When I was preparing for my PMP certification exam, I never thought that total float and free float were different concepts. I used to think that these two terms were just synonyms to each other.
Fortunately before the exam, while searching on Google, I came to know the difference between these two terms. Therefore, I believe that there will be many PMP aspirants like me thinking the same way as I was.
I am writing this blog post to express my understanding on the topic. I insist you go through this concept carefully; otherwise you may face some difficulty in analyzing the network diagrams and the critical path.
Total float is what we are aware of, and is commonly referred to as a float.
Total float is the amount of time that an activity can be delayed without delaying the project completion date. On a critical path, the total float is zero.
Total float is often known as the slack.
You can calculate the total float 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).
Now we come to free float. This is going to be a bit different, and might be new to you!
Free float is the amount of time that an activity can be delayed without delaying the Early Start of its successor activity.
You can calculate the free float by subtracting the Early Finish date of the activity from the Early Start date of next activity (ES of next Activity – EF of current Activity).
Keep in mind that if an activity has only one successor activity, free float and total float will be the same.
A note on convention used in the example:
There is disagreement whether the first day of the project should be “1” or “0”.
In fact, both conventions are correct, and you are free to choose what you like. I choose to start my project from day “1”.
The following are my reasons for doing so:
- This convention is followed the PMBOK Guide.
- It seems more logical to me to say, “Hey, today is my first day of the project!” instead of saying, “Hey, today is my zero day of the project.”
Anyway, you’re free to select your choice.
Since mathematically these two situations are different, the formula to be used to calculate the free float will also be slightly different. Don’t worry; the result is same in both cases.
If you choose to start your project from day 1
Free float of Activity X = Early Start of next Activity – Early Finish of Activity X – 1
If you choose to start your project from day 0
Free float of Activity X = Early Start of next Activity – Early Finish of Activity X
The difference between these two formulas is very easy to remember. If you start from day “1,” you have to subtract “1” from the formula.
If you start from day “0”, there is no need to subtract “1.”
Now we will move on to the examples. I will start with a simple example, and then go on to a more complex situation.
While giving you these examples, I’m assuming that you’re aware of drawing a network diagram, identifying the critical, and calculating the Early Start, Early Finish, Late Start, and Late Finish dates of activities.
Don’t worry, if you’re having problems with these calculations, I have written a blog post on the critical path method explaining all these things. Please visit this blog post to understand these concepts better and then come back here again.
Visit: Critical Path Method
Okay, let’s get started.
In the above network diagram, we have two paths:
- The first path is A->B->D with 20 days duration, and
- The second path is A->C->D with 12 days duration.
Obviously, the path A->B->D is the critical path because it has the longest duration.
Calculating the Total Float
As we can see, the given diagram has only two paths, path A->B->D and path A->C->D.
The path A->B->D is a critical path; therefore it will not have a total float.
Since the path A->C->D is a non-critical path, it can have a total float.
You have two methods to calculate the total float. In the first method, you subtract the duration of the non-critical path from the critical path.
In the second method, you 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) on any activity.
The first method of finding the Total Float
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 8 days.
The second method of finding the Total Float
On the path A->C->D, Activity A and D lie on the critical path; therefore, they will not have a total float. Only Activity C can have a total float.
As stated earlier, we can calculate the total float by using either finish dates or start dates. Here, I will show you both ways to find it.
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
So both durations are the same, which means both formulas will provide you with the same result.
Calculating the Free Float
From the figure, you can see that only Activity C can have a free float, because other activities are lying 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 8 days.
Now it is time to move on to a more complex example.
For the below given network diagram, identify which activities can have a free float and calculate the free and total float for those activities, considering 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 are converging on one common activity.
Activity D will not have a free float because its successor activity E is starting on next day of completing of activity D.
You will select the activity with the larger Early Start date.
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 see here that the free float for Activity G is 2 days, and the total float is 15 days, both are different.
Please note: if in the exam you’re asked to calculate the float for any activity, you have to calculate the total float. Total float is commonly referred to as a float.
Here is where this blog post finishes. If you have any thoughts, share them through the comments section.
Update: This blog post has been updated to correct an error pointed out by Mr Murali and Mr Bryan.
image credit => Master isolated images/ FreeDigitalPhotos.net