P6 Float Calculator

Float, also called slack, tells you how long an activity can slip before it causes a problem. Primavera P6 reports two kinds: total float, which protects the project finish date, and free float, which protects the next activity's early start. This calculator works out both from activity dates, so you can understand and check the values P6 shows.

How to use it

1. Enter the activity's early start (ES), early finish (EF), late start (LS) and late finish (LF).
2. Enter the earliest early start of its successors for free float.
3. Read total float, free float and whether the activity is critical.

Formulas

• Total Float = LF - EF (equivalently LS - ES)
• Free Float = earliest ES of successors - EF (for finish-to-start links without lag)

Worked example

Activity Install ductwork: ES = day 10, EF = day 15, LS = day 18, LF = day 23. Its successor Ceiling grid has ES = day 17. Total float = 23 - 15 = 8 days: the ductwork can slip 8 days without delaying the project. Free float = 17 - 15 = 2 days: after 2 days of delay, the ceiling grid starts later, even though the project finish is still safe.

Reading float in P6

• Activities with total float of zero or less are critical; negative float means the schedule cannot meet a constraint or deadline.
• P6 can define critical as longest path instead of a float threshold, which is better when calendars or constraints distort float.
• Float belongs to the path, not to one activity; if one activity uses it, successors on the same path lose it.
• Watch near-critical activities, for example those with 1 to 10 days of float.