Calculate the float (or slack) between the task and the next successor task. If there are multiple successors, then it will select the one with the closest start date.
Free Float (FF): The amount of time that a schedule activity can be delayed without delaying the early start date of any immediately following schedule activities.
FF = Successor Task Early Start (ESs) – Predecessor Task Early Finish (EFp) – 1 – Lag
We calculate Free Float by subtracting the EF of the predecessor from the ES of the successor minus any lag between the two minus 1 since it ends the day before the early start of the successor. Let’s understand this conceptually. Any value greater than zero represents the amount of time the predecessor can delay without impacting the successor. If it has multiple successors, the predecessor’s Free Float is the minimum value of all those calculated.
The example below shows how this is not the same as TotalFloat.