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


Early Start, Early Finish and Late Start, Late Finish

Why do we need to know the early and late number for an activity? By knowing how early an activity can start and how late it can finish gives you the flexibility to juggle resources between other activities that can potentially impact critical path, and hence the project completion date. The shorter a network path is in relation to critical path, more schedule flexibility you will have on activities on that path.

Early start and Early finish

Indicates the earliest time an activity on a network path can start and earliest it can finish. If you decide to start an activity on its early start (assuming previous activities on that network path are completed on their early finishes), that activity can finish on its early finish (if it does not slip). And when the last activity on a network path is completed by its early finish, you have all the resources of those activities at your disposal to deploy on other high risk activities.

Calculating Early start and finish (take a FORWARD pass through network path)

Remember!: Always start with the critical path and then go with paths with descending order of their total duration.

Step 1: Early start of first activity on critical path is always 1. Write it at the top left corner of that activity box (see the image below).

Step 2: Add its activity duration to this early start number and reduce it by one. Write the resulting number on the top right corner of activity box.

Step 3: Take the subsequent number of this early finish and write as early start for next activity. Continue this till you reach the end of critical path.

Step 4: Select the network path with second highest total duration, and calculate early starts and finishes. If you find an activity with early start and finish already written do not overwrite them. Do the same for remaining network paths.

Note: If you find two activities converging on a single activity (say, activity-G), it indicates that the activity-G will start only AFTER converging activities finish. So, you will take the largest value amongst the early finish of these two activities and write subsequent number as early start of the activity-G.

Why add duration to early start and then reduce by one, to get early finish? Because the duration of an activity includes both start and finish days (or any other unit of measurement you use). So, if first activity duration is 5 days, and early start is 1 then early finish is 1+6-1 = 6. That is 6 days including the start and finish days.

early start early finishFigure 6: Early start and finish

As you noticed, early start number is written at the top left corner of activity box, and early finish on the top right corner. The critical path with early start and early finish days will look like this –

calculating early start and early finishFigure 7: Early start and early finish for critical path

Late start and Late finish

Indicates the latest time an activity on a network path can start and latest it can finish. Knowing how late the last activity on the network path can start and still finish within the time to not impact critical path, will let you decide how much of flexibility you want to exercise on its schedule. However, once the last activity on the network path starts on its late start day it should not slip, else it will impact project completion date.

Calculating Late start and finish (take BACKWARD pass through network path)

Remember!: Start with the critical path, beginning at the last activity’s late finish.

Step 1: Late finish of last activity on the critical path is same as its early finish. Write this number at the bottom right corner.

Step 2: Calculate late start of this activity as the late finish minus activity duration plus 1. This calculation has the same reason – start and finish are both included in the duration. Write this number at the bottom left corner.

Step 3: Write this late start of the activity minus 1, as the late finish of previous activity. Continue this way all way till you reach the late start of first activity on the critical path.

Step 4: Select the network path with second highest total duration, and write late starts and finishes beginning at the last activity of that path. Do the same for remaining network paths.

Notes:

  • If you find two activities converging on a single activity (say, activity-C), take the smallest value amongst the late start of these activities and write previous number as late finish of the activity-C.
  • If you find an activity with late start and finish already written do not overwrite them.

late start late finishFigure 8: Late start and Late finish

Late start number is written at the bottom left corner of activity box, and late finish on the bottom right corner. The critical path of our example with late starts and late finishes will look like this –

calculating late start and late finishFigure 9: Late start, finish for the critical path

Let us go back to our example and calculate early/late start/finish for the entire schedule network diagram.

final schedule network diagramFigure 10: Early start, finish and Late start, finish for the entire schedule network diagram

This has been a lengthy lesson, let us summarize in the next page (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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