Critical Path vs Longest Path

critical path vs longest path

When it comes to your project’s Critical Path, there are few topics more important to your client. The expectation is that YOU, dear scheduler, are the expert. So YOU had better know your project’s Critical Path inside and out.

But wait! ……Is it the Critical Path you should be reporting on, or the Longest Path?

The terms Critical Path and Longest Path are often used interchangeably when discussing your project’s super-high-priority sequence of work to deliver on time. It’s confusing to many. And there are some great debates about the differences.

The good news is that there IS an important distinction between your project’s Critical Path and the Longest Path and we’ll explain it to you here.


What is a project’s Critical Path?

Well, it depends.

“It is the path of 0 Total Float through the project.” – popular definition.

Well, the above definition for Critical Path IS true but only in under a very strict set of circumstances. When IS it true?

A. No Project Deadline. 

When we set a deadline on our project (ie: a Must Finish By date in Primavera P6), it results in the appearance of negative Total Float if you are late, and positive float if you are early. Thus, the path of 0-float activities vanishes.

B. No Constraints. 

Constraints affect an activity’s Total Float value. Constrained activities can show negative Total Float and can drive negative float to their successors or predecessors. Applying contraints throughout a project will affect what activities are Critical. And so, again the path of 0-total float through the project eludes you.

C. No Actuals.

A project with actuals may not show a path of 0-float either. The actuals and the order of execution  can affect the activities’ Total Float as well.


So it seems that the age-old “path of 0 Total Float” definition doesn’t fit in our complex project environments.

And if you care to do some digging online, you’ll find that our attempts to clear up the concept of “Critical Path” has led to numerous new ways to define the term “Critical” for a project. Search and you are likely to run into terms like:

  • Criticality
  • Critical Chain
  • Near-Critical
  • Critical Risk

So which activities are critical?  It all boils down to this – it depends.

What will be your project’s Critical Path depends on how you end up defining the term “Critical” and specifying what activities are “Critical” in your eyes. You may have activities that you deem “Critical” regardless of what the software says their Total Float values are. You may have activities that have positive Total Float but are Critical in the client’s eyes. You may want to track a specific path of activities through a project. This is called real-world project controls.

“Critical Path is a path through your project which has activities YOU consider critical.”


What is a project’s Longest Path?

There’s more clarity to be found here. The longest path is THE LONGEST path – there’s no debating the term “longest”.

The Longest Path is the path through a project network from start to finish where the Total Duration is longer than any other path.

It makes sense to focus on a project’s Longest Path as it is definitely the path where deviations from plan will affect the project’s finish date.


Is there overlap between Longest Path and Critical Path?

Let me say it this way – the Longest Path is a Critical Path, but not all Critical Paths are the Longest.

Because we can alter Critical Path through constraints, actuals, deadlines and other mechanisms, often we end up with a path that has more Critical activities than would be on the Longest Path.

If you start with a baseline schedule that has no actuals, no constraints and no deadline, and you perform some CPM scheduling on it, the path of 0 Total Float will be the Longest Path. If you add up the durations of all activities on that 0-float path, it will have the longest Total Duration.

If you calculate the Longest Path for a project that does have actuals, constraints, a deadline, etc, you will come up with a path of activities that are “important”, need I say “small-c critical” to the timely completion of the project.


Which one should I use?

I would strongly suggest you get to know your project’s Longest Path and monitor it. What the client wants to see might be something else, but the timely completion of Longest Path activities leads to your project finishing on time.

Most software packages let you track both Critical Path and Longest Path. Track both.

Lastly, I would advocate educating yourself on how the software that you use defines Critical Path and what sorts of things (constraints, relationships, lag,  etc.) will affect an activity’s Total Float.


What Do You Think?

The Critical Path vs Longest Path debate is long from over. What are your thoughts about which one to use and why? Let us know in the comment section.


Michael is an avid project controls blogger and is the Chief Learning Officer here at Plan Academy. Michael has taught 1000s professionals how to use project controls software like Primavera P6 over the past 10 years through his online courses and tutorials. Michael is a member of AACE, the Guild of Project Controls and holds his PMP certification from PMI.