In a project activity network diagram, the formula for float (or slack) of an activity (there is no float of project) is extremely simple, yet I have trouble recalling whenever I need them. The names “Total Float” and “Free Float” can be very misleading themselves, and doesn’t tell much of what it really means. So here is my final attempt to remember what they are once and for all, just by using common sense.
Total Float and Free Float are properties of a single activity, in units of days (or time). When people unfamiliar with the term “total float” tries to guess what it means, they may tend to think it has something to do with summing up all the “free floats” in the project. No, that’s not what they terms mean. The good news however, is that you probably already understand the float concept with your own terminology.
What Are Floats Used For?
Total Float of an activity answers the question “How many days can I delay this activity without delaying the end project date?”. Using common sense alone, you know that if a project is at the brink of being delayed, all activities must then be starting at their “late start date”. Hence if you delay an activity by the “Total Float” number of days, you will be forcing all subsequent activities to start at their “late start date”.
Free Float of an activity answers the question “If I finish this activity early, how many days do I have left before the next activity starts?” OR “How many days can I delay this activity without forcing the next activity to start later than planned.”. When all activities are planned, common sense tells you that everyone hopes to start the activities at the earliest date possible. When there are parallel tasks in progress, and if we are on the easier task with a shorter duration, we want to know how many days we can delay before the longest/slowest task completes. This gives us some slack to slow down our pace, without affecting the earliest start date of the next activity.
Total Float of an Activity = LF – EF
Note that if this activity finish late, subsequent activities must all start late and hence project schedule is at risk. But, the total float is still a property of the activity, not of the project itself.
Free Float of an Activity = ES of next activity – EF of the current activity
Note that this formula gives the answer to “If we finish early, how many more days till the next activity starts.”
EF = Early Finish Date
ES = Early Start Date
LF = Late Finish Date
LS = Late Start Date