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In creating our CPM diagram we calculated the Total Float for each Activity. Total Float is a measure of how tightly the Activity is tied to the finish date.The calculation for each Activity is TF=LF-EF. We can now put TF onto the Activity list as follows:

 from: HOW TO MAKE A CPM DIAGRAM. Example #1


Work Package         Activity              Predecessor              Successor           Duration (days)               Total Float        Free Float

        Start up                 1                           Start                           2                             0                                         0                     0

        Start up                 2                               1                              4                             3                                         0                     0

        Start up                 4                               2                           7,5,9                         1                                          0                    0

             A                        7                               4                               8                            6                                         3                     0

             A                        8                               7                              12                           1                                         3                     3

             B                        5                               4                                6                           2                                         7                     0

             B                        6                               5                               12                          1                                         7                     7

             C                        9                               4                               10                          8                                         0                     0

             C                       10                              9                               11                          1                                         0                     0

             C                       11                            10                               12                          1                                         0                     0

         Finish off             12                          8,6,11                          13                          2                                          0                     0

         Finish off             13                             12                            Finish                      5                                         0                     0

Also shown on the above table is Free Float. Free Float is a measure of how tightly the Activity is tied to the next Activity, its Successor. Let's look at this a bit closer.

Total Float (TF): The amount of time that an Activity can be delayed, from its Earliest Start, without delaying the Project Finish, or violating a schedule constraint

  • TF=LF-EF

  • Every Activity on a branch path has identical Total Float

  • The Total Float for a branch path an be called the path slack for that branch

  • For every Activity on the Critical path TF=0

  • A positive number for TF indicates how far away that Activity is from the Critical path, and from delaying the project finish.

Free Float (FF): The amount of time that an Activity can be delayed, from its Earliest Start, without delaying the Earliest Start of any successor, or violating a schedule constraint.

  • FF=ES next - EF 

  • The equation means the FF for an Activity is the ES for the next Activity minus the EF for the Activity in question.

  • Activities on a branch path can have different Free Floats

  • Free Float will be zero if its immediate successor can start as soon as the Activity is finished

  • For every Activity on the Critical path FF=0

  • For Activities that intersect the Critical path, their FF=TF

  • A positive number for FF indicates how far away that Activity is from delaying the earliest Start of its successor.

          These videos provide a nice summary for the topic of Floats.

               They will load faster in Firefox than in Internet Explorer.

Channel 4
What is Total Float (Total Slack) and how to calculate it in a network diagram
Play Video
What is Free Float (Free Slack) and how to calculate it in a network diagram
Play Video

By now you realize you cannot determine Float until you make a CPM Diagram. Then you can list the Floats on the Activity list. Sometimes Total Float and Free Float are extracted from the Activity list to  make a Float Table. Especially in large complicated projects, referring to a Float Table is a quick way to recognize how a delayed Activity might impact the next Activity and the project finish date (if at all). A Float Table is a handy tool for quickly identifying risks and implications from possible/proposed changes to the schedule, without having to re-calculate the whole CPD.

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