Lead Time and Lag Time in Project Scheduling Network Diagram

lead time and lag timeThe concept of Lead Time and Lag Time is very important in project scheduling network diagram. As a Project Manager, you must know about the Lead Time and Lag time in order to better understand your project plan and execute it accordingly.

Although, this concept is not very difficult, a few people still find it difficult to understand. Therefore, in this blog post, I’m going to give a simple explanation about Lead Time and Lag Time with real world examples.

Okay, let us get started.

Usually, when the first activity finishes, a second activity starts. This type of activity sequence is called Finish to Start and it happens the majority of the time. However, there are many instances when other situations arise; e.g. a second activity starts while first activity is about to finish, or a second activity will start a few days after finishing the first activity.

finish to start activity

These two situations are termed as Lead and Lag.

Lead Time

When the first activity is still running and second activity starts, this is called Lead, and the balance of time for the first activity is known as Lead Time. Lead Time is the overlap between the first and second activity.

finish to start with lead time

For example, let us say that time duration for the first activity is 20 days, and for the second activity it is 15 days. As the first activity completes its first 15 days, you start working on the second activity. (Please note that the first activity still has 5 days to finish itself completely.)

In this case we’d say that Lead Time is 5 days, or Finish to Start activity has 5 days Lead Time.

Let us take a real world example.

You’re constructing a two-floor building, and now you have two activities in sequence; i.e. electrical work and painting.

However, as you complete the electrical work of ground floor, you start painting it, and electrical work for first floor continues.

Lag Time

When first activity completes, if there is then a delay or wait period before the second activity starts, this is called lag and the delay is known as the Lag Time. Lag Time is the delay between the first and second activity.

finish to start with lag time

For example, the time duration for the first activity is 3 days, and for the second activity it is 2 days. After completing the first activity you wait for one day, and then you start second activity.
(Please note that, here you start second activity after one day of completing the first activity.)

In this case we say that Lag Time is one day, or Finish to Start activity with one day delay or lag.

And the real world example for lag time is as follows:

Suppose you have to paint a newly constructed room. So the first activity would be applying the primer coating and then you will go for the final painting. However, after applying the primer coating, you must give it some time to dry properly. Once the primer coating dries, you can start final painting. The time given for coating to dry itself is called the lag time.

Here, this blog post about lead and lag time finishes. I hope that you would have enjoyed reading it.

Feel free to posts your thoughts and concerns through the comments section.

image credit => digitalart / FreeDigitalPhotos.net 

Comments

  1. Arturo says

    Hi Fahad,

    you have explained carefully de Lag an Lead Times concepts regarding Finish to Start relationship betwen activities: excellent work.

    Although I’m totally agree with you that this type of relationship betwen activities is the most usual, there are the Start to Start and Finish to Finish relationships as common use in building schedules, above all Start to Start ones.

    An example easy to understand of Lag Time applied to a Start to Start relationship is when the program need to wait to begin one activity X days (or hours) after the begining of the precedent activity due e.g. to workload circunstances, etcetera.

    I’m sure you can explain this better as you have the opportunity to design figures in the blog posts creation process, thanks.

    • Fahad Usmani says

      Lead time and lag times can be utilized in all kind of relationship whether it is finish to start or start to start or something else. I chose the most commonly used relationship to express my ideas.

      Anyway, you always have option to post your ideas and thoughts in comments section.

      Thank you Arturo once again to add value in the post.

  2. says

    Just a few corrections.
    First: In a Finish-to-Start relationship, if A is the first activity and B is the second (predecessor / successor) B CAN start after A is finished. It does not HAVE to start. The relationship identifies that B cannot possibly start before A is finished, but can start any time after that.
    Sorry, but the above electrical painting example is not a good example of the application of lead.
    Lead is not considered great to use anyway, as who can really predict the end of A in the first place.
    So – Electrical starts work, after a certain percentage has been done, we can start drywalling, and once that is finished then you can start painting. Electrical to Drywall – Start-to-Start plus lag expressed as a percentage. Then drywall to paint also start-to-start with a percentage.

    • Fahad Usmani says

      You are right that in Finish to Start relationship, second activity starts when first activity finishes. But if there is a lead time then second activity will start before the first activity finishes. I’m not saying this, it is the definition of the lead time; moreover, using lead time is a good or bad it depends on the situation, project plan and the project manager.

      Regarding my example of painting and electrical works, there may be many ways to accomplish the same task. It again depends on the project plan and how the project manager builds the network diagram and sequence the activities.

  3. says

    Mr. Fahad,
    This post and all other Post in your Blogs are truly excellent.
    Great work for people who are going to appear in PMI-PMP.
    You must add some Primavera/ MS Project Training post /video also.
    Thanks

    • Fahad Usmani says

      Hello Sandeep, thank you for your comment. Although I have some working knowledge of Primavera but I’m not an expert in it.

      Regarding videos, I may add them in future..

  4. Sameer Morar says

    Hi Fahad,

    thanks for the fantastic explanation of the lead and lag times. I would love to see how this is effected in the forward and backward passes in network/precedence diagram calculations and thier effect on start and finish times of activities.

    Regards,

    • Fahad Usmani says

      Hello Sameer,

      It is in my to do list. I am planning to write a complete guide to Critical Path Method where I will explain how to calculate Critical Path, forward pass, backward pass, how to calculate early start, early finish, etc.

  5. Daniel Koshy says

    Hello Fahad
    Could you please explain lag and lead for a start-start relationship? I have a doubt about it, is there anyway I can insert a picture to better explain my doubt? Can I just send you an email regarding this?

    regards
    Daniel

  6. Shaik Akhil says

    Hello Fahad
    Excellent Work….It is very easy to understand. Can you explain about independent float and interference float?
    regards
    Shaik Akhil

    • Fahad Usmani says

      Interfering Float = Latest Finish Time of Activity – Earliest starting Time of next activity.

      An activity will have an Independent float if it is started at Latest starting time but finishes before Earliest finishing time.

      Independent Float = EF – LS – time required for activity

  7. Gaurav says

    Hi Fahad,
    Activity A —> Activity B (FS with Lead of 10% )
    So does this mean that with Lead… FS is no more FS actually..because Activity B has started before Finish of Activity A ?

    Thanks

  8. Jayadev says

    Dear Fahad,

    Thank you for your blog ! you have beautifully explained the concept ! This is very useful !

    Regards,
    Jayadev

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