third party’s needs, e.g. needs of a contractor or shareholder.

STEP 3. Create the Network Diagram

Now, you can make your task line-up visual. The good old pen-and-paper method may work well; a more sophisticated way of doing this is by using a network diagram such as a PERT or Gantt chart (more information to come on these later in this article) and connecting your tasks in the chart.

STEP 4. Estimate Duration

  • Clearly define the beginning and end date for each task.
  • Taking into account order and dependencies, set each task’s estimated duration.

EDITOR’S NOTE: to calculate the time for each task, and prioritize them easily, use a time tracking app. This will help you to accurately monitor and evaluate time on your project.

STEP 5. Perform Resource Leveling

The main aim of resource leveling is to allocate resources efficiently so that the completion of a project lies within the given time, and no resource conflicts take place. Resource conflicts may lead to:  

  • Delays in the completion of specific tasks.
  • Difficulties in assigning resources.
  • Inability to change task dependencies.
  • Removing tasks as needed.
  • Adding more tasks as needed.
  • Overall delays and budget overruns of projects.

An app with time tracking capabilities can make leveling HR resources easier by allowing a team to more accurately predict future team efficiency based on past completion times, particularly in roles requiring specialization. 

STEP 6. Determine the Critical Path

Find the longest sequence of project tasks in the diagram. This sequence is the critical path for your project!

Image credit: Researchgate

Variables Within Critical Path

Let’s take a look at some terms and concepts you’ll need to understand to be able to calculate critical path.

Float/Slack

Float (sometimes called slack) of a task is the duration that it can be delayed without delaying the subsequent task, completing the project, or violating a schedule constraint.

Early Start and Early Finish

Early start and early finish are just that: the most initial time a task on a path can begin, and the earliest it can finish. 

Beginning a task on its early start is conditional upon the completion of tasks on the same path. If the prior task can finish on it’s early finish time and it completes the last task on the path, the team can begin with an early start on the next task, freeing up more time to use on other, more unpredictable tasks.

Forward Pass Method of Determining Early Start:

Beginning with a project start date, you can assess an early starting date for each subsequent task on the project path by following it from left to right in the diagram.

Steps to calculating Early Start dates:

  1. Record the length of time the prior task took for completion, and add that to the start date of the current task. 
  2. Add any extra time the project took to complete, or subtract any time it took to finish early. 
  3. Take into consideration the resources needed for each task (there should be a resource calendar available to show this), including equipment, HR hours, or any other resources required, and add any time that may be needed to accommodate those resources. (Things like off-days, maintenance, etc.)
  4. Based upon the above three steps, determine the date calculated and use that for the early start date of the next task. 

Late Start and Late Finish

Late start and late finish are the adverse. They’re the latest beginning, and ending dates a task can be assigned without derailing the other tasks on the path or the whole project altogether.

Begin with the project plan’s completion date, then the critical path, then paths of descending order of completion dates. Work right to the left through the network diagram and subtract the time each task will take on each path to calculate the latest date that task could begin while still meeting the project’s completion date. 

Just as with the forward pass (early start/finish, left to right), resources must be taken into account when expediting the backward pass. 

Steps to calculating Late Starts dates

  1. Assign the overall project’s completion date as the finish date of the task before it on the critical path, then on all subsequent paths from longest to shortest times. 
  2. Subtract any extra time that task took for completion, or add any spare time left over to the estimated late finish date. 
  3. Factor in resources, just as with early start/finish and subtract the time those contribute to each task. 
  4. List the date calculated from the above three steps as the late start date for the previous task on the path. 

To calculate late start dates, begin with the project completion milestone and assign it as the finish date of its predecessor activities. Follow the steps above to estimate the late start dates of predecessor activities, assuming finish-start relationships.

Tools And Software to Calculate Critical Path

From sophisticated software, to pen and paper, it’s up to you and your team to decide what is most productive and helpful to use. Below, we’ll take a glance at some CP software, templates, and diagram generators available on the web to help get you started, stay organized, and be successful when determining critical path for your project.

We’ll also get into how to use a PERT or Gantt chart to estimate critical path, and provide a clear, visualized model for the CP of your project. 

Tools to calculate critical path

With a quick google search, you’ll discover there are a host of tools out there to calculate CP, as well as some critical path templates to illustrate your project’s critical path.  

Capterra provides feature comparisons and ratings between the most popular project management software programs available right now. Many, if not all of them, feature customizable templates and Gantt chart generators to make finding the critical path for your project easier. 

MS Project has a pretty comprehensive suite of resources for the critical path process, from calculators, to Gantt chart generators, to in-house tutorials. You can learn more about finding critical path in MS Project here.

PERT chart for estimating critical path

A PERT chart is an activity-on-arrow visual outline of a project’s schedule. It shows the sequence of tasks on a path and determines the time it will take to complete each, and which of those may be completed simultaneously. 

Like the critical path method, a PERT chart will contain information like float between activities, early start/finish dates, and late start/finish dates.

Usually, PERT comes into play at step four of the Critical Method process- determining task completion times. Many project managers use PERT because it estimates completion times based on the most likely scenario rather than unrealistic time frames.  

When using PERT, you will estimate the shortest, longest, and most likely length of time a task may take if it’s predicted to take longer than anticipated.

Terms you’ll need to know to use a PERT chart include:

  • Nodes: Usually circles that represent project tasks and milestones
  • Arrows: Arrows represent the sequence of a task. Arrows on the chart can diverge when tasks can be completed at the same time.
  • PERT Event: The beginning or the end of a task
  • Slack: How long a task can be delayed without interrupting other tasks or the whole project
  • Critical Path Activity: An activity where no slack is available 
  • Lead Time: The time it will take realistically to complete a task without delaying or interfering with the next task 

Here’s a helpful video on how to create a PERT chart:

Gantt chart for estimating critical path

Gantt charts are best for smaller projects, linking dependent tasks instead of establishing a network diagram showing interconnecting independent tasks as a PERT chart does for larger, more complex projects. 

While PERT charts usually come into the picture during the planning of a project to determine the most efficient scheduling, the Gantt chart model is generally used as a project is progressing. It illustrates the length of the duration of a task, who is involved and responsible for the task, and reveals scheduling conflicts. In a Gantt chart, you can revise and edit your project’s begin and end dates throughout the project.  

To create a Gantt chart, or bar chart, check out this video:

What’s the difference between a PERT and Gantt chart view?

Gantt and PERT charts are both visual depictions of a project’s completion, illustrating it’s tasks, the timeline for those tasks, and variants that can impact the schedule, such as resource leverage and dependencies. 

The primary difference between a PERT chart and a Gantt chart is its timeline representation. PERT uses a network diagram, or flow chart, while a Gantt chart uses a bar chart as a representation of a timeline.

Critical Path Method: In Closing

Due to its incredible effectiveness, critical path is a project staple in organizations everywhere. We hope this article has helped make learning how to find critical path much more straightforward. Happy planning!

Featured image credit: twproject

Mike Kulakov

IT entrepreneur, executive and a former engineer. Responsible for company growth as well as the team’s motivation. Big fan of playing tennis, snowboarding, traveling, reading books, and (of course) I live and breathe our product.