Free float vs total float sounds like the kind of distinction that exists to be asked about in an exam. It is not. They answer two genuinely different questions, they disagree on real plans, and the one your software prints by default is the one that will get you a phone call from a subcontractor.
I ran both numbers over a 21-task house renovation — the sample plan I use for all of this — and then a third number that no CPM tool prints, because it needs information the critical path method throws away. Here is all three, on the same plan.
Total float: how far it can slip before the finish date moves
Total float is late start minus early start. Run the forward and backward passes over your dependency list and you get, for every task, the earliest it could begin and the latest it could begin without pushing the project end out. The gap is the float.
On the renovation, Roofing can start as early as working day 30 and as late as day 39. Nine days of total float. Delay it a week and the handover date does not move. That is a useful thing to know and it is why total float is the number everything prints.
Free float: how far it can slip before anything else moves
Free float asks a narrower question — how long can this task slip before it disturbs any of its own successors? Microsoft’s definition in Project is exactly that: the amount of time a task can be delayed without delaying any successor tasks. The arithmetic is one subtraction: the earliest early start among your successors, minus your own early finish.
Roofing finishes on day 35. Exterior painting, which waits on it, starts on day 35. Free float: zero. So Roofing has nine days of total float and not one day of free float. Slip it by a single day and the painters move, even though the project still lands on the same date.
Where the two disagree, and why
On this plan they differ on only three tasks out of twenty-one — Roofing 9 against 0, Driveway 9 against 0, Windows and exterior doors 2 against 1. That ratio is normal. Free float and total float agree on most tasks and then disagree loudly on exactly the ones in the middle of a chain.
The reason is that total float is shared. Driveway has nine days and so does Landscaping, which waits on it. Those are not eighteen days. It is nine days belonging to the pair of them, spendable once. Whoever slips first spends the lot, and the second one finds out afterwards. Free float is the portion that is unambiguously yours — the slack you can use without a conversation.
Which is why the two numbers suit two different jobs. If you are the project manager asking does the end date still hold, total float is your number. If you are the person about to tell a trade they can come Thursday instead of Monday, free float is.
A word on negative float
If you impose a deadline earlier than the date the backward pass produces, the arithmetic keeps working and float goes negative. Minus four days does not mean a task has slack in some strange direction; it means the plan is four days short of feasible and nobody has decided what to give up yet. Negative float is a message about the deadline, not the task.
The third kind: the float you have once the crew is counted
Here is the part that made me build a tool rather than a spreadsheet. Both floats above come out of a method that assumes anything not forbidden by a dependency can run in parallel — that if four tasks are technically ready on Tuesday, four crews are available on Tuesday.
The renovation has three general crew. On the critical path method’s schedule, the busiest day wants five, and fourteen separate days are over capacity. Re-schedule the same tasks so no day ever exceeds three people, and every float on the plan changes:
- Roofing — 9 days of total float, 0 in practice. The crew that would roof is drywalling.
- Exterior painting — 14 days on paper, 6 once the painters’ other work is honoured.
- Rough electrical — 0 days of total float, and 3 in practice. It gains room, because the finish date it is measured against moved out five days and it is no longer on the binding chain.
That last one is the one people find surprising, and it is the clearest sign that these are different quantities rather than a correction to the same one. Count the tasks that genuinely cannot move and you get thirteen out of twenty-one, against twelve on the CPM schedule — and it is not the same twelve. Roofing, Windows, Driveway and Landscaping join the list; Rough electrical, Interior painting and Flooring drop off it. Anyone managing this job off the CPM float column would be watching three tasks that no longer matter and ignoring four that now do.
Working it out on your own plan
Total float you can get from any critical path tool. Free float takes one more subtraction and is easy enough by hand for a small plan, though it is awkward in a spreadsheet, because you store predecessors and free float needs successors — you end up searching every other row’s predecessor list for your own ID.
The crew-constrained one is not arithmetic at all. Finding the schedule that respects every dependency, never exceeds the people you have and still finishes as early as possible is an NP-hard problem, which is why no spreadsheet formula produces it and why the levelling buttons in commercial packages use priority rules and do not tell you how far off they landed.
The scheduler I put up reports all three: total float from the CPM pass, the crew-constrained float beside it, and which tasks are actually immovable once your resource list is honoured — solved with a constraint solver, and it tells you when it has proved the answer optimal. Free and no signup. If your plan is a construction schedule, the template with the crew column is the shape it expects.
None of this makes total float wrong. It is the right answer to the question it asks. It is just worth knowing which question that is before you hand somebody nine days they do not have.
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