Is the amount of time that an activity may be delayed from its scheduled start date without delaying a succeeding activity or the entire project?

Leads, lags and float are concepts used in schedule development process. The process of schedule development includes: identification of all activities, sequencing all activities based on dependency, estimating duration of each activity and finalizing the schedule. Leads, lags and float are used as part of activity sequencing process. All activities once sequenced will form a schedule network diagram. Let us first look at the definition of all these three attributes.

Float

Float (also known as slack) is the amount of time by which the start of an activity can be delayed without delaying the project completion time. Every task will have following set of start and finish time. 

• Earliest start time (ES) - The earliest time, an activity can start once the previous dependent activities are over.
• Earliest finish time (EF) - This would be ES + activity duration.
• Latest finish time (LF) - The latest time an activity can finish without delaying the project.
• Latest start time (LS) - This would be LF - activity duration.

Float time of an activity can be calculated by taking the difference between Late Start (LS) and Early Start (ES) OR between Late Finish (LF) and Early Finish (EF).

Float = LS-ES  OR 

=LF-EF

A positive float time indicates the flexibility we will have in delaying the specific activity without delaying the project completion time.

Typically, while doing scheduling, the critical path tasks will have zero float and the non-critical path tasks will have a positive float. That means non-critical path tasks can be delayed to certain extent without compromising on the project completion time. Float time information of tasks is very useful to the project team for taking scheduling decisions when there will be resource constraints.

Lag:

Lag is the amount of wait time between two tasks. Or in other words, lag is the amount of time by which a successor activity will be delayed. Lag can be used in all the four logical relationships in scheduling, such as Finish-to-start (FS), start-to-start (SS), finish-to-finish (FS) and start-to-finish (SF).

In below example, Task A and B have a Finish to Start (FS) relationship. Ideally both A and B should get finished on the 12th day. But when we insert a waiting of time of 2 days before B can start, then both A and B will get completed only on the 14th day.

Lead:

Lead is the amount of time a successor task can be accelerated. Lead can applied only on finish-to-start relationship between two activities. We can see the below example. In the below example, task B can start 2 days before the completion of task A. Hence the start of task B, which ideally would have been on 6th day, will not start on 4th day.

Conclusion:

Float, lead and lag are very important concepts and information for the scheduling team. A PMP training course ensures you get a hold of these concepts. These are used to optimally identify the dependencies and the associated constraints. Float information is useful in resource allocation when there are resource constraints. Lead is used for accelerating start of tasks (fast tracking) for reducing project timelines. Lag is used for ensuring that required idle or wait time after a task is appropriately provisioned.

Today’s topic of discussion is total float vs free float.

Until I started my PMP exam preparation, I used to think that total float and free float were synonymous.

While reading the Head First PMP book, I came to understand that a network diagram has two different types of floats. 

Total floats and free floats are important in developing a network diagram. A better understanding of both will help you draw one and analyze a critical path.

Let’s get started.

Total Float Vs Free Float

Let’s start with total float.

Total Float

Total float is also known as “float.”

Total float is how long an activity can be delayed without putting off the project completion date.

On a critical path, the total float is zero. Total float is often known as the slack.

You can calculate it by subtracting the Early Start date of the activity from its Late Start Date.

Total Float = Late Start date – Early Start date

Or 

You can get it by subtracting the activity’s Early Finish date from its Late Finish date. 

Total Float = Late Finish date – Early Finish date

Free Float

Free float is how long an activity can be delayed without delaying the Early Start of its successor.

You can calculate the free float by subtracting the Early Finish Date of the activity from the Early Start Date of the next activity.

Free Float = ES of next Activity – EF of Current Activity

Please note that if two activities converge into a single activity, only one of these two activities may have a free float.

A Note on the Convention Used in the Example

You can refer to the first day of your project in two ways. Some experts consider it “one,” while others consider it “zero.”

Both conventions are correct, and you are free to choose. I decided to refer to the first day of the project as “one.”

Here are my reasons:

• The PMBOK Guide follows this convention.
• It seems more logical to me to say, “Hey, today is the first day of the project!” instead of saying, “Hey, today is the zero-day of the project.”

Anyway, you can follow your preference.

The formula used to calculate free float differs for these two situations; however, the result is the same.

I’m assuming that you know how to draw a network diagram, identify the critical path, and calculate the Early Start, Early Finish, Late Start, and Late Finish dates of activities.

If you struggle with these calculations, I have a blog post on the critical path method. Read that post, and then come back.

Examples of Total Float Vs Free Float

Here are two examples of how to calculate free float and total float. The first is easy, and the second one is tougher.

Example: 1

In the above network diagram, you can see two paths:

1. The first is A->B->D with a 20-day duration.
2. The second is A->C->D with a 12-day duration.

Path A->B->D is the critical path because it has the longest duration.

Calculating the Total Float

Path A->B->D is a critical path; therefore, it will not have a total float.

Path A->C->D is a non-critical path, so it can have a total float.

There are two methods to calculate the total float. For the first, subtract the duration of the non-critical path from the critical path.

For the second method, find the total float for any activity by subtracting the Early Start date from the Late Start date (LS – ES) or subtracting the Early Finish date from the Late Finish date (LF – EF) for any activity.

First Method

Total float = duration of the critical path – duration of the non-critical path

= (duration of the path A->B->D) – (duration of the path A->C->D)

= 20 – 12

= 8

Hence, the total float is eight days.

Second Method

On path A->C->D, Activity A, and D are on the critical path; therefore, they will not have a total float. Only Activity C can.

We can calculate the total float by using either the finish dates or start dates. I will show you both ways.

First, we will go with the Late Finish and Early Finish dates:

Total float for Activity C = (LF of Activity C – EF of Activity C)

= 15 – 7

= 8

Now, the second formula:

Total float for Activity C = (LS of Activity C – ES of Activity C)

= 14 – 6

= 8

The durations are the same, so both formulas will give you the same result.

Calculating the Free Float

From the figure above, you can see that only Activity C can have a free float because all others are on the critical path.

Let’s find it.

Free float of Activity C = ES of next activity – EF of Activity C – 1

= 16 – 7 – 1

= 8

Hence, the free float for Activity C is eight days.

Now we will discuss a more complex example.

Example: 2

Find which activities can have a free float for the below-given network diagram. Calculate it and the total float, considering the duration in days.

We know that:

Free float = ES of next activity – EF of current activity – 1

In the above diagram, Activity G can have the free float because Activity D and G converge.

Activity D will not have a free float because its successor, Activity E, starts the day after the completion of Activity D.

Free Float for Activity G

We know the formula for free float:

Free float of Activity G = Early Start of Activity E – Early Finish of Activity G – 1

= 6 – 3 – 1

= 2

Total Float for Activity G

Total float for Activity G = Late Finish of Activity G – Early Finish of Activity G

= 18 – 3

= 15

You can see here that the free float for Activity G is two days, and the total float is 15 days. 

Summary

Total float and free float are important concepts in schedule management. Total float is commonly referred to as float. Activities on a non-critical path will have a total float. When two activities converge, one of them will have a free float. 

Please note that you have to find the total float if you are asked to calculate the float for any activity on the exam. 

Here is where this post on total float vs free float ends.

How do you use total float and free float in your project? Please share your thoughts in the comments section.

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