Free Float
Free float is the number of days a task can slip without delaying even the earliest start of its immediate successor — a narrower notion than total float, which is measured against the project finish.
How it differs from total float
Total float is how far you can slip "to the end of the project"; free float is how far you can slip "to the next task." Narrowed to the simple case — FS relationships with zero lag, and the convention that finish and start dates each include their own day — the formula is free float = min(successor ES) - EF - 1. If a task finishes on day 8 (EF=8) and its successor starts on day 12 (ES=12), the three days between — 9, 10, and 11 — are its free float. Once SS, FF, SF, or lag enter the picture, the point that holds the successor changes, and the formula changes with the relationship type. A task with zero free float can still hold total float.
Why it matters
It turns promises between teams into numbers. "We can run three days late without touching your kickoff" is the translation of three days of free float. Slip on total float alone and the finish date may hold, but the downstream team's start moves — waiting costs and lost trust follow. A natural division of labor: total float for weekly reporting, free float for coordination between teams. The more interfaces a project has — the more points where deliverables cross to another team or vendor — the more free float matters as a management number.
Common misconceptions
Most confusion comes from mixing the two floats. The baselines differ: total float measures against the project finish, free float against the immediate successor's start. The slack you may spend without bothering anyone ends at free float; beyond it, total float is a shared resource you spend only after talking to the rest of the path. Reading the empty space between Gantt bars as free float is another mistake — that space can contain lag and non-working days.
wbsgantt computes and records total float; free float is not derived separately.