Simple Formula Questions Part 2 – Calculating Float

By Vinai Prakash

Many calculation type PMP questions are pretty straight forward. All you need to know is the right formula, and have a good understanding of the basic concepts to apply the formula in the right way.

In the first part of the series, I talked about the Three Point Estimate Questions. Today, I am going to tell you about Free Float and Total Float – as they apply to the Critical Path Method, under the Time Management Knowledge Area, and how to calculate Float for the PMP Exam.

First things first… on the PMP Exam, Float is also written as Slack. They mean the same thing. Float = Slack, and Slack = Float.

Total Float or Slack: It is the amount of time that an activity can be delayed without delaying the project’s end date. This gives you to total time or “buffer” you have to play with for this activity, before the project gets delayed.

Free Float: This is the amount of time an activity can be delayed, without impacting the early start date of its successor. For this to happen, you need to have 2 or more activities having a common successor.

There is a third kind of Float, called the Project Float. This means the amount of time a project can be delayed, without impacting the externally imposed date by the customer and/or management.

Example:
Let’s check this out in an example. A customer asks you to Setup a Booth for an upcoming exhibition, which opens on November 1 this year. You need 4 days to setup the booth.

If now we are in August, you have 2 full months (60 Days) to Complete this Project, but you need only 4 days. Therefore, you have a Project Float of 60-4 = 56 Days.

Is Float Good or Bad?

Having some float or slack on the project is actually good for the project. This means that you use this “extra” time to tackle unforseen emergencies, delays, and problems, or test & improve the quality of the product. If the Float is Zero, then the Project is very risky. Any single point of failure, and the entire project can be delayed. The risk on the project then becomes quite high.

Critical Activities (activities on the Critical Path) usually have a Float of Zero.

This means that these activities CAN NOT be delayed, and if there is any delay, then the entire project will get delayed. Therefore, a Project Managers needs to focus all the attention on activities on the critical path, and near critical path.

How to Calculate Float on the PMP Exam?

There are 2 ways to calculate Float, and the answer is the same with either method.

Float = Late Start (LS) – Early Start (ES)

Float = Late Finish (LF) – Early Finish (EF)

TIP: Both formulas start with LATE. You can use Late Start or Late Finish. Just remember this, and you can get this formula correct.

Many questions need you to calculate Float. Others may simply state the Float, and then ask you to interpret it.

Sample PMP Question:

For the Barracuda project, you have a early start of 3, late start of 5 and early finish of 25. What is the float of the Project?

In this case, we do not need all the three values. Float can simply be calculated as Late Start (5) – Early Start (3). So the answer is 2. This is a positive number, and gives you a bit of a breather…

Mind you, Float can be negative too. In this case, the activities are already at risk of delaying the successor activities and the entire project.

Hope this gives you an idea of the concept of Float. These questions are easy to attempt in the PMP Exam, if you know the basics well.

All the Best!

Cheers,
Vinai, PMP

PS 1: The PMP Exam has changed from August 31, 2011. Are you aware of the changes? Please check out my detailed post about this.

PS 2: I am conducting an Online PMP Coaching workshop – with daily Video lessons, Discussions, Questions. If you can spare an hour a day, you can be ready to sit for the PMP exam in just a few  weeks. Check out the Lesson Plan here.

About Vinai Prakash


Vinai Prakash is the founder and editor of PMChamp PMP Exam Tips. "I love coaching and training PMP® aspirants to pass the PMP Exam in their first attempt, without stress, and without hassles through classroom & online PMP coaching workshops." Learn more about Vinai here.


Comments

  1. Toni says:

    I am confused. In this sample question What type of float were you asking for? Previously you had given the formula LS-ES or LF-EF for calculating formula.
    How did you know to use EF-ES?
    Is saying float of the project, the same as saying project float?

    Sample PMP Question:
    For the Barracuda project, you have a early start of 3, late start of 5 and early finish of 25. What is the float of the Project the same as saying project Float?

    In this case, we do not need all the three values. Float can simply be calculated as Early Finish (25) – Early Start (3). So the answer is 22. This is a positive number, and gives you a bit of a breather…

    • Vinai says:

      Hi Toni,

      You have raised a good point. As you know, many PMP questions are ambiguous. If nothing is given specifically, you should assume that they are asking for the Project Float. Yes, “float of the project” is the same as “Project Float”. Do note that PMP questions may not exactly state things clearly as per the PMBOK guide.

      In the example, there are three values given – ES, LS & EF. Since we have 2 formulas, we could use either of them. But to use the second formula, we need both Finish values – Early Finish and Late Finish. However, the question failed to supply the “Late Finish” value. So we can’t use this formula. This leaves us with only the first formula – and here we find that both values are present – Early Start & Early Finish. Thus, we chose to use this formula, and got the 5-3 = 2 days answer.

      Hope this clarifies. If you still have any doubt, feel free to post it here.

      Cheers,
      Vinai

  2. Ajay says:

    As per the formula, you mentioned correctly as
    Float = Late Start (LS) – Early Start (ES)

    In the sample we have both the values ” you have a early start of 3, late start of 5 ”
    So the float should be 5-3 = 2

    Also, 22 days seems the duration of activity. If you need late finish , it should be Float + LF i.e. 2+25 =27

    So the answer should be Float = 2. Is there something I am missing?

  3. Muammer Cavusoglu says:

    Please continue to write posts about “Simple Formula Questions”
    Because they are very simple and useful.
    Thanks

    Muammer

  4. Ranjan says:

    Hi Vinai,

    YOUR EXPLANATION IS REALLY GOOD BUT 2ND PARA OF YOUR REPLY TO TONY
    “This leaves us with only the first formula – and here we find that both values are present – Early Start & Early Finish. Thus, we chose to use this formula, and got the 5-3 = 2 days answer.” IS NOT CORRECT. THOUGH YOUR ANSWER IS CORRECT BUT IT’S A CASE OF ERROR IN TYPING. INSTEAD OF Early Start & Early Finish, U SHOULD HAVE WRITTEN EARLY START AND LATE START.

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