The concept of Lead Time and Lag Time is very important in the project scheduling network diagram.
As a Project Manager, you must know about 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 some 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 the first activity is about to finish, or a second activity will start a few days after finishing the first 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. The balance of time for the first activity is known as Lead Time. Lead Time is the overlap between the first and second activity.
For example, let us say that the 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 while the electrical work for the first floor continues.
Lag Time
When the 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.
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 the 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.
Let’s look at a real-world example for lag time:
Suppose you have to paint a newly constructed room. 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 your final painting. The time given for coating to dry itself is called the lag time.
This is where this blog post about lead and lag time finishes. I hope that you enjoyed reading it.
Feel free to posts your thoughts and questions through the comments section below.
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.
Arturo says
You are welcome Fahad.
Abu Kareem says
Fahad, thank you so much for clearing these two up with the easy examples!
Fahad Usmani says
You are welcome Abu.
rao says
some of my friends are telling that Negative Lags are not desirable. But in my view negative lags are desirable if the relationship permits it. could someone help me in clarifying it.
thanks and regards,
Fahad Usmani says
In other words you can call it lead.
rao says
thank you Fahad,
please tell me that my view is correct or not.
Fahad Usmani says
Yes, both are needed.
Peace says
Well explained with real practical examples. Was struggling
Fahad Usmani says
I am glad it helped you Peace.
Darya Duma 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.
Ali says
You are awesome… the post is very informative .
Thanks bro.
Fahad Usmani says
Thanks Ali.
Sandeep Mishra 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..
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.
noor says
Hello,
you have explained lead and lag time in very easy way and now i can apply it on my proj.
thanks
Fahad Usmani says
You are welcome Noor.
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
Fahad Usmani says
Yes you can send me the mail.
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
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
Fahad Usmani says
It is still FS with 10% lead.
Jayadev says
Dear Fahad,
Thank you for your blog ! you have beautifully explained the concept ! This is very useful !
Regards,
Jayadev
Fahad Usmani says
You are welcome Jayadev.
aliakbar says
please can U explain & solve problem as an example to understand using lead & lag calculate the minimum total duration between two milestone.
Fahad Usmani says
Okay, I will write about it soon.
Muhammad Anjum says
LEAD TIME:
In case of SS:
First activity = 20 days, Second activity = 15 days
If, second activity starts after 5 days starting first activity (SS5), it means 15 days are lead time because it is the overlap period in between first & second activity. Am I right o wrong ?
If, second activity starts before 5 days starting first activity (SS-5), it means 10 days are lead time because it is the overlap period in between first & second activity. Am I right o wrong ?
In case of FF:
First activity = 20 days, Second activity = 15 days
If, second activity finishes after 5 days finishing first activity (FF5), it means 10 days are lead time because it is the overlap period in between first & second activity. Am I right o wrong ?
If, second activity finishes before 5 days finishing first activity (FF-5), it means 15 days are lead time because it is the overlap period in between first & second activity. Am I right o wrong ?
LAG TIME:
Is lag time only delay (gap between finishing of first and starting of second activity) in between two activities (as you wrote) or is it difference also in between two activities ?
If, it is only delay in between two activities, then it means that lag time doesn’t exist in all above cases.
If, it is difference also, then lag time is 5 days in all above cases.
Your expertise is requested in all above cases that am I right or wrong ?
Fahad Usmani says
Hello Anjum,
I will reply you once I go through this concept in detail.
Muhammad Anjum says
Sure, and I’ll wait for.
Hoa says
Excellent explanation. Thank you very much, Fahad.
Fahad Usmani says
Thanks Hoa.
Rajesh Rajagopalan says
This is simple and Outstanding
Fahad Usmani says
Thanks Rajesh.
Nicholas says
Hi there,
Thank you for the explanation. It is very helpful indeed! Would that be possible to tell me about the benefits of find the phase difference between lead time and lag time?
Thanks
Fahad Usmani says
Hello Nicholas,
I did not understand your question. If possible, please elaborate little further.
Sameh says
Dear Fahad
could you pls explain to us the answer of question no 75 in the followed link
http://www.oliverlehmann.com/pmp-self-test/75-free-questions.htm
actually i understand nothing from this question !!
Ahmad says
Dear Sameh,
the answer for that question is that there is a dangle between the activity “E” and the finish milestone.
if the activity “E” has been on progress for 40%, then the activity “D” will start, lets assume that activity “E” has reached 50%, then no effect of it on the path, and activity “D” can be completed, but the project will not be completed without finishing all the activities.
so, the activity “E” has a relation like (SS/40%) with activity “D”. it has no efffect on the project after acheiving 40% of it.
that will result in an open end activity.
Hope i have explained in a simple way.
Regards,
kisholoy says
Excellent articulation of lead and lag , It clears all the doubts
Keep it up the good work
regards
kisholoy
Fahad Usmani says
Thanks Kisholoy for your comment.
neethish says
sir.,
I understood the leads and lags from your explanation ,thank you very much for that,so i would like to know ,is there any mitigation software or tools to mitigate leads and lags once if we are identified using primavera or any other project management software.
It will be great help and very informative if i could get any answer for this .
thank you for your time
Fahad Usmani says
I don’t know if any tool is available for this task but the risk management is more like people oriented process.
Irene says
Thanks for your post. It is very useful for me.~~
Teachers cannot always cover all things in class.( ?-? )
Fahad Usmani says
You are welcome Irene.
NDEH Stanley says
Thank you Fahad. Well explained as always.
Fahad Usmani says
You are welcome NDEH.
mashael says
well done Fahad .
but i still have a doubt of utilizing lag time in SS relationship. would you kindly provide me of the real world example.
Fahad Usmani says
It is not necessary to have an mandatory SS relationship.
It is also possible that you can have two activities in parallel which were planned as FS type of relationship.
For example. you can start the wood work along with the electric work simultaneously.
Khalid says
Its really a nice and very detailed explanation, although I am not a planner but answered a lot of questioned in my mind.
Thanks Mr. Fahad.
Fahad Usmani says
You are welcome Khalid.
Tefera M says
Thank you for your good explanation.
Fahad Usmani says
You are welcome Terera.
Edison M Braun says
Friends:
What do you think about the effects of the use of leads and/or lags on the performance of a quantitative risk analysis? I don’t know how to associate uncertainties to leads or lags, whether would be a possibility of doing it.
Many times leads and lags may be uncovering real activities which do have uncertainties….
Christian Okolo says
Thank you. This is perhaps the simplest and best explanation of Lead and Lag I have read.
Fahad Usmani says
Thanks Christian for your visit and leaving comment.
Thazin Phyo says
It is the good explanation. As it includes the examples, I can understand easily.
Thank you for your explanation.
Fahad Usmani says
You are welcome Thazin.