Difference between Total Float and Free 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.
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.
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
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:
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: