Scheduling Fundamentals
Free Float
Free float is how long an activity can slip before it delays the very next activity, as opposed to total float, which measures how long it can slip before it delays the project finish.
Definition
Total float and free float answer two different questions. Total float asks: how much can this activity slip before the project finish date moves? Free float asks the narrower question: how much can this activity slip before its immediate successor has to move? An activity can lose free float while its total float stays completely unchanged -- a sign the schedule is tightening up locally, activity by activity, well before anything shows up as a threat to the finish date. That's exactly why it's tracked as a separate figure rather than folded into total float: a string of activities quietly losing free float across several updates in a row is often the earliest visible warning that a section of the schedule is under real pressure, long before any single activity crosses into negative float or starts driving the critical path.
Example
An activity retains 20 days of total float from one update to the next -- unchanged -- but its free float drops from 12 days to 2, because its immediate successor's own start date moved up. The project finish looks unaffected, but the local sequencing has gotten much tighter.