What is the Difference Between Total Float and Free Float?

Written on . By Tracy Mah

I’m often asked the question about the differences between Total Float (TF) and Free Float (FF). The Total Float and Free Float of an activity may not always be the same number of days. They can be different based on its predecessor and/or successor activity relationships. By definition:

Total Float (TF) = the maximum amount of time an activity can be delayed from its early start without delaying the entire project. Late Date – Early Date = Total Float

More simply, the range an activity can move around without affecting the end date of the project.

Free Float (FF) = the maximum amount of time an activity can be delayed without delaying the early start of any of its succeeding activities.

More simply, the range an activity can move around without affecting the start of any activity after it.

When a project is scheduled, Primavera undergoes some calculations that determine the earliest and latest dates each activity can start and finish. The Total Float value for each activity is then calculated from the difference between its late dates and early dates. The Total Float is then used to determine the ultimate date the project can finish at. Activities along the critical path are usually the sequence of activities in the project that will take the longest time to complete (i.e. the longest path). The critical path activities are the red activities in the Gantt chart.

Example Scenarios

In this scenario below, let’s concentrate on what’s happening with Activities B and D. Both activities have a Finish-to-Start relationship with Activity A, meaning neither can start until Activity A is completed. However, the successor path for Activity B and D are slightly different which in turn gives different results for their Total Float and Free Float values.

Figure 1: Initial Project Scenario

 

Scenario 1: Activity B Delayed 1 Day

Activity B has a Free Float of 0 days, which means that if Activity B starts even one day late (i.e. starts on Day 7 instead of Day 6), it will delay the start of its successor Activity C by one day (i.e. Activity C will start beginning of Day 12 instead of Day 11).

  • Notice that it doesn’t affect the end date of the project.

Figure 2:  Scenario 1 – Activity B Delayed 1 Day

 

Scenario 2: Activity B Delayed 7 Days

Activity B has a Total Float value of 5 days and is scheduled to be completed for the end of Day 10.

  • If Activity B was delayed more than the total float of 5 days, say for instance it completed 7 days late (i.e., Activity B finished end of day 17 instead of day 10), then it will have a cascade affect and delay the successor activities along its float path and ultimately delay the project finish date by 2 days as well.
  • Notice that the Activity path of BàCàE becomes critical giving the project -2 days Total Float.

Figure 3:  Scenario 2 – Activity B Delayed 7 Days

 

Scenario 3: Activity D Delayed 5 Days

Activity D has a Free Float of 5 days. This means that Activity D has a range of 5 days that it can start without actually affecting the start of its successor Activity E (i.e., Activity D can start anytime between Day 6 to Day 11 without affecting Activity E’s planned start date of Day 16).

  • Activity D has a Total Float of 10 days. The Total Float and Free Float have a difference of 5 days. Even if Activity D was delayed 8 days (i.e., start on Day 13), it will delay the start of Activity E by 3 days (i.e., start on Day 19), but it won’t affect the end date of the project.

Figure 4:  Scenario 3 – Activity D Delayed 5 Days

 

Scenario 4: Activity D Delayed 14 Days

If the start of Activity D was delayed more than the Total Float value of 10 days, then it will push back the end date of the project.

  • Activity D was planned to start on Day 6 but if it delays by 14 days and starts on Day 20, then it will cause Activity E’s start date to be delayed to Day 25 and finish on end of Day 29.
  • This means the project’s finish date is the end of Day 29 (i.e., Total Float=-4d, 4 days late).

Figure 5:  Scenario 4 – Activity D Delayed 14 Days

 

These 4 scenarios show how the Free Float and Total Float are different for an activity and how a change in that activity’s start date will have a cascade effect on its successor activities and project end date. Just remember Free Float is the range an activity can ‘float’ around without affecting any of its successor activities and Total Float is the range it can ‘float’ around without affecting the project’s scheduled end date.

Posted in Primavera, Scheduling