Float is one of the very basic concepts essential for building network diagrams and in turn a project schedule. Further, there are different types of floats such as Free float, Total float, Project float, Interfering Float, Independent Float. As a Project Manager, you should have a clear understanding of these concepts in order to create efficient project schedules. So, lets see..
The term “Float” implies “Fluid”, which in turn implies “Flexibility“. In Project Scheduling, Float refers to the amount of scheduling flexibility. Float is also popularly called “Slack“.
Types of Float
The different types of float are
- Total Float or Float
- Free Float
- Project Float
- Interfering Float (INTF)
- Independent Float (INDF)
Total float is the amount of time an activity can be delayed without delaying the project completion date.This is the type of Float that is commonly referred to as “Float”.
PMBOK defines Total Float as
“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.”
So, Total Float is about flexibility at the project level. It is about the flexibility that an activity has in its execution without delaying the Project finish date.
Total Float Formula
Total float is calculated by subtracting the Early Start date of an activity from its Late Start date (Late Start date – Early Start date), or Early Finish date from its Late Finish date (Late Finish date – Early Finish date).
Total Float or Float = LS – ES or LF – EF
PMBOK Defines Free Float as “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”
So, Free Float is about flexibility at the activity level. It is about the flexibility that an activity has in its execution without delaying its successor activity.
Free Float Formula
Free float is calculated by subtracting the Early Finish date of current activity from the Early Start date of its successor activity (ES of successor Activity – EF of current Activity).
Free Float = ES (of successor) – EF (of current)
Total Float vs Free float
While Total Float is how much an activity can be delayed without affecting the project Finish date, Free Float is about how much an activity can be delayed without affecting its successor activity.
This type of Float is not mentioned in the PMBOK, but is present in some reference books.
It is the amount of time a Project can be delayed without delaying the externally imposed project finish date by the customer, or the project finish date previously committed to by the Project Manager.
Free & Total Floats are about the time an activity can be delayed, while Project float is the amount of time a Project can be delayed.
Interfering Float (INTF)
Interfering Float is the amount of time a schedule activity can be delayed or extended from its early start date without delaying the project finish date. However, it should be understood that delaying an activity into interfering float will delay the start of one or more following non-critical activities. If an activity is delayed for the amount of the Free and Interfering Float, then its successor activities are critical.
Interfering Float Formula
Interfering Float = Total Float – Free Float
Independent Float (INDF)
Interfering Float is the maximum amount of time an activity can be delayed without delaying the early start of the succeeding activities and without being affected by the allowable delay of any predecessor activity.
Independent Float Formula
Independent Float (INDF) = Earliest Successors’ Early Start – Earliest Predecessors’ Late Finish – Activity’s duration
when the INDF is a negative value, we set the value to zero.