Difference between Total Float and Free Float
There are a few types of floats in project management. I will explain the difference between total float vs free float in this post. PMP aspirants always get confused with these similar sounding terms. To compound the confusion, this concept is not explained well in many books. In addition to the difference, I will also answer the following questions.
- What is the meaning and definition of different floats?
- What is the purpose of floats in project management?
- What are the important formulas to calculate floats?
Note: This post is written assuming that you understand the concept of Project Network Diagrams. In addition, you have already performed Forward Pass and Backward to arrive at critical path method; you are familiar with Early Start (ES), Late Start (LS), Early Finish (EF) and Late Finish (LF) dates.
Generally speaking Float is also referred to as Slack. In some literature you might find these terms written as Total Slack and Free Slack.
Let us understand the concept with the help of a small example. Refer to the following network diagram.
Total Float (TF) is sometimes simply written as Float.
It is the amount of time an Activity can be delayed without impacting the Project Finish Date.
e.g. if TF for an Activity Alphais ‘n’ days, it means Activity Alpha can be delayed by ‘n’ days without impacting the Project Finish Date. It can be calculated by using either of the following formulas
TF = LF – EF
TF = LS – ES
Both the formulas will produce same result. e.g. In the above diagram TF for Activity P and Activity Q are 0 and 1 respectively.
In simple terms we can say that TF is the Flexibility is starting or finishing of an Activity. Here are a few other pertinent points about TF:
- There is difference between Buffer (Contingency Reserve) and TF. These are not same.
- It is calculated separately for each activity in the project network diagram. In the above diagram, TF is written in lower middle box for its respective activity.
- It is calculated for each single activity and not for the entire path.
- It is shared among the activities that are on the same path. If one activity uses complete TF, the other activities on the same path will have zero TF (i.e. no flexibility). e.g. On Path QY if Q uses 1 unit of TF then Y will have n0 TF.
- In a project network, the activities on the Critical Path have least amount of TF. Generally it is zero but it could be negative also. In the above diagram PXZ is Critical Path.
Free Float (FF) is another type of Float.
It 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 Alpha & Activity Beta have a Finish to Start relationship and Activity Beta is the Successor. If Activity Alpha has a FF of ‘n’ days, it means Activity Alpha can be delayed by ‘n’ days without impacting the Early Start of Activity Beta. It can be calculated by using one of the following formulas.
FF = min(ES of Successors) – (ES of Activity in Question) – (Duration of Activity in Question)
FF = min(ES of Successors) – (EF of Activity in Question)
The first formula is a generic formula and will work everywhere but the second one will work only if you have used 1 method to do critical path analysis.
Refer to the above network diagram again.
FF for Activity X is 0 as any delay in X will delay ES of X.
FF Activity Q is 0 as any delay in Q will delay ES of Y.
FF Activity R is 4 as ES of Y will not be impacted till R is delayed by 4 days or less.
Here are a few other pertinent points about FF:
- Lag and FF are different.
- FF is useful when there is an imposed start date for a Successor activity.
- All the Successor activities should be considered while calculating the FF.
- TF and FF of an Activity can be different.
- The above formula for FF is applicable for Finish to Start relationships only.
Over To You
Don’t let you mind Float (wander)…Please leave a comment if you are still confused about TF or FF.