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


Wrapping up

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We looked at few concepts in this post –

  • Critical path
    • Critical path in a network schedule diagram is the network path with longest total duration
    • Critical path sure is the longest network path, but this indicates Shortest Duration of the project – which means any activity slipping on this path will cause a delay in project completion
    • It is calculated by simply adding up duration of all activities on each network path and finding the one with largest number
  • Float
    • Float (also known as Slack) of an activity is the duration that it can slip without delaying the next activity or the project end date
    • Float for every activity on critical path is zero
    • Float for every activity on a network path is the difference between its total duration and that of critical path (excluding the activities that have float calculated already)
    • Float is calculated for network paths in the descending order of their total duration, starting with critical path
  •  Calculating Early and Late Starts and Finishes
    • Early start and finish are calculated by forward pass through the network path, and Late start and finish are calculated by backward pass
    • Both are calculated first for the critical path
    • Both are calculated for remaining network paths in the descending order of their total duration
    • During calculation if you come across any activity with duration already calculated, do not overwrite

Here’s a video explaining nicely the concept of Critical path, Float and network diagram using Forward and Backward pass.
Note that he uses the convention of starting on day 0 and so NOT adding a day to start next activity. On the contrary, I use the convention of starting an activity on day 1 and adding 1 to start next activity on subsequent day, as this feels natural for our understanding.
You can use any convention you feel comfortable with, just know that the question on exam might present scenario using any of these conventions – so knowing both will help you answer them easily.

These calculations are quite simple once you get a hang of it. Good luck!

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{ 30 comments… add one }
  • base_speed April 8, 2014, 7:35 pm

    thank you very much, it greatly helped…

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

      I’m glad you found this useful!
      -Shiv

      Reply
    • Tony July 13, 2016, 6:07 am

      Thank you for the free service

      Reply
  • NOni May 31, 2014, 5:25 am

    Thanks 2 much Mr. Shiv.

    I was realy need it.

    Reply
  • 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.

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

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

      Reply
  • 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.

    Reply
    • 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.

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

    great written practical illustration

    Reply
  • aaaaa December 27, 2014, 7:49 pm

    Wonderful. Helped me a lot. Thanks

    Reply
  • khadar January 27, 2015, 12:19 am

    thank you for your help

    Reply
  • 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.

    Reply
    • 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

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

    seriously helped me alot. thank u so much

    Reply
    • 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

      Reply
  • 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

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

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

      Good luck!
      Shiv

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

    Very useful example

    Reply
  • 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.

    Reply
  • 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~

    Reply
    • Shiv Shenoy February 6, 2017, 7:36 pm

      Thank you Shannon, for the kind words. I’m glad to see that you are finding this useful!
      Best,
      Shiv

      Reply
      • magy danny April 5, 2017, 1:45 pm

        thanks so much for that good example

        Reply
  • 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 🙂

    Regards,
    Pankaj

    Reply
    • Shiv Shenoy May 1, 2017, 6:25 pm

      Thank you Pankaj, that is the idea! 🙂

      Cheers,
      Shiv

      Reply
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