Lead Time and Lag Time in Project Scheduling Network Diagram

July 25, 2020

Lead Time and Lag Time both play an important role in network diagram development.

Although the concept is easy, a few people still find it difficult to understand. In this blog post, I will explain Lead Time and Lag Time with simple, real-world examples.

What is Lead and Lag Time?

When the first activity finishes, a second activity starts. This is a Finish to Start sequence, and it is widely used in a network diagram. There are many instances when the second activity starts when the first activity is about to finish, or a second activity will start a few days after the end of the first activity.

These two conditions are known as Lead and Lag. They are an integral part of a network diagram. You apply Lead and Lag after you sequence activities and identify the dependencies.

According to the PMBOK Guide 6th Edition, “Lead time is the amount of time whereby a successor activity can be advanced with respect to a predecessor activity.”

Put simply, when the first activity is still running when the second activity starts, it is Lead. Lead Time is the overlap between the first and second activities.

For example, assuming that the time duration for the first activity is 20 days and 15 days for the second activity, the first activity is on its 15th day and you have started the second activity. Please note that the first activity still has five days to be finished.

Here, we would say that Lead Time is five days.

Let’s look at a real-world example.

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

However, as you complete the electrical work on the ground floor, you start painting it while the electrical work on the first floor continues.

We commonly use Lead with Finish to Start relationships. In this dependency, the predecessor activity must finish before the successor activity starts.

Lag Time

According to the PMBOK Guide, 6th edition, “A lag time is the amount of time whereby a successor activity is required to be delayed with respect to a predecessor activity.”

Put simply, when the first activity completes and there is a delay before the second activity starts, this is called Lag. The delay is known as Lag Time.

Lag Time is the delay between the first and second activities.

For example, the duration of the first activity is three days and two days for the second activity. After completing the first activity, you wait for one day, and then you start the second.

Here, we say that the Lag Time is one day.

Please note that you started a second activity one day after completing the first activity.

Let’s look at a real-world example.

Suppose you have to paint a room. The first activity is applying the primer coating, and then you will do the final painting. However, you must let the primer dry before applying the paint.

Hence, you start your final painting two days after. The time given for the coating to dry is called the Lag Time.

Lag can be used with all types of activity dependency.

Summary

Lead Time and Lag Time play an important role in developing the schedule baseline. Lead and Lag can be used in any type of dependency in a network diagram. They are very helpful and offer project managers flexibility in schedule development.

How do you apply Lead Time and Lag Time in your schedule network diagram? Please share your experiences in the comments section.

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• Richard says:

When I use SF, the task starts on the correct date but overlaps by one day. For example if task 124 is one day on 3/23/21 and task 121 is enters as 124SF with duration of three days, it shows task start on 3/20 and end on 3/23

• Fahad Usmani says:

Hello Richard, it is something with software settings.

• Mignonne says:

Thank you for this information it helped me tremendously. I understood how to memorize the definition for lead and lag, but didn’t have a clear knowledge of how these 2 words applied to a project….now I know

• Fahad Usmani says:

Great Mignonne.

• Emma R Anderson says:

This was most appreciated! You provided very clear examples and explanations!

• Fahad Usmani says:

Thank you Emma for your visit and leaving comment.

• Onesimo Bumhira says:

You have unpacked lead time, cycle time and lag time to the extend that a layman can understand the differences.

• Fahad Usmani says:

Thanks Onesimo for your comment.

• Kog Amatus Niminye says:

Good post. I really got the picture clear for better distinction.

Please I am preparing for PMP exam by end of June, 2019, any good exam prep materials for me?

• Roshni Vishwas says:

Greetings!!

Reading your blogs are really helping me to understand the things.

Preferred logic
Preferential logic
Hard logic
Soft logic

• Fahad Usmani says:

Hello Roshni, explaining these topics will require a separate blog post. I have noted it, and may soon publish a blog post on it.

• Siyad Ahmec says:

I am studying for an project tools and techniques examupcming and I have found this useful. Thank you so much for sharing

• Fahad Usmani says:

You are welcome Siyad.

• Tope says:

Clear and distinct

• Fahad Usmani says:

Thanks Tope.

• Eve says:

Thank you for the easy clarification.

However, I have a question on how this can be reflected/ translated on the schedule plan?

• Fahad Usmani says:

This is inbuilt in schedule. While developing the schedule you define it.

• renjith says:

Very informative and well explained..thank you

• Fahad Usmani says:

You are welcome Renjith.

• Serena says:

Thank you so much for this! I was struggling to understand this after studying the PMBOK guide. You just cleared it up with your simple explanation. I may pass this test afterall!!

• Fahad Usmani says:

You are welcome Serena.

• Mounir says:

Use of lag and lead time should be used for technical/specification requirements only,otherwise you are manipulate in float.
Advise to use lag/lead time = 0.

• Fahad Usmani says:

• Bob says:

• 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.

• 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.

• 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….

• Tefera M says:

Thank you for your good explanation.

• Fahad Usmani says:

You are welcome Terera.

• 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.

• Fahad Usmani says:

You are welcome Khalid.

• 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.

• NDEH Stanley says:

Thank you Fahad. Well explained as always.

• Fahad Usmani says:

You are welcome NDEH.

• 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.

• 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.

• 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.

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,

• 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.

• Rajesh Rajagopalan says:

This is simple and Outstanding

• Fahad Usmani says:

Thanks Rajesh.

• Hoa says:

Excellent explanation. Thank you very much, Fahad.

• Fahad Usmani says:

Thanks Hoa.

• Muhammad Anjum says:

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.

• 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.

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

Regards,

• Fahad Usmani says:

You are welcome Jayadev.

• Gaurav says:

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.

• Shaik Akhil says:

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

• Daniel Koshy says:

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.

• 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.

• Sameer Morar says:

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.

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..

• Ali says:

You are awesome… the post is very informative .

Thanks bro.

• Fahad Usmani says:

Thanks Ali.

• 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.

• Arturo says:

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:

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.

• Jagapathi says:

Hi,

I see that lead time also has the below definition:
A lead time is the latency between the initiation and execution of a process. If your working area is software development, then the definition from JIRA golossary may be more understandable for you: Lead time is the time taken from when an issue is logged until work is completed on that issue

I am confused. Can you please explain the differences and the contexts in which they would be used.

• Fahad Usmani says:

This blog post discusses lead and lag time in a network diagram.

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