Project Management guide · Scheduling and time
Float and Slack in Project Management: Types and Formulas
Float, also called slack, is the amount of time an activity can be delayed without causing a delay. Total float is measured against the project finish date; free float is measured against the next activity. Activities with zero total float are on the critical path.
Key takeaways
- Float is how long an activity can slip without causing a delay.
- Total float protects the project finish; free float protects the next activity.
- Float is shared along a path, so using it early reduces it for later activities.
- Negative float shows how much time must be recovered.
Types of float
| Type | Definition | Formula |
|---|---|---|
| Total float | Delay allowed without delaying the project finish | LS - ES or LF - EF |
| Free float | Delay allowed without delaying any successor's early start | Earliest ES of successors - EF |
| Project float | Time between the calculated finish and a required deadline | Required finish - calculated finish |
| Negative float | The schedule cannot meet a required date | Total float below zero |
Worked example
In our standard six-activity network (see forward and backward pass), the early and late dates give the following floats:
| Activity | ES | EF | LS | LF | Total float | Free float |
|---|---|---|---|---|---|---|
| A | 0 | 3 | 0 | 3 | 0 | 0 |
| B | 3 | 7 | 3 | 7 | 0 | 0 |
| C | 3 | 5 | 7 | 9 | 4 | 0 |
| D | 7 | 12 | 7 | 12 | 0 | 0 |
| E | 5 | 8 | 9 | 12 | 4 | 4 |
| F | 12 | 14 | 12 | 14 | 0 | 0 |
- C: total float 9 - 5 = 4 days. Free float = ES of E (5) - EF of C (5) = 0, so any slip in C pushes E.
- E: total float 12 - 8 = 4 days. Free float = ES of F (12) - EF of E (8) = 4.
- The 4 days are shared along the path C-E. If C slips 3 days, E has only 1 day left.
- If the sponsor requires completion by day 16, project float is 16 - 14 = 2 days. If the deadline is day 12, the critical activities have -2 days of float.
How to use float
- Calculate float after every schedule update.
- Use free float first when you need to delay an activity, because it does not affect anyone else.
- Use float to smooth resource peaks through resource levelling.
- Watch near-critical paths with low float; a small delay can make them critical.
- Agree who owns float. Many contracts say it belongs to the project, not to whichever party uses it first.
Negative float
Negative float appears when the calculated dates are later than a required date or constraint. It tells you how much time you must recover. Typical responses are crashing or fast tracking, reducing scope, or negotiating a new date through change control.
Common mistakes
- Treating float as spare time that can always be used. It is shared along a path.
- Date constraints that hide or distort float.
- Ignoring float when assigning people, so a low-priority task with float takes a resource needed on the critical path.
- Reporting only total float when free float would show the immediate impact on the next team.
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Frequently asked questions
What is float in project management?
The amount of time an activity can be delayed without delaying either its successors (free float) or the project (total float).
Is slack the same as float?
Yes. The terms are used interchangeably; slack is more common in PERT literature and some tools.
How do you calculate total float?
Total float = late start - early start, or late finish - early finish.
How do you calculate free float?
Free float = the earliest early start of the activity's successors minus its own early finish (allowing for any lags).
What does zero float mean?
The activity is on the critical path: any delay delays the project.
What is negative float?
Float below zero, showing the schedule is behind a required date by that amount.
Can free float be greater than total float?
No. Free float is always less than or equal to total float.
What is project float?
The difference between the project's calculated finish and an imposed deadline, sometimes called positive project float.
Who owns float?
It depends on the contract. Many organisations treat float as a shared project resource managed by the project manager.
Does float change during a project?
Yes. As activities finish early or late, float is recalculated and can grow, shrink or become negative.