Total float and free float: read both
Two numbers, two different questions. Confusing them makes a blocking task look like a comfortable one.
What each one measures
| Total float | Free float | |
|---|---|---|
| The question | How far can this task slip without moving the project finish? | How far can it slip without moving its first successor? |
| What it looks at | The project finish, or the deadline that bounds it | The early start of its successors |
| When it is zero | The task is on the critical path | Any delay gets in the way of what follows immediately |
| It can be negative | Yes, when a constraint or a deadline cannot be met | No |
An example
A task has 10 days of total float and 0 of free float. Moving it by a day does not push the project finish out, but it blocks its successor the next morning. On site, that is a task everyone believes is flexible and is not.
The reverse does not happen: free float never exceeds total float.
Why in working days
Float expressed in calendar days means nothing: five days across a four-day holiday weekend are not five days of work. Float is computed in working hours, then converted using the calendar of the task concerned.
What float does not say
- 200 days of float does not mean the task is comfortable. It nearly always means a link is missing.
- Negative float does not mean the project is late. It means an imposed date cannot be reached with the links as declared.
- Float says nothing about resources: two tasks with comfortable float can need the same crew on the same day.