When I was preparing for my PMP exam, I was very confused about the difference between the two CPM concepts - Total Float and Free Float. I understood Float, but not the two types. So, I did some research and finally made-up an example to understand the concepts myself. Today I'm sharing the example that I prepared almost 2 years ago. I hope it will help PMP aspirants on their exam.
Let's look at the PMBOK Guide definitions first.
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.
Now we'll look at the 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 Early Start, Early Finish, Late Start and Late Finish for activities on a Network Diagram.
Problem: What is the Total Float and the Free Float for Activity F and J?
Total Float = LF - EF (or LS - ES)
Free Float = ES of next activity - EF
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.