Negative float in Primavera P6 — what it means and how to fix it

What it is

Negative float means the late dates come before the early dates: by logic, the activity cannot finish until some day, and a date in the file says it must finish earlier. The number is the gap. Minus fifteen is fifteen working days to find.

It is not an error in the schedule. It is the schedule telling you, correctly, that you are late against something — the question is against what.

Where it comes from

• A constraint. "Must finish on", "Finish on or before", or a mandatory date on a milestone that the logic can no longer reach. This is the usual cause.
• The project finish date. Under the scheduling option "Calculate float based on finish date of each project", every activity is measured against the project's Must Finish By date. If that date is fixed and the logic has slipped past it, the whole back half of the programme goes negative at once.
• Progress. The data date has moved on, work that should be done is not, and the remaining durations push everything downstream past a date that was fine last month.

How to find the cause

Filter on Total Float less than zero and sort ascending — the most negative activity is the end of the problem, not the start. Open it, look at its constraint and its successors, and walk the driving relationships backwards until the float changes. Where it stops being negative is where the binding date sits.

The Schedule Log, written every time you press F9, lists the constraints and the activities with negative float. It is the fastest way to see whether one date or many are responsible.

What to do about it

Four options, in the order to try them.

• Check the constraint is real. A planner-imposed "finish on or before" that nobody remembers setting is not a deadline; remove it and the float corrects. A contract date is real, and you move on to the next three.
• Resequence. Look for finish-to-start links that could be start-to-start with a lag — second-fix does not need every room first-fixed before it begins. This is where most recovery lives.
• Shorten durations where crews can be added. Honestly: a duration cut with no change in resource is a wish, and the next update will put the float back.
• Report it. If none of the above closes the gap, the negative float is the size of the delay, and the file saying so is the record you want to have kept. Hiding it by softening the constraint removes the evidence, not the problem.