How to Calculate Critical Path, Float, Early Start & Late Start, and Early Finish & Late Finish


critical path method

This is a part-post from Developing Project Schedule project management activity post. You will learn how to calculate critical path, float/slack, early & late starts, and early & late finishes of a network path in this article.

For easier understanding this post is divided into four parts that can be navigable at the bottom of each part.

I suggest you get yourself a cup of your favorite beverage (I was going to say Coffee), this one is going to be a long ride. ๐Ÿ™‚

As we saw in Developing Project Schedule project management activity, Critical path is made up of series of activities from beginning to the end, where each activity has a dependency over previous activity in such a way that delay in any one activity causes delay in all subsequent activities, causing the project to slip.

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In other words, critical path is the longest path in your project’s schedule network diagram, and is the SHORTEST possible duration for the project.

How to Calculate Critical Path

Let us take a simpler example than John’s home construction example we saw in previous lessons.

This project involves assembling a train set.

Calculating Critical Path is a simple 4-step process.

Step 1: Find Activities

Activities for this project are as below (output from Define Activities process) –

  1. Assemble two-tier bridge
  2. Join winding tracks
  3. Assemble and add train station
  4. Place standalone items around
  5. Assemble and add construction site
  6. Join train engine and bogies
  7. Place the train on the track
  8. Start the engine and let it chug!

Step 2: Build Schedule Network Diagram

Sequence activities and build schedule network diagram (output from Sequence Activities process) . This is how it looks, with individual activity duration in minutes –

critical path trainsetFigure 2: Assemble train set – schedule network diagram

Step 3: Find all Possible Paths

Find all possible paths through the diagram, there are 3 in our case –

A -> B -> F -> G -> H

A -> B -> C -> D -> G -> H

A -> B -> C -> E -> G -> H

Step 4: Calculate Duration for Each Path

Let us see the duration for each of these paths –

A -> B -> F -> G -> H —> 10+20+4+2+2 = 38 minutes

A -> B -> C -> D -> G -> H —> 10+20+5+10+2+2 = 49 minutes

A -> B -> C -> E -> G -> H —> 10+20+5+2+2+2= 41 minutes

The network path with longest total duration is the critical path!

Critical path is the shortest duration required to complete the project successfully.

In our example this is the second path:ย  A -> B -> C -> D -> G -> H, which comes to 49 minutes.

critical path firstpathFigure 3: Assemble train set – Critical Path

Note that sum of durations of all activities on critical path comes 49 minutes, and sum of duration of ALL activities on the project is much longer. If managed well, the whole project can be completed within the critical path’s duration.

In the next page, let us see how to calculate Float for activities..

(please use the page numbered link below to navigate)..

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{ 43 comments… add one }
  • Moses Clifford Hinsch March 4, 2023, 1:34 am

    So wonderful!

  • Danika October 4, 2021, 7:25 pm

    Absolutely amazing notes! I wish you were my lecturer, Iโ€™d ace this subject!! Thank You for this

  • Majaliwa January 24, 2021, 11:19 pm

    Thanks so much for this

  • Carol December 26, 2020, 12:35 am

    Thank you. This is so helpful

  • Daniel Keegan March 6, 2020, 9:45 pm

    Hi,

    Is it possible to have a late start earlier than the early start? Thanks in advance

    • Shiv Shenoy March 10, 2020, 4:39 pm

      No, because in reverse pass we take late start of an activity as late finish of previous activity in the path. The narrowest difference between late start and early start is zero (as in case of very first and very last activity, and few more possibly in between) but never negative. Watch the video in the last part of this article to understand this concept visually.

  • Aljoker January 15, 2020, 10:59 pm

    Thank u so much

  • Arvind December 9, 2019, 11:21 pm

    very very thanking you sir

  • Dhiraj November 22, 2019, 3:01 pm

    Nice explanation with diagrams.

  • Ken October 2, 2019, 6:29 pm

    thanks for helping me understand how to calculate the critical path

  • Osagie March 23, 2019, 2:06 am

    Easy to understand and very interesting. Thanks a million times.

  • Joel February 24, 2019, 5:40 pm

    It has helped me so much

  • Pankaj April 28, 2017, 9:45 am

    Hi Shiv,
    Just started to read your posts, feel you have done amazing work.
    Post going through your posts, i believe this could substitute all the trng materials out there ๐Ÿ™‚

    Regards,
    Pankaj

  • Shannon February 5, 2017, 9:15 pm

    I was really impressed with the easy to understand breakdown. Both the written explanation, diagrams, and videos were very comprehensive and easy to follow. There were no gaps in the explanation and demonstrations. Huge help to me. Thank you~

  • Bernard Bartholomew January 21, 2017, 9:12 am

    Thanks a lot of shiv this was more than helpful me. You have done a great job. May continue to wax strong.

  • M. Gibson January 10, 2017, 1:18 pm

    Very useful example

  • Bhushan April 11, 2016, 12:28 am

    Wonderful, doubt if one can find anywhere anything better on the above subject on the net.
    Great work Shiv, thank you so much, keep up the good work.

    Regards,
    Bhushan

    • Shiv Shenoy April 19, 2016, 12:34 pm

      Hey Bhushan, am glad to know you are finding these notes useful.

      Good luck!
      Shiv

  • MOHIT CHUGH November 17, 2015, 4:45 pm

    seriously helped me alot. thank u so much

    • Shiv Shenoy November 18, 2015, 9:43 am

      Hi Mohit,
      Glad to see you are finding these useful.
      Best wishes for the exam!
      Best,
      Shiv

  • Hamed Barhumi March 12, 2015, 2:54 pm

    Hi Shiv, hope you are doing fine, I would appreciate sending me your posts to my e-mail, im applying for PMP certification & interseted in project scheduling hints & Tips..

    Yours,
    Hamed B.

    • Shiv Shenoy March 17, 2015, 5:49 pm

      Hi Hamed, please refer to the Start Here page from the menu above. You can also schedule a free Skype call with me using button at the bottom of this page. Over this call we can discuss what should be your examination approach, and I can share few tips that might be useful.

      Cheers,
      Shiv

  • khadar January 27, 2015, 12:19 am

    thank you for your help

  • aaaaa December 27, 2014, 7:49 pm

    Wonderful. Helped me a lot. Thanks

  • json crown October 27, 2014, 5:08 pm

    great written practical illustration

  • Nick August 27, 2014, 6:58 am

    Major error on page 4. Critical path is the LONGEST path out of all paths in the specific precedence network. You have it listed as shortest.

    • Shiv Shenoy August 27, 2014, 4:31 pm

      Nick, the longest path through network diagram is the critical path – which is the shortest path for the completion of the project.

  • 555PPS June 10, 2014, 10:21 am

    Is the calculation in Step3/figure5 correct?
    I can’t see the path with a duration of 31.

    • Shivshanker Shenoy June 10, 2014, 2:29 pm

      Hi! Thanks so much for pointing out the typo, I have fixed it now.
      Regards,
      Shiv

  • NOni May 31, 2014, 5:25 am

    Thanks 2 much Mr. Shiv.

    I was realy need it.

  • base_speed April 8, 2014, 7:35 pm

    thank you very much, it greatly helped…

    • Shivshanker Shenoy April 9, 2014, 6:28 pm

      I’m glad you found this useful!
      -Shiv

    • Tony July 13, 2016, 6:07 am

      Thank you for the free service