When I was preparing for my PMP Exam, I never thought that total float and free float are different concepts. I used to think that these words are just synonyms to each other.
However, fortunately before the exam, during some search on the Google, I accidentally 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.
Regardless of whether you’re a project planner, or a project manager, it is important for you to have a common understanding of these terms otherwise you may face some difficulty in analyzing the network diagrams, and the critical path.
Total Float is the amount of time that an activity can be delayed without delaying the project finish date. On a Critical Path Total Float is zero. Total Float is often known as the Slack.
You can calculate the total float by subtracting the Early Start from the Late Start (LS – ES), or Early Finish from the Late Finish (Late Finish – Early Finish).
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 of the activity from the Early Start of next activity (ES of next activity – EF of current activity).
Please note that if you consider “0″ as your project’s starting day then the above formula is okay. Otherwise, if you count the first day of your project as “1″, then you must subtract “1″ from the above formula.
In this case:
Free Float = ES of next activity – EF – 1
The PMBOK Guide count first day the first of activity as “1″. Therefore, I am also following them in my example. Anyway, feel free to choose the option that works best for you.
Please keep in mind that you will find free float only when two or more activities converge on a single activity, or you can say that when an activity has many predecessor activities, the predecessor activities will have a free float.
In the above network diagram, we have two paths:
The first path is A-B-D with 20 days duration.
The second path is A-C-D with 12 days duration.
Obviously, path A-B-D is the critical path because it has the longest duration.
Calculating the Total Float
You have two methods to calculate the total float. In the first method, you can subtract the duration of the non-critical path from the critical path. In the second method, you can find it by subtracting the Early Start from the Late Start (LS – ES), or subtracting the Early Finish from the Late Finish (LF – EF).
First method of finding the Total Float
Total Float = duration of the critical path – duration of the non-critical path
= (duration of path A-B-D) – (duration of path A-C-D)
= 20 – 12
Hence, the total float is 8 days.
Second method of finding the Total Float
Total Float for Activity C = (LF – EF) or (LS – ES)
= 15 – 7
(Please note that, on path A-C-D, activity A & D lie on the critical path; therefore, they will not have a total float. Only activity “C” can have a float.)
Calculating the Free Float
Calculating the free float is very easy. From the figure, you can see that only Activity B and C have a common successor. This means, these two activities can have the free float; however, the activity B is on a critical path, it can’t have a free float. Therefore, only activity C will have it. Let’s find it.
Free Float of Activity C = ES of next activity – EF – 1
= 16 – 7 – 1
Hence, the free float for activity C is 8.
While writing this blog post, I assumed that you have a solid understanding about the network diagram, the critical path, and you know how to calculate Early Start, Early Finish, Late Start, and Late Finish.
Anyway, if you are not very aware of these things, please keep be patient as I am going to write a blog post covering all of it. Once I do that, I promise — You will love it.
Let me know in comment section if you have something to say about the free float and the total float.
image credit => Master isolated images/ FreeDigitalPhotos.net