Total Float Versus Free Float

total float vs free float

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

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).

Free Float

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.

However, if an activity has more than one successor activity, it may have a different free float.

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.

Example: 1

total float and free float

In the above network diagram, we have two paths:

  1. The first path is A->B->D with 20 days duration, and
  2. 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

= 8

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

= 8

Now, the second formula:

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

= 14 – 6

= 8

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

= 8

Hence, the free float for activity C is 8 days.

Now it is time to move on to a more complex example.

Example: 2

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.

CPM-Diagram-Late-Dates-path-GHI- updated

We know that,

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

In the above diagram, Activity D and G can have the free float because only these activities have more than one successor activities.

Now since these activities have two successors, which activity will you select to use in the formula?

You will select the activity with the larger Early Start date.

Free Float for Activity D

Activity D has two successors: Activity B and E. Activity B has a larger Early Start date than Activity E. Therefore, we will select the Early Start of Activity B.

Free float for activity D = ES of Activity B – EF of Activity D – 1

= 11 – 5 – 1

= 5

Total Float for Activity D

We know that,

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

Or

Total float = Late Start of Activity – Early Start of Activity

Since we can use any of the formulas to calculate total float for the Activity D, we can go with the first formula.

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

= 10 – 5

= 5

Have you noticed that the total float and free float for Activity D is the same?

This is just a coincidence; they need not to be the same. They can be different.

Free Float for Activity G

Again, activity G has two successor activities: Activity E and H. Since the Early Start date of Activity E is greater than the Early Start date of Activity H, we will choose Activity E to determine the free float for Activity G.

Hence,

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 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.

image credit => Master isolated images/ FreeDigitalPhotos.net 

Comments

  1. Kishore Vaishnav says

    Can you please give me an example where total float of an activity is not same as free float of that same activity.

    • Fahad Usmani says

      Hello Kishore,

      In fact, I was also planning to draw a network diagram where you can see that the total float and free float are different but in that case, calculation will little lengthy. So I avoided it.

      I covered concept to calculate the total float and free float in this example and I hope once you understand it, you can calculate them in any scenario.

      Even if you’re having any problem with it, send me scanned copy of diagram, I will try to explain it to you, or wait for my other blog post in CPM where I will calculate the total float and free float for more complex diagrams.

  2. Ayushman says

    I am unable to see the network diagrams on any of the questions of explanations like the one above for total and free float.

  3. says

    Hi Fahad, Thanks for the explanation and a good diagram, now i can calculate them. Looking ahead for the next post in CPM for more complex diagrams and their calculations. thankyou

  4. jayatheertha says

    hi,

    can you please explain how do we calculate a float for a particular node, like what is the float at node B?

    • Fahad Usmani says

      In the given example we have two paths; i.e. ABD and ACD. The duration of the path ABD is 20 days, and the duration of path ACD is 12 days.

      Obviously, the path ABD is the critical path, and float on critical path is always zero. Therefore the float for activity B is zero.

      However, you can calculate the float for the path ACD, which is (20 – 12); i.e. 8 days. Therefore flat for activity C will be 8 days.

  5. Jerry says

    Do you have anything describing total float vs. free float when you have start-to-start, finish-to-finish, and start-to-finish links along with leads and lags?

    • Fahad Usmani says

      I took the simplest example to explain the concept. As of now I don’t have any start-to-start, finish-to-finish, finish-to-start and start-to-finish relationship diagram with leads and lags explaining the float concept.

      But in future I might write on it.

  6. Muhammad Abbas says

    Also There is one other type of float in which is interferring float ” is the maximum amount of time an activity can be delayed without delaying the entire project but delay the early start of the succeceding activities.

  7. Muhammad Abbas says

    one other type of float in independent float :”is the maximum amount of time an activity can be delayed without delaying the early start of the succeeding activity and without being affected by the allowable delay of the preceding activities”.

  8. Samuel Odozi says

    Thanks for your detailed explanation on Total Float vs Free Float. Sir, i’ve not gotting the concept of RACI Chart and thus cannot answer PMP questions on it, pls i need you assistance.

    • Samuel Odozi says

      Thanks for your detailed explanation on Total Float vs Free Float. Sir, i’ve not got the concept of RACI Chart and thus cannot answer PMP questions on it, pls i need you assistance.

      • Fahad Usmani says

        RACI stands for responsible, accountable, consulted, and informed. It is a responsibility assignment matrix where you list the name of person and mention that whether he is responsible, accountable, consultant, or informed for a particular process.

  9. Mike Grisham says

    I’m attempting to calculate the date by which a task must be completed so it won’t delay the subsequent task(s), in Project 2007. I’m using Free Float added to the Finish date. Most of the returned dates make sense. However, a few don’t. I haven’t identified a pattern to the exceptions yet. Any thoughts on this?

  10. Daniel Koshy says

    Hello Fahad
    Seriously this blog and notes has prevented me from tearing my hair :). I initially just knew the definition of free float but never had to calculate it on any of the questions I came across, I came across it on the first pmp exam which I took and failed. Finally found a free float calculation question on oliver lehmann today and was confused by the answer. I found more confusing answers on pmzilla, different people had different opinions on how to calculate it. I was really frustrated seeing so many answers and not knowing for sure how to calculate it. even the first website that came up on google when I typed “how to calculate free float” explained it, but I was still not confident in trusting it. However your blog explains it completely with an example. I am confident now. Thanks a lot man..really really appreciate it. I will keep looking for more questions that I have on your blog. This is my second time preparation for the pmp. I have gone through 5 books in total: Rita, achieve pmp success, kim heldman and pmp head first. The last 2 I have just gone through the questions, since I felt the explanation is the same across the books. I am in the process of completing as many sample exams I can come across, I have been doing okay averaging 70% across all, some tough some easy. Sorry….digressed from my initial comment..thanks a lot once again.

    - Daniel

    • Fahad Usmani says

      Hello Daniel,

      I am sorry to hear that you have failed the exam in your first attempt. I hope you will pass the exam in your next attempt. Just work on improving your weak spot.

      Don’t go for many books. Two books with the PMBOK Guide is more than enough to pass the exam.

      Fahad

  11. khaled_elagmy says

    Hi
    I think if there is more than one successor activity,we must take the least in early start not the greater … I want explain this point why u take the greater….it seems not logical to me

    • Fahad Usmani says

      I think you are talking about the forward pass, when an activity has two predecessor activities, and we take the greater early finish date to calculate the early start of successor activity.

      Okay.

      Let us say activity C has two predecessor activities, activity A and activity B. It means activity C can not be started until its predecessors activities complete. If activity A finishes in 5 days and Activity B finishes in 10 days, activity C will only start after 10 days, because until both activities complete, activity C can not be started.

      Hope it helps.

  12. Joe Tse says

    Hi Fahad,

    First of all, thank you very much for the detailed explanations and the diagrams about the float. It helps me to understand a lot more in the topic. However, same as khaled_elagmy, I am still not sure a point about the free float.

    Take your example 2, you said that Activity G has a free float of 2. Recalling the definition of free float: “the amount of time that an activity can be delayed without delaying the Early Start of its successor activity”, if Activity G is going to delay 2 days, then obviously the ES of Activity H will be affected (it will become 6), which violated the definition. Therefore, the free float of Activity G cannot be 2.

    Did I miss anything in understanding the free float?

    • Fahad Usmani says

      One path will have to free float, other will not.

      In example 2, Activity G will have a free float for the path Start-G-E-F-End.

      Activity G will not have a free float for the path Start-G-H-I-End.

      Hope it is clear now.

  13. HB says

    hi fahad…appreciate your explanations….these are of great help..just a small clarification
    ( maybe late in this chain since i discovered your blog only now!!)…
    the free float example 2, raises a basic question…aren’t free floats applicable only for those activities which share a common successor? if yes, then activities G and D, which have a common successor in E, ought to be evaluated for free float. It will be seen that G is the one having a free float of 2 days in comparison to D which will have 0 free float.

  14. Lax says

    Hi Fahad,

    Thanks for your explanation. Since 2 years unable to clear the doubt.

    I had same strong doubt like Joe Tse and HB. After google search, i found small note that boost for your explanation.

    NOTE : Free Float can only occur when two or more activities share a common successor, or in other words, when activities converge on a Network Diagram.

    In Example 2. the activities D and G are common successor. Still that Free Floots effect to E and H activities. On view of this confusion i came to one conclusion. The following modified note will going to solve the confusion.

    Note : Free Float can only occur when two or more activities share a common successor, or in other words, which activities has Divergence and immediate convergence in that path only has free float.

    In Example 2.
    Activity D has free float only on Divergence immediate convergence path -> Start-D-B-C-End.
    Activity G has free float only on Divergence immediate convergence path -> Start-G-E-F-End.

    I hope this clear the doubt.

    Thanks

    • Fahad Usmani says

      Hello Lax, for free float, you have remember only two things.

      1) An activity can have a free float if it has two or more successor activities. (not when two or more activities share a common successor)

      2) If an activity has two successor activity, it have a free float only for one path which has larger early start date.

  15. Ahmed says

    Dear Mr Fahad

    Thanks for nice explanation,

    Can you explain the examples of network diagrams with leads and lags e.g Finish to Finish, Start to finish, Start to Start.

    • Fahad Usmani says

      Hello Ahmad,

      The example given in this blog post is enough for the PMP exam. Anyway, I have noted your request and once I get some time, I will update this post with more examples as you requested.

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