This is one of the most misunderstood concept. Generally PMP aspirants either think Total Float and Free float is one and the same thing or at times some even think total float is combination of all the floats at different stages of the project. I have seem some of the PMP trainers and PMP professionals at times confused about the differences between the two. Let’s start from very basic and understand what is a float first.
Float is the flexibility at a project or activity level to delay a task to yield positive project benefit without delaying the project. In order to better understand float, we need the schedule network diagram which is graphical representation of relationships between different activities within the project. This network is built by having activities connected to its predecessor (an activity that logically comes before the given activity or activities) and successor (an activity that logically comes after an activity or activities). Precedence Diagramming Method is created using early start (ES), early finish (EF), late start (LS) and late finish (LF). for each activity. Early start how soon a activity can be started and early finish is how early an activity can be finished which is calculated by adding early start with activity duration. Late finish is how late an activity can be started and late finish is how late an activity can be finished without impacting project schedule. Unlike early finish which is calculated by adding duration of activity to early start (forward pass), late start is calculated by subtracting duration of activity from late finish (backward pass).
Now since we have all the necessary ingredients to calculate float, lets get back to it and understand total float and free float.
Total Float is the amount of time that a schedule activity can be delayed or extended from its early start date without delaying the project ﬁnish date or violating a schedule constraint. Total float is at the project level and is calculated by subtracting ES (Early Start) from LS (Late Start) or subtracting EF (Early Finish) from LF (Late, Finish). Their rarely is the confusion with this formula. Lets now talk about Free Float and understand what it is.
Free Float is the amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint. It is calculating by subtracting EF of current activity from ES of nearest successor activity. Free float is calculated at the activity level.
*Note: Always remember that Free float will always be less or equal to Total Float. If it is coming greated than Total Float then recheck your calculation.