What Is Float And How To Calculate Total & Free Float?

In one of the recent discussion on our Forum, I realized PMP® aspirants have lots of questions on float of activities.There is a lot of confusion about different types of floats which can exist in a schedule network diagram. Total float and free float are considered same by many aspirants. It’s high time we drill this down so that all the confusions go off.

The term float is used to express the flexibility. This flexibility can be at the project level or at the activity level. Few points/definitions to be understood before we get into float of an activity.

1. During a schedule network diagram – activity can be either predecessor or successor activity corresponding to the dependent activity we refer to
   • Predecessor – An activity that logically comes before a dependent activity in a schedule
   • Successor – A dependent activity that logically comes after another activity on a schedule.
2. Critical Path – The critical path is the sequence of activities that represents the longest path through a project, which determines the shortest possible project duration
   • All activities on the critical path are called critical path activities
3. Schedule Network Diagram – is a graphical representation of the logical relationships, also referred to as dependencies, among the project schedule activities
4. PDM (Precedence Diagramming Method) – A technique used for constructing a schedule model in which activities are represented by nodes and are graphically linked by one or more logical relationships to show the sequence in which the activities are to be performed

I am not going into details of above and details of how to create a network diagram, ES, EF, and LS& LF for activities.

Now let’s get back to float. There are 2 types of floats which can exist i.e.

1. Total Float
2. Free Float

Let’s see what these floats are all about

Total Float

“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 this is talking about flexibility we have at project level. Now in a network diagram, we can have multiple paths to reach the end. So this float talks about flexibility one path has as compared to others.

Simple formula to calculate total float is our usual formula i.e. LS (Late Start) – ES (Early Start) or LF (Late, Finish) – EF (Early Finish)

Free Float

Now comes the real challenge, Free Float is different to Total Float and yes, it’s not a small difference which can confuse one with another.

“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”

Simple formula to calculate Free Float is ES (of successor) – EF of current activity

Here we are talking about activities and how much an activity can be delayed as compared to its successors

Looking at above lets bottom out few differences which can help us understand what are these two floats all about

Total FloatFree FloatCalculated at path level of activitiesCalculated at the activity levelDefines flexibility of a path w.r.t project end dateDefine flexibility of activity w.r.t its successor startFormula LS-ES or LF-EFFormula ES (of successors) – EF of current activity -1Can come into existence if network diagram has multiple path and there are activities which are not there on Critical PathCan come into existence if successor is having more than one activity converging on it or the successor activity is having a constraint applied

Let’s look at one example to compile the stuff. Look at below network diagram

What does this diagram depict?

1. There are 3 paths ACE, BCE & BDE. ACE will be the critical path with total float 0 and the critical path length 18
2. Activities A, C & E will be having even free float 0 (no kind of float/flexibility for critical path activities)
3. Total float for B is 1 (LF-EF OR LS-ES) & activity D is 6
4. Out of B & D which activity can have free float? Activity B is not satisfying free float definition. I.e. B can be delayed w.r.t C (6-4-1=1) but not w.r.t D (5-4-1=0). So if “ANY” part of the definition is not satisfying i.e. B can’t be delayed without impacting ANY successor of it.
5. Free float for activity D = 14-7-1 = 6. This activity satisfies the definition along with point 4 and also there is no dependency/constraint in example which can hinder activity D having flexibility

I hope now the basic difference between these floats is clear and Float mystery is not more floating for you . If you still face any doubts or confusions you may refer to two points- first the definition and second the above table. Happy reading and do post follow up questions.