Which of the following is the maximum amount of time of activity can be delayed from its early start without deleting the entire project?

Float is one of the very basic concepts essential for building network diagrams and in turn a project schedule. Further, there are different types of floats such as Free float, Total float, Project float, Interfering Float, Independent Float. As a Project Manager, you should have a clear understanding of these concepts in order to create efficient project schedules. So, lets see..

The term “Float” implies “Fluid”, which in turn implies “Flexibility“. In Project Scheduling, Float refers to the amount of scheduling flexibility. Float is also popularly called “Slack“.

The different types of float are

1. Total Float or Float
2. Free Float
3. Project Float
4. Interfering Float (INTF)
5. Independent Float (INDF)

Total Float 

Total float is the amount of time an activity can be delayed without delaying the project completion date.This is the type of Float that is commonly referred to as “Float”.

PMBOK defines Total Float as

“The amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint.” 

So, Total Float is about flexibility at the project level. It is about the flexibility that an activity has in its execution without delaying the Project finish date.

Total Float Formula

Total float is calculated by subtracting the Early Start date of an activity from its Late Start date (Late Start date – Early Start date), or Early Finish date from its Late Finish date (Late Finish date – Early Finish date).

Total Float or Float = LS – ES or LF – EF

Free Float

PMBOK Defines Free Float as “The amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint”

So, Free Float is about flexibility at the activity level. It is about the flexibility that an activity has in its execution without delaying its successor activity.

Free Float Formula

Free float is calculated by subtracting the Early Finish date of current activity from the Early Start date of its successor activity (ES of successor Activity – EF of current Activity).

Free Float = ES (of successor) – EF (of current) 

Total Float vs Free float

While Total Float is how much an activity can be delayed without affecting the project Finish date, Free Float is about how much an activity can be delayed without affecting its successor activity.

Project Float

This type of Float is not mentioned in the PMBOK, but is present in some reference books.

It is the amount of time a Project can be delayed without delaying the externally imposed project finish date by the customer, or the project finish date previously committed to by the Project Manager.

Free & Total Floats are about the time an activity can be delayed, while Project float is the amount of time a Project can be delayed.

Interfering Float (INTF)

Interfering Float is the amount of time a schedule activity can be delayed or extended from its early start date without delaying the project finish date. However, it should be understood that delaying an activity into interfering float will delay the start of one or more following non-critical activities. If an activity is delayed for the amount of the Free and Interfering Float, then its successor activities are critical.

Interfering Float Formula

Interfering Float = Total Float – Free Float

Independent Float (INDF)

Interfering Float is the maximum amount of time an activity can be delayed without delaying the early start of the succeeding activities and without being affected by the allowable delay of any predecessor activity.

Independent Float Formula

Independent Float (INDF) = Earliest Successors’ Early Start – Earliest Predecessors’ Late Finish – Activity’s duration

when the INDF is a negative value, we set the value to zero.

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.

Which of the following is the maximum amount of time an activity can be delayed from its early start without delaying the entire project?

What is the maximum amount of time an activity can be delayed without extending the overall project completion time?

Is the amount of time a schedule activity can be delayed without delaying the early start of any successor?

Which of the following is an activity that can be delayed for start without affecting the time duration of the preceding activities is represented?