Project Time Management: Free Float vs Total Float for PMP Exam


Project Time Management: Free Float vs Total Float for PMP Exam

In project time management, when using the Critical Path Method  (CPM) to determine the project schedule, there are two types of float that are easily confused by many Aspirants: Free Float and Total Float.

Free Float vs Total Float

A Float/Slack is a schedule flexibility an activity has in a project network diagram which allows the delay of the activity without causing a specific delay:

  • Free Float: the amount of time (float) an activity on the schedule network diagram can be freely delayed without affecting the early start of the of following/successor activity
    • for activities NOT on the critical path
    • Free Float can only be non-zero when two or more activities have a common successor activity
    • Free Float = ES of successor activity – EF
      where ES = Early Start, EF = Early Finish

  • Total Float: the total amount of time (float) an activity on the schedule network diagram can be delayed without affecting the project finish date
    • for activities NOT on the critical path
    • Total Float = LF – EF (or LS – ES)
      where ES = Early Start, EF = Early Finish, LS = Late Start, LF = Late Finish

Note the following:

  • Activities on the Critical Path have ZERO free float or total float. 
  • Total Float and Free Float for an activity may be the same or different depending on the other activities in the schedule diagram.
  • When you are asked to calculate the “Float” for an activity in the PMP® Exam, you are asked to calculate the “Total Float“.

Illustrated Example

Let’s take the project of exam study and preparation as an example to illustrate the concept of Free Float and Total Float. Let’s suppose getting certified consists of the following three activities:

  • Activity A — Get 35 Contact Hours (duration: 4 weeks)
  • Activity B — Apply for the exam and pass the audit (duration: 6 weeks)
  • Activity C — Write mock exams (duration: 3 weeks)

Below is a simplified Schedule Network Diagram:

Schedule Network Diagram

Schedule Network Diagram

  • Total Float of Activity C = LF – EF = 10 – 3 = 7 weeks
  • Free Float of Activity C = ES of successor activity – EF = 7 weeks
  • Though the Total Float and Free Float is the same in this case, there are possibilities that that can be different!

Summary

To distinguish between Free Float and Total Float, always remember the following definitions of these:

  • Free Float is the amount of time an activity can be freely delayed without affecting the early start of the of successor activity
  • Total Float is the total amount of time an activity can be delayed without affecting the overall project finish date

Note: Float can be negative (i.e. Negative Float) which indicates that the activity must start before the predecessor activities have finished to catch up with a target finish date — which is probably caused by a delay in predecessor activities.

recommended PMP resourcesAdditional FREE PMP® resources: 47+ Commonly Confused Term Pairs with detailed explanations. If you found this article useful, you may wish to reference other Commonly Confused Term articles.

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Edward Chung

Edward Chung aspires to become a full-stack web developer and project manager. In the quest to become a more competent professional, Edward studied for and passed the PMP Certification, ITIL v3 Foundation Certification, PMI-ACP Certification and Zend PHP Certification. Edward shares his certification experience and resources here in the hope of helping others who are pursuing these certification exams to achieve exam success.

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5 Responses

  1. Jj says:

    also the float is calculate as ES – LS and EF – LF they have to be the same difference 8 – 0 is 8, the other is 10-3 = 7. That is a bogus example? What am i missing?

  2. Jj says:

    Not sure how you got the Free float to = 10? wouldnt ES of succesor be “0” for activity C and minus 3 for the EF of activity C give you -3?

  3. Stephanie says:

    why do some websites explain free float as ESsuccessor – EF -1, while others do not include the -1?

    • Edward Chung says:

      Yes, that depends on whether you count the beginning as day 0 or day 1. If you start from day 1, you will need the ‘-1’. But for simplicity, I adopt day 0 as the start of the process and there is no need to ‘-1’.

      Wish you PMP success!