Total Float vs. Free float

Difference between Total Float and Free Float

total float

During my training programs, I am frequently asked questions about Total Float and Free Float. PMP aspirants always get confused with these similar sounding terms. To compound the confusion, the concept of free float is not explained well in the books I have read.

Let us answer the following questions.

  • What is the need of calculating 2 different Floats?
  • What is the utility of Free Float?
  • Why is the meaning of ‘Total’ in Total Float?
  • How is Total Float and Free Float calculated?

Note: This post is written assuming that you understand the concept of Network Diagrams. You have already done Forward Pass and Backward Pass to determine Early Start, Late Start, Early Finish and Late Finish.

Let us try to answer what is Total Float and how is it different from Free Float. Let us understand the concept with the help of a small example. Refer to the following network diagram.

free float

Click to Enlarge

Generally speaking Float is also referred to as Slack. In some literature you might find the terms Total Float & Free Float written as Total Slack & Free Slack respectively.

Total Float

Total Float is sometimes simply written as Float. Total Float is the amount of time an Activity can be delayed without impacting the Project Finish Date. e.g. if Total Float for an Activity X is ‘n’ days, it means Activity X can be delayed by ‘n’ days without impacting Project Finish Date.

Total Float can be calculated by using either of the following formulas

Total Float = Late Finish – Early Finish

OR

Total Float = Late Start – Early Start

Both the formulas will produce same result. e.g. In the above diagram Total Float for Activity A and Activity B are 0 and 1 respectively.

In simple terms we can say that Total Float is Flexibility is starting or finishing of an Activity. Here are a few other pertinent points about Total Float:

  • There is a difference between Buffer (Contingency Reserve) and Total Float.
  • Total Float can be calculated for any Activity in Project Network. In the above diagram, Total Float for activities are written in lower middle box.
  • Total Float is calculated for one single Activity and not for entire Path.
  • Total Float is shared among the Activities on the same Path. If one Activity uses Total Float, other Activities on the same Path will have zero Total Float (no flexibility). e.g. On Path BC if B uses 1 unit of Total Float then C will have 0 Total Float.
  • All the Activities on the Critical Path have least amount of Total Float in Project Network – generally it is zero but it could be negative also. In the above diagram ADE is Critical Path.

Free Float

Free float is another type of Float. Free Float is the the amount of time an Activity can be delayed without impacting the Early Start date of any of its Immediate Successors. e.g. Consider Activity X & Activity Y have a Finish to Start relationship and Activity Y is Successor. If Activity X has a Free Float of ‘n’ days, it means Activity X can be delayed by at least ‘n’ days without impacting the Early Start of Activity Y.

Free Float can be calculated by using the following formulas

Free Float = (Early Start of Successor) – (Early Start of Activity in Question) – (Duration of Activity in Question)

Refer to the above Network Diagram again. Free Float for both Activity A and Activity B is 0. A delay in Activity A will not delay Activity C but it will delay Activity D. A delay in Activity B will delay Activity C.

Here are a few other pertinent points about Free Float:

  • Lag and Free Float are not same.
  • Free float is useful when there is an imposed start date for the Successor activity.
  • All the Successor activities should be considered while calculating Free Float.
  • Total Float and Free Float of an Activity can be different.
  • The above formula for Free Float is applicable for Finish to Start relationships only.

You should also refer to my other articles on difference between similar Project Management Terms.
I have also compiled a PMP® Formulas Pocket Guide. You can download it free. It is a comprehensive guide to all PMP® Exam formulas.
You can refer to Max Wideman Glossary to read some other standard definitions of Estimate to Complete.
Did you that there is something called Independent Float?
Don’t let you mind Float (wander)…Please leave a comment if you are still confused.
Praveen Malik, PMP is a certified Project Management Professional (PMP®) with a rich 20+ years of experience. He is a leading Project Management Instructor and Consultant. He regularly conducts Project Management workshops in India & abroad.

2 thoughts on “Total Float vs. Free float

  1. Looks like your description over activity D in the middle, careful!, It’s the activty Duration and it’s not Early Start or ES. Am I right?

    This could confuse the readers.

    Robert A. Romero

  2. Looks like your description over activity D in the middle, careful!, It’s the activty Duration and it’s not Early Start or ES. Am I right?

    This could confuse the readers.

    Robert A. Romero

    August 13, 2011 8:50 PM

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