Planner answers

What causes negative float in a P6 programme?

Short answer: Negative float appears only when a date the logic cannot meet is imposed: a finish-on-or-before or mandatory constraint, or the project's Must Finish By date. The amount of negative float is how late the forecast is against that date.

Total float is late finish minus early finish. Late dates come from the backward pass, which starts at the project finish or the Must Finish By date and is pulled earlier by constraints. When the forward pass (logic and durations) ends later than such a date, the late dates fall before the early dates and float turns negative.

How to find the source

  1. Check Project → Dates → Must finish by.
  2. Filter activities with constraints (Finish On or Before, Mandatory Finish, Start On or Before).
  3. Trace the chain with the lowest float back to the constraint.

The free problem finder lists the constraints and project dates pulling the float below zero. If the date is contractual, the negative float is the delay to recover or to claim; if it is not, remove the constraint.

Find what drives the negative float →

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