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

December 2, 2019
Seema Sonkiya
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  • Project Time Management
  • PMP®
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Project managers usually apply schedule adjustments to develop an efficient CPM schedule. Float and Free Float is one of the sources to make such adjustments. The term Float expresses flexibility that helps in such schedule adjustments. This flexibility can be at the project level or the activity level gives different choices.

During PMP® coaching sessions, I observe a lot of confusion around Float and Free Float that exists in a schedule network diagram. Many PMP® exam aspirants consider total float and free float the same. In this blog, I am addressing the confusion of Float and Free Float raised by PMP aspirants.

Let’s discuss first a few points/definitions of Schedule Network Diagram, PDM & Critical Path

1. Schedule Network Diagram –

It is a graphical representation of logical relationships among project activities. So, this diagram shows dependencies among those project activities. These dependencies reflect the order in which we schedule project activities. Here, activity can be either predecessor or successor corresponding to a 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. PDM (Precedence Diagramming Method) –

PDM is one of the Schedule Network Diagrams or schedule models. In this model, nodes represent activities. One or more logical (or precedence) relationships link these nodes graphically to show the sequence we perform these activities. So, we assign precedence relationships based on the dependencies between each activity. PDM has four types of precedence relationships to complete the network diagram – ES, EF, and LS & LF. Here, I am not going into the details of creating a network diagram using these relationships.

3. Critical Path –

In a PDM, a critical path is a tool to determine minimum project duration. It is the sequence of activities to represent the longest path through a project. This longest path determines the shortest possible project duration. All activities on the critical path known as critical path activities.

Now let’s get back to float. There are 2 types of floats that exist in a PDM i.e.

  • Total Float
  • Free Float

Let’s see what these floats are all about

Based on PMBOK® Guide Sixth Edition 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.

So, the Total Float talks about available flexibility at the project level. A network diagram can have many paths to reach the end. So, this float talks about the flexibility one path has compared to others.

Now, the question is..

How is total float calculated?

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

Let’s make it simple further to see –

How is float time calculated in critical path analysis? And what is total float in the critical path method? Is total float a slack?

The Total Float and slack are synonymous.

The Total Float is the difference between the project completion date and the total duration of critical path activities.

In other words, suppose you have a project to finish in 25 days. You see that your calculated critical path activities on the schedule network diagram will take 22 days. So, you have a project float of +3 days. Here you can see, a Float can be a positive or negative number.

We will discuss negative float later in this post.

In summary,

Find the second-longest sequence of activities in the network diagram. Subtract its total duration from the duration of the critical path sequence. The difference between the two durations will give you the float for each activity in the second sequence.

I see another confusion here – is Float the same as lag? The reason for this confusion lies in the concept of the ability to delay an activity. Lag is a delay in the successor activity. Total Float is the amount of time by which you can delay an activity from its early start date without delaying the project finish date.

Then what is the difference between lag and total float?

Both have a different purpose when we look at it from a realistic project management scenario:

We use Lag to put delay deliberately to meet already available dependency or pre-requisite. Like you cannot start putting wallpaper until the final coat of paint gets done and dried. In this case, you need to delay the wallpaper work by a few days. You can see it is like necessity or dependency.

But the float is the flexibility we have that helps make the desired choices in a schedule. It’s up to the project manager if she wants to use it or not. e.g., in a project, some paths have six days of float. The project manager uses it to make the best possible decision for the project. So here, the Project manager may choose to finish an activity on that path by six days in advance. Afterward, she can put the resource somewhere else. Or, she may first use the resource somewhere else and then put it back to the activity of that path. In that way, she still can meet the dates and constraints.

In summary, we can’t bypass Lag as it is deliberate. Float is a flexibility that we can plan and manage to optimize the best possible value for the project.

Free Float

Free Float is different from Total Float. And yes, it’s not a small difference that 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

The obvious question is –

How do you calculate free float and independent float?

A simple formula to calculate Free Float is ES (of successor) – EF of current activity
Here we are talking about activities and how much activity can be delayed as compared to its successors.

Let’s take an example to see how to calculate free float –

If you use the formula, LS-ES or LF-EF will get activity two has eight days float. But this float is converting into free float.. let’s see how…

You cannot delay activity two by eight days without slowing activity three’s early start of 17th.. so the float of activity two is =6 days.

For an activity, the total float can become free float if-

The successor has more than one activity converging on it, or the successor activity has a constraint

All activities do not have a free float. You need to see if there is a free float available for an activity.

Let’s look at one more example to compile the stuff. Refer to the below network diagram:

What does this diagram depict?

  1. There are 3 paths ACE, BCE & BDE. ACE will be the critical path with 0 Total Float. The critical path length is 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 the 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 the example which can hinder activity D having flexibility.

In summary,

What is the difference between total float and free float?

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

Now come to the negative float:

Many PMP® certification aspirants ask me –

Can a critical path have negative float? Can a float be negative?

Yes, why not?

Float expresses flexibility, but negative float snatches that flexibility.

So, Negative Float is an imposed constraint. For example, you estimated to finish an activity in 10 days. But, due to an upcoming festival, you have to finish it in five days. Your logic needs ten days, but the constraint is not allowing you to go beyond five days. So, you have a negative float of five days.

It is a temporary situation because you cannot leave the project with a negative float. Here your flexibility has been snatched, and you need to get up for some actions. You need to pull the project to meet stakeholders’ expectations. You need to find ways to get this negative float to settle down. It should come to the 0 from a negative number to get rid of the negative float.

PMBOK® Guide Seventh Edition does not define usages of float explicitly. But, it appreciates all the concepts described in PMBOK® Guide Sixth Edition. Also, the New PMP® Exam based on PMP® Exam Content Outlines 2021 balances the focus on scheduling between predictive and adaptive approaches. The adaptive scheduling methods recommend continuous planning for short durations using

  • Backlog Refinement,
  • Iteration Planning, and
  • Release Planning techniques.

We have an elaborated Online Live Virtual program that has all the required content to master adaptive and predictive approaches. Moreover, you do not have to refer to any book or material with our program. Our participants are getting great results and learning by just following our program and contents.

Finally, If you need a discussion around Float and Free Float concepts with some practical project management scenarios, watch the following video right away.

PMP

I hope now the basic difference between these floats is clear and Float mystery is not more floating for you. Happy reading and do post follow up questions here on our DISCUSSION FORUM.

Name Date Place
PMP Certification and Training 21 Jan – 26 Feb 2023 India More Details
PMP Certification and Training 16 Feb – 17 Mar 2023 Bangalore More Details

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