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

train-setThis is a part-post from Developing Project Schedule project management activity post. 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. :)

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.

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 (refer to the picture above).

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) –

A. Assemble two-tier bridge B. Join winding tracks C. Assemble and add train station D. Place standalone items around E. Assemble and add construction site F. Join train engine and bogies G. Place the train on the track H. 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:  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..

Pages: 1 2 3 4

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{ 7 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
  • 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

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