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 the development of 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 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 activity from its Late Start Date.
Total Float = Late Start date – Early Start date
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 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 are converging 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 to be “one” and others think of it as “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 is different 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.
In the above network diagram, you can see two paths:
- The first is A->B->D with a 20-day duration.
- 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.
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
Hence, the total float is eight days.
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
Now, the second formula:
Total float for Activity C = (LS of Activity C – ES of Activity C)
= 14 – 6
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
Hence, the free float for Activity C is eight days.
Now we will discuss a more complex example.
For the below-given network diagram, find which activities can have a free float. Calculate it and 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
Total Float for Activity G
Total float for Activity G = Late Finish of Activity G – Early Finish of Activity G
= 18 – 3
You can see here that the free float for Activity G is two days, and the total float is 15 days.
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.