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Float (slack or buffer) is extra time that a task could consume beyond its duration estimate without impacting other aspects of the project.   Total float is extra time without impacting the end date of the project and free float is extra time without impacting another project task.

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Quick reference

Float, Slack, Buffer

Float (slack or buffer) is extra time that a task could consume beyond its duration estimate without impacting other aspects of the project. Total float is extra time without impacting the end date of the project and free float is extra time without impacting another project task.

When to use

Float (slack or buffer) is a valuable project management resource.  Some tasks have it and some do not.  Knowing which is which helps to make day-to-day prioritization and risk management decisions.  Float is calculated as part of calculating critical path.  If the critical path has not been calculated, you cannot know with certainty how much float you have or which tasks have float.

On schedule driven projects, I calculate the critical path and determine where there is float available.  I preposition float as a risk mitigation response, allocating a portion of float into the schedule immediately following high risk tasks. There is no float available for critical path tasks if a project has been scheduled to complete as soon as possible.  (By definition, the critical path is the path with no float.)

Instructions

To calculate float (slack or buffer) first calculate the critical path.  Subtract the “Early Finish” value from the “Late Finish” value.  If the values are the same, the float is zero and the task is on the critical path. If the value is negative, an error has been made somewhere, recheck your calculations.

Float is created through the project planning process.  Based upon the network diagram for the task schedule and the task duration estimates, float can be calculated.  The amount of float available to a task can be changed by changing the network diagram or changing the duration estimates of that task or other tasks in the project.  Float management is an excellent form of risk management on schedule driven projects.

Float is a very valuable resource for resolving issues.  When an issue arises, resources can be “borrowed” from tasks with float to work on solving the issue.  As long as those resources are returned to their original task before all the float is consumed, the project will not be delayed.

Float can be categorized as Free Float or Total Float.  The calculation referenced above calculates Total Float.  This is the float available to a task before a delay would impact the end date of the project.  The best way to calculate Free Float is to subtract the task Total Float from the successor task Total Float.   Free Float is the amount of float available to a task before it begins to impact any other task, not just critical path tasks.

Definitions

Total Float:  "The amount of time that a scheduled activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint." PMBOK® Guide

Free Float: "The amount of time that a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint." PMBOK® Guide

This definition is taken from the Glossary of the Project Management Institute, A Guide to the Project Management Body of Knowledge, (PMBOK® Guide) – Sixth Edition, Project Management Institute, Inc., 2017.

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  • 00:04 Hi, I'm Ray Sheen, I'd like to talk about Float, Slack, Buffer.
  • 00:09 Those terms are used interchangeably, but
  • 00:11 they're an important concept in schedule planning.
  • 00:14 To understand the concept of float, slack, or buffer,
  • 00:18 let's work with the metaphor of a window.
  • 00:21 Every task has a time window in which to accomplish the task.
  • 00:25 The task cannot be started until the window opens and the task must be finished
  • 00:30 before the window closes for the project to remain on schedule.
  • 00:33 As long as the task is accomplished while the window is open,
  • 00:36 the project will not be delayed.
  • 00:38 Sometimes, the window is open much longer than is needed to complete the task.
  • 00:43 You might have a six week window to do a five day task.
  • 00:47 Sometimes, the window is open for
  • 00:49 exactly the length of time that required to complete the task.
  • 00:53 In that case, you may only have five days to do a five day task.
  • 00:57 When that happens, you need to start the task the minute the window opens so
  • 01:01 that you are done when the window closes.
  • 01:03 Now I know, you're just thinking sometimes
  • 01:06 the time window was smaller than the duration of the task, no it's not.
  • 01:11 If you think that's the case during a project plan, you need to change the plan.
  • 01:15 Don't start a project that you already know will fail.
  • 01:19 If you don't have enough time, you should see the stakeholders to either
  • 01:22 get the time boundary extended or the scope reduced or more resources added.
  • 01:28 Extra time within a time window is referred to as float, slack, or
  • 01:32 sometimes it's called buffer.
  • 01:34 I don't care which term you use,
  • 01:36 I'll be using float in the remainder of this lesson.
  • 01:38 Be aware that the term buffer is use with a different definition
  • 01:42 when you're doing the critical chain project scheduling.
  • 01:46 So why do we care?
  • 01:47 How do we use float to help us manage a project?
  • 01:50 Float is created based upon the schedule plan that you've established for
  • 01:53 the project.
  • 01:55 If you have multiple paths of project activities,
  • 01:57 you're almost guaranteed to have float.
  • 02:00 The predecessor, successor relationships between the tasks and the estimates for
  • 02:04 the task durations will determine how long each of those paths will take.
  • 02:08 The longest path is the critical path and has no float.
  • 02:12 All the other paths have float.
  • 02:14 Changing the project schedule will change the float.
  • 02:18 Either changing relationships or task estimates, or adding or deleting tasks.
  • 02:22 If the duration of one of the paths changes, the float will change.
  • 02:27 I use my float as a schedule reserve.
  • 02:29 I want to know which paths to float and how much they have so
  • 02:33 I know which paths have schedule reserve.
  • 02:35 Paths with no float aren't by definition critical paths.
  • 02:39 However, normally the majority of task are not on the critical path and
  • 02:43 they have some float to work with.
  • 02:45 So what do I do with that reserve?
  • 02:48 I'll use it in one of three ways.
  • 02:50 One approach is to position float immediately following a high risk task.
  • 02:54 It maybe high risk because of estimate uncertainty or
  • 02:57 maybe high risk because of technical difficulty.
  • 03:00 It may even be high risk because of resource constraint.
  • 03:02 Whatever the reason, the float is there to accommodate
  • 03:05 any unplanned delay in that task without impacting any other task.
  • 03:10 I'll also positioned some of the float right before a major milestone is due.
  • 03:14 Often as you approach a milestone,
  • 03:16 there will be little last minute problems at surface.
  • 03:19 The float time is there to resolve those without impacting the milestone.
  • 03:23 And finally, I use float to resolve resource over allocation problems.
  • 03:27 If I have a resource over allocation at a particular time in the project schedule,
  • 03:32 I try to use float to move one of the more active task at that time period
  • 03:36 to a later time period when that task has time within its window.
  • 03:41 There are two ways to calculate float, Total Float and Free Float.
  • 03:45 Let's compare the two.
  • 03:47 The Project Management Body of Knowledge, the PMBOK Guide defines Total Float as
  • 03:51 the amount of time that a scheduled activity can be delayed or extended
  • 03:55 from the earliest start date without delaying the project finish date or
  • 03:59 violating a schedule constraint.
  • 04:01 This is calculated for us with the critical path calculation and
  • 04:04 its the latest time minus the earliest finish time for each task.
  • 04:09 Of course, when those times are equal the answer is zero and
  • 04:13 that task is then on the critical path, there is no float.
  • 04:17 But as long as we have a positive number, there is Task Total Float.
  • 04:21 The impact of exceeding the value of Task Total Float
  • 04:24 is the cause of delay to the end date of the project.
  • 04:27 Using up only a portion of the float will likely impact successor tasks,
  • 04:32 although it will not impact the end of the project.
  • 04:34 Now, we compare that Task Total Float with the other type of float, Task Free Float.
  • 04:41 The Project Management Body of Knowledge, the PMBOK Guide, defines Free Float as
  • 04:45 the amount of time that a schedule activity can be delayed without delaying
  • 04:49 the early start date of any successor or violating a schedule constraint.
  • 04:54 That means that this is the amount of float that a task can use
  • 04:57 without impacting anything else from the project.
  • 05:00 It's calculated by starting with the Task Total Float value and
  • 05:03 subtracting the successor Task Total Float value.
  • 05:07 If both task are on the same non-critical path with no other relationship
  • 05:11 interventions, they will normally have the same Total Float value.
  • 05:15 The Free Float value is then zero.
  • 05:18 A delay of the first task impacts the start of the second.
  • 05:21 The impact of exceeding the task Free Float is on the next task or
  • 05:24 abandon the project.
  • 05:26 Using up only a portion of task Free Float will not impact the schedule or
  • 05:30 anything else in the project.
  • 05:32 Now if you're planning on taking the PMP exam, be certain that
  • 05:35 you understand the differences between these and how to calculate them.
  • 05:41 The use of float is an excellent way to manage schedule risk in your project.
  • 05:45 If your project has schedule constraints, it's imperative that you understand which
  • 05:49 tasks and paths have float and which ones don't.
  • 05:54 This is your schedule reserve.

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