What is the difference between Total Float and Free Float?
Total Float is the amount of time that an activity can be delayed from its early start date without delaying the project finish date. Free Float is the amount of time that an activity can be delayed without delaying the early start date of any successor activity.
PMBOK Guide definitions
Total Float: The total amount of time that a schedule activity may be delayed from its early start date without delaying the project finish date, or violating a schedule constraint.
Free Float: The amount of time that a schedule activity can be delayed without delaying the early start date of any immediately following schedule activities.
What is the formula for Total Float?
Total Float = LF - EF (or LS - ES)where ES = Early Start, EF = Early Finish, LS = Late Start, and LF = Late Finish
What is the formula for Free Float?
Free Float = ES of next activity - EF
Float calculation QuizLet's understand the concepts with the help of an example. Refer to the Network Diagram in the figure first. I'm assuming that you know how to work with a Network Diagram and how to calculate ES, EF, LS, and LF for activities on a Network Diagram. Click on the figure to download the network diagram as a PDF file.
Problem: What is the Total Float and the Free Float for Activity F and J?
Solution: The calculations are really simple.
For Activity F:
Total Float = LF of F – EF of F => 8 – 5 = 3
Free Float = ES of G – EF of F => 5 – 5 = 0
For Activity J:
Total Float = LF of J – EF of J => 8 – 3 = 5
Free Float = ES of G – EF of J => 5 – 3 = 2
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 our example, only activities F and J can have Free Float.
Hope you'll do well if you get such a question on your exam. If you have any doubts, I'm here to help.
While you are at it, also learn about Negative Float.
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Image credit: Flickr / miguelvirkkunen