I have been wanting to draft a post for you and explain the basics of calculating float, total float and free float in network diagram. This is finally it!

One of the areas that appeared many times for my exam and has been supported in forums such as PMHangout and iZenbridge is….Network Diagram.

During my bootcamp, my instructor actually spent half a day going through this section out of the four days training we had. So you can imagine that network diagram is one important topic for PMP exam.


In project management, float or slack is the amount of time that a task in a project network can be delayed without causing a delay to: subsequent tasks (“free float”) project completion date (“totalfloat”)

Total Float
Total float is referred to the difference between the early (ES) and late start dates (LS) (Formula: LS – ES) or the early(EF)  and late finish dates (LF) (Formula: LF – EF). Total float often measures from the beginning to the end for the whole project. This hopes to provide a simpler statement to what you will read from PMBOK 5th edition, The total amount of time that a schedule activity may be delayed from its start date without delaying the project finish date, or violating a schedule constraint.
You may often hear other terms such as “float” or “slack”, associated to Total Float.
I would cover how to calculate critical path using forward and backward passing in a separate post.

Let us look at the diagram for example:



If activity C1 has duration of 2 days and is occurring concurrently with activity C3 which has duration of 3 days, activity C1 has 1 day of total float. Meaning, it can be delayed up to one day impacting the project finish date.
However, if activity C1 is delayed by 2 days, there is now a negative float situation: -1 days. This means that the project will now take one more day than anticipated.

Free Float
Now let us look at a different type of Float – Free Float. According to PMBOK, free float is the amount of time that a schedule can be delayed without delaying the early start date of immediately following scheduling activities.

This is only calculated on the last activity on a path. So the node with Free Float is always the activity right before the finishing activity for Activity on Arrow type of Network Diagram or the finish for Activity on Node type of network diagram.

How to identify and calculate Free Float?
Free float is measured by subtracting the early finish (EF) of the activity from the early start (ES) of the following activity.

One thing to remember in the Exam:
• Free float on the critical path is always zero.
• If anything else, it is likely to be a red herring question to trick you.

If you are preparing for PMP exam now, please read through PMP Certification Guide and the directory of free PMP sample questions to help you prepare for your exam.


Does this post help? Leave me a comment if you have additional explanation to add to this post.

Let’s help others to ace PMP 🙂