In project management, float or slack is any flexibility in the schedule that an activity has in a project that allows the delay of the activity. There are two types of float: free float and total float. What’s the difference?
Free float is the amount of time an activity on the schedule network diagram can be freely delayed without affecting the early start of the following activity. Free float is often associated with activities NOT on the critical path.
Free Float can only be non-zero when two or more activities have a common successor activity.
- Free Float = ES of successor activity – EF
- ES = Early Start, EF = Early Finish
Total float is the total amount of time an activity on the schedule network diagram can be delayed without affecting the project finish date. Total float is meant for activities not on the critical path.
- Total Float = LF – EF (or LS – ES)
- ES = Early Start, EF = Early Finish, LS = Late Start, LF = Late Finish
It should be noted that activities on the critical path have zero float. Total float and free float for an activity may be the same or different depending on the other activities in the schedule diagram.
To distinguish between free float and total float, always remember the definition for each. Float can be negative (i.e. negative float) which indicates that the activity must start before the predecessor activities have finished to catch up with a target finish date — which is probably caused by a delay in predecessor activities.
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2 weeks ago, one of our readers asked how to calculate float time in critical path analysis. Therefore, we decided to write this article. Total Float vs Free Float is one of the most important concepts in PMP Certification training. Basically, TF and FF are two critical path method terms. You use both of them while analyzing the project network diagrams and determining the critical path of a project. Although they sound quite similar, they refer to two different calculation method. In order to analyze project network diagrams, project schedules, and the critical path in the right way, the differences between these two terms should be known. In this article, we will analyze the differences between total float and free float with the help of an example to provide you a better understanding.
Note that they are also used by planning engineers while performing delay analysis and preparing claim files.
Before to start, let’s take a glance at both concepts. Total Float (TF) and FF definitions are given below.
Total Float (TF)
While analyzing a project network diagram, schedulers often say “the float of the activity is five days, two days, etc.” But what is the TF and why do we use it?
According to PMI’s PMBOK Guide, 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.”
In other words, total float for any activity is defined as the difference between its latest start date and earliest start date.
In a network diagram total float of the critical path is zero.
TF can be calculated by subtracting the Early Start date of activity from its Late Start date or Early Finish date from its Late Finish date.
• TF: LS – ES
• TF: LF – EF
Note that the TF is also known as the slack.
Free Float (FF)
Free Float is different than Total Float. According to PMBOK Guide, 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”
Simply we can say that Free Float is the amount of time that an activity can be delayed without delaying the “Early Start” date of its successor. Different than TF which is calculated at the path level, FF is calculated at the activity level.
• Free Float: ES (of successors) – EF of current activity -1
Total Float and Free Float Example
For better understanding, let’s analyze a simple total float and free float example.
Assume that, we made forward and backward pass calculation and determined the critical path of the below network diagram.
The Critical Path is “A-C-E” path because it has the longest duration hence The TF of this path is zero.
TF: LS – ES = 1-1 = 0 TF: LF – EF = 7-7 = 0 TF of Activity D = 18-11 =7
TF of Activity B = 7-6=1
When I was preparing for my PMP exam, I was very confused about the difference between Total Float and Free Float in Critical Path Method (CPM). I understood Float, but not the two types. So, I did some research and finally made-up an example to understand the concepts myself. Today I’m sharing the example that I prepared almost 2 years ago. I hope it will help in PMP Certification aspirants on their exam.
What is the difference between Total Float and Free Float?
Total Float is the amount of time that an activity can be delayed from its early start date without delaying the project finish date. Free Float is the amount of time that an activity can be delayed without delaying the early start date of any successor activity.
PMBOK Guide definitions of Total Float and Free Float
Total Float: The total amount of time that a schedule activity may be delayed from its early start date without delaying the project finish date, or violating a schedule constraint. Free Float: The amount of time that a schedule activity can be delayed without delaying the early start date of any immediately following schedule activities.
What is the formula for Total Float?
Total Float = LF - EF (or LS - ES)
where ES = Early Start, EF = Early Finish, LS = Late Start, and LF = Late Finish
What is the formula for Free Float?
Free Float = ES of next activity - EF
Quiz Time - Calculate Total Float and Free Float
Let’s understand the concepts with the help of an example. Refer to the Network Diagram in the figure first. I’m assuming that you know how to work with a Network Diagram and how to calculate ES, EF, LS, and LF for activities on a Network Diagram. Click on the figure to download the network diagram as a PDF file.
Problem
What is the Total Float and the Free Float for Activity F and J?
Solution
The calculations are really simple.
For Activity F:
Total Float = LF of F – EF of F => 8 – 5 = 3 Free Float = ES of G – EF of F => 5 – 5 = 0
For Activity J:
Total Float = LF of J – EF of J => 8 – 3 = 5 Free Float = ES of G – EF of J => 5 – 3 = 2
Pro Tip
Free float can only occur when two or more activities share a common successor, or in other words, when activities converge on a Network Diagram. In our example, only activities F and J can have Free Float.
In the next section, let’s address the question posted by Dr. PDG in the comments.
Who owns Total Float and who owns Free Float?
Total Float belongs to the project, whereas Free Float belongs to individual activities. If the project is being done under a contract, it’s debatable whether Float belongs to the buyer or seller.
How should the Project Manager use Float on the project?
‘Float’ is a project resource. It should be used judiciously, mainly to cover the risks or other unforeseen issues on the project.
Float is a project resource.
Hope you’ll do well if you get such a question on your exam. If you have any doubts, I’m here to help.
While you are at it, also learn about Negative Float.
Image credit: Flickr / miguelvirkkunen