Total float and free float

Lesson 3.3 used total float. There is a second kind, and the difference matters every time something slips.

	Total float (TF)	Free float (FF)
How far can the activity slip without delaying…	the project finish	any successor’s early start
Calculated from	late dates: LF − EF	the successors’ early dates

For a finish-to-start relationship, free float is the gap between this activity’s finish and its successor’s start:

FF = ES(successor) − EF − lag, taking the smallest gap if there are several successors

Shared float

Float belongs to a path, not to a single activity. Look at the office fit-out in the lab. After the strip-out, two paths run to Final clean:

• Walls and ceilings: A1020 Build partition walls (3 days), then A1030 Install ceilings (4 days). This path is critical.
• Lighting: A1040 Order light fittings (2 days), then A1050 Install lighting (3 days). This path is 2 days shorter.

ID	Activity	ES	EF	TF	FF
A1040	Order light fittings	5	7	2	0
A1050	Install lighting	7	10	2	2

Both show 2 days of total float, but it is the same 2 days. A1040 has no free float, because A1050 starts the moment it finishes. A1050, the last activity on the path, has the gap before Final clean.

So if the light fittings arrive a day late:

• A1050 moves by a day, because A1040 had no free float.
• Handover does not move, because the path had total float.
• The path now has only 1 day left, for both activities.

If A1040 uses up both days, the lighting path becomes critical. One more day and Handover slips.

Using float well

• Free float is the safe kind. An activity can use its free float without affecting anyone else.
• Total float is shared. If the first activity on a path uses it, the later ones lose it. That is why many contracts say float belongs to the project, not to whoever reaches it first.
• Big float numbers deserve a second look. Large float often means missing logic (lesson 2.1), not a relaxed plan.

Your turn

In the lab, delay A1040 one day at a time. At 3 days, A1050 moves and Handover doesn’t. Find the longest duration that still keeps Handover on Day 14, then push one day further and watch the finish slip to Day 15.