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


Calculating Float for Activities

Now, any schedule will have some flexibility, or elbow room, called Float.

Float of an activity is the duration that it can slip by without delaying the subsequent task or completion of the project, or violating schedule constraint.

It is the elbow room that a task has. Amount of time an activity on a network path can slip without causing a delay in early start of any successor activity and without violating schedule constraint is called Free Float.

Total amount of time an activity can slip from its early start date without causing delay to project completion or violating schedule constraint is called Total Float.

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The simple 3-step process to calculate float of ALL activities in your schedule network diagram –

Step 1: Arrange the paths in decreasing order of their total duration, starting with Critical path

For the above example it would be as below, the first being critical path itself –

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

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

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

The float for each activity on the critical path is zero.

Step 2: Find float for activities on the second longest path

This would be the difference between total duration of critical path and next longest path. In our example this would be 49-41= 8 minutes. Assign this to ALL activities on this path, which do not already have a float. In this example that would be only activity.

critical-path-trainset-secondpathFigure 4: Float for second path

Step 3: Do the same to all remaining paths, for unassigned activities

Now calculate difference between critical path and third path’s total duration, and assign this to activities on the third path – excluding any which already have a float assigned in previous step. In our example this would be 49-38 = 11 minutes.

Do this step for rest of the identified paths and you have float for all activities.

critical-path-trainset-thirdpath1

Figure 5: Float for third path

In the next page (navigate from page link below), we will see how to calculate Early Start, Early Finish and Late Start and Late Finish for all activities on the network path..

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

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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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