What Is Float (Slack) in a Project Schedule?
Two tasks can look identical on a Gantt chart, yet one is a crisis if it slips a day and the other can drift a week with no harm. The difference is float. It is one of the most useful numbers in a schedule and one of the least understood. Here is what float is, the two kinds you will meet, and how to read it off your own plan.
Float in one sentence
Float, also called slack, is how long a task can be delayed without delaying something else. That "something else" is either the whole project or just the next task, and which one you mean gives you the two types of float covered below.
A task with plenty of float is flexible: start it late, stretch it, and nothing downstream notices. A task with zero float is unforgiving: any slip pushes the finish date. Knowing which is which is the difference between worrying about the right things and worrying about everything.
Float and slack are the same thing
You will see both words. "Slack" is common in general use and in tools like Microsoft Project; "float" is the term in formal scheduling and the critical path method. They mean exactly the same measurement. This guide uses float, but read slack wherever you prefer.
How float is calculated
Float falls out of two passes over the schedule. A forward pass gives every task its earliest start and finish. A backward pass, working from the deadline, gives every task its latest start and finish without pushing the end date. Float is the gap between them:
In plain terms: float = latest start minus earliest start (the same number you get from latest finish minus earliest finish). If a task can start on the 3rd at the earliest and the 7th at the latest without moving the deadline, it has four days of float.
Total float vs free float
The one distinction that trips people up. Both are real, and they answer different questions.
| Total float | Free float | |
|---|---|---|
| Delay before... | the project finish moves | the next task moves |
| Measured against | the deadline | the following task's earliest start |
| Size | larger or equal | smaller or equal |
| On the critical path | zero | zero |
Free float is the slack you can use without affecting anyone else at all. Total float is the slack you can use before the project itself is late, and spending it may quietly eat into a later task's freedom. When in doubt, protect free float and spend total float knowingly.
A worked example
- Build: days 6 to 10. Review waits on it, so Build has zero float, it is on the critical path.
- Write docs: needs only 2 days but has the same window until day 10. Earliest finish day 7, latest finish day 10, so it has 3 days of total float.
Notice you never had to guess. Float is calculated, not felt, which is why a tool that computes it beats a chart you shade by hand.
Why float is worth knowing
Float turns a schedule from a picture into a decision tool. The tasks with zero float are your critical path: the chain that sets the finish date, where every day of delay is a day off the end. The tasks with float are your flexibility: where you can absorb a surprise, shift a resource, or let something wait.
Managers who track float stop firefighting evenly across the whole plan and start protecting the few tasks that actually matter. It is the single most efficient way to spend limited attention.
How to see float on a Gantt chart
You do not calculate float by hand on a real project. In a tool that computes the schedule, it is already there:
- Link your tasks with dependencies so the tool knows the order, this is what makes float meaningful.
- Turn on the critical path. The highlighted tasks are the zero-float chain; everything not highlighted has some float.
- Slip a non-critical task and watch: if the finish date does not move, you have just seen its float in action.
- Use the space. Move a person from a high-float task to a zero-float one when the critical path is under pressure.
You can try this in the free gantts.app editor: build a few linked tasks, turn on the critical path, and drag a non-critical bar to feel where the slack is.
The short version
Float is how much a task can move before it costs you. Zero float means it is on the critical path and every slip hurts; positive float means you have room to breathe. Free float protects the next task, total float protects the deadline. Learn to read it and a busy schedule suddenly tells you exactly where to look.
Templates that use this
Keep reading
- Critical Path Method (CPM): Steps, Formula & Example
- Gantt Chart Dependencies Explained: FS, SS, FF and SF
- What Is a Gantt Chart? Definition, Examples & Uses
Frequently asked questions
What is float in project management?
Float, or slack, is the amount of time a task can be delayed without delaying the project (total float) or the next task (free float). Tasks on the critical path have zero float, so any delay to them pushes the finish date.
Is float the same as slack?
Yes. Slack is the everyday term, common in tools like Microsoft Project; float is the formal scheduling term used in the critical path method. They describe the same measurement.
What is the difference between total float and free float?
Total float is how long a task can slip before the project's finish date moves. Free float is how long it can slip before the next task has to move. Free float is always less than or equal to total float.
How is float calculated?
Float equals the latest start minus the earliest start (equivalently, latest finish minus earliest finish). Those values come from a forward pass and a backward pass over the schedule, which a scheduling tool does for you.
What does zero float mean?
Zero float means the task is on the critical path: it has no room to move, so any delay to it delays the whole project. Those are the tasks to protect first.