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 two activities are converging to a single activity, one of these two activities may have the 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.

updated total float and free float

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.

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 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 

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. 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.

  13. Murali says

    Dear Fahad,

    I beg to differ on your explanation on FF. As per the definition of FF, “it is the duration by which an activity can be delayed without impacting the ES of the successor activity or successor activities.

    Therefore for activity D the FF is ZERO, as any delay in activity D will impact the ES of Activity E, eventhough it doesnot impact activity B. Similarly the FF of activity G is ZERO, as any delay in activity G will impact the ES of Activity H, eventhough it doesnot impact activity E.

    As I understand FF of an activity is NOT calculated by taking the largest ES date of its successors, and it is just governed by the above DEFINITION.

    Most importantly, the activities having FF in the network are:

    Activity F= 31-18= 13 days

    Activity I=31-13= 18 days

    Loading the above network to a CPM software could thoroughly validate my explanation.

    • Fahad Usmani says

      Total Float and Free Float are different.

      An activity will have a free float only when it has two successors activities.

      Hope it helps.

      • Murali says

        I am sorry to say your explanation on FF is flawed. It is NOT true that an activity will have a FF only when it has 2 successor activities.

        Infact FF can only occur when 2 or more activities share a common successor and not the other way. As I said before the only activities having FF in the network are activities F and I.

        I am only replying for the benefit of others, as I have seen in this string many people (Khaled, Joe, HB, Sadek etc) raising the same question to you on FF.

        • Fahad Usmani says

          I meant that, to have a free float an activity should have more than one successor activity. It can have two successor activities or three…

          For more clarification, you can visit the pmbok guide, page 177, which says..

          free float is the amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraints.

          • Fahad Usmani says

            Hello Murali,

            Let me explain you in this way.

            The PMBOK Guide says that

            “free float is the amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraints.”

            And you says that Activities F and I will have a free float. How they can have it because they are last activities on their respective path, and therefore they are not affecting the early start of their successor activities.

            Now you are saying that activity D is not having the free float.

            Let us see if the PMBOK Guide’s definition is applying here.

            Activity D is finishing on 5 day.

            Its successor Activity is B, which is starting on 11 day.

            Now you can clearly see that activity D can be delayed for 5 days without affecting the early start of Activity B. This is exactly the PMBOK Guide is saying. Please read the definition again.

            Hence the Activity D will have free float of 5 days.

            Is it clear?

            • Bryan says

              If there was an activity before END, then the F and I would have a free float that would be the same as total float as you explained in the first simple example.

              On the other hand, if you had an activity right after START branching out to A, D and G, then the activity would not have a free float despite the fact that it had multiple successors.

              If you delete H and I from the diagram, G would have free float of 2 days and total float of 15 days even though it does not have multiple successors. It shares E as successor with D.

              In your diagram, D and G would violate E and H respectively if they are delayed though it would not affect critical path right away.

              I am not sure if D and G really have a free float when they violate one of the successors as long as it does not affect critical path.

              I would appreciate if you check again and clarify.

              • Fahad Usmani says

                Hello Bryan,

                You are right, free float will occur when two or more activities shares a common successor.

                I have amended this blog post as well.

            • Murali says

              Dear Fahad,

              Let me explain my position as below:

              “free float is the amount of time that a schedule activity can be delayed without delaying the early start date of “ANY” successor or violating a schedule constraints.”

              Please note the word ‘ANY’ from the above definition:

              For activity D, it has 2 successors namely activity B and activity E. All the while in your explanation you are talking about FF of activity D with respect to Activity B (first successor) only, what about its impact on activity E (the second successor)????

              Can we delay the activity D without delaying the ES of Activity E?, and the answer is NO. Therefore as per definition the activity D has Zero FF.
              The same logic goes to activity G, which also has ZERO FF, as it is delaying the ES of activity H.

              Nowhere in the defintion it talks about taking the greatest ES amongst its successors while calculating the FF of an activity.

              Nowhere in the defintion it talks about taking only ONE successor while calculating the FF of an activity.

              Also it is NOT MANDATORY to have multiple successors to have a FF, even activities with a single successor can have FF provided you have a CONVERGING network.

              That is the reason why FF is more stringent than a TF. The reason being, for a FF all the activities are to be completed as per their ES dates.

              Therefore in a project, the TF is owned by the Owner/client and the FF is owned by the Contractor.

              Now coming to activities F & I, both of them have a successor activity END (which is a milestone activity with duration ZERO). As the CP duration is determined as 31 days, the activity F has a FF of 31-18= 13 days and activity I has a FF of 31-13=18 days. Coincidentally they are equal to their respective Total Floats as well.

              As I mentioned in my first reply, please update this logic in a CPM software like P3, P6 or MS Project and they will validate my explanation.

              • Fahad Usmani says

                Please send me your network diagram at fahad@pmstudycircle.com

                You are right that when two or more activity will converge, they may have free float. I have corrected this error in my blog post.

                Regarding your second point:

                You are saying that Free Float of F = 31 – 18,

                This means Free Float of F = Late Finish of F – Early Finish of F

                Are you sure?

                I believe you are calculating total float and mixing it with free float.

                Formula to calculate free float is

                Free Float of an activity = ES of next Activity – EF of Activity

                • Murali says

                  As the activity END is a milestone activity (Zero duration), its ES is 31 and EF is 31.

                  FF of activity F= ES of activity END – EF of Activity F
                  =31- 18= 13 (In other words activity F can be delayed by 13 days without delaying the ES of activity END or project completion)

                  FF of activity I= ES of activity END – EF of Activity I
                  =31- 13= 18

                  Now coming to Total Floats,

                  TF of activity F= LF of activity F – EF of activity F
                  = 31 – 18 = 13 days

                  TF of activity I= LF of activity I – EF of activity I
                  = 31 – 13 = 18 days

                  Therefor for activities F & I, the TF and FF are the same.

                  Hope this clarifies.

        • Fahad Usmani says

          Yes, you’re right Murali that free float will occur when two activities share a common successor.

          Thanks for pointing out this mistake in my blog, post and I thank you for the same.

          • Murali says

            Dear Fahad,

            You are welcome.

            I am happy that I could contribute to your popular website for prospective PMPs.

            Please continue your good work for the PM community.

  14. Ali says

    Your explanation on every topic is wonderful. I will practice the calculation with the similar example you used for your calculation and then I think I’ll be able to perform all the calucations with difderent examples.

  15. Vijay Kumar says

    I’m very lucky to get the explanations and examples so easily explained by you…..really appreciated and thanks a lot for writing such a wonderful blog….thanks again…

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