What is float in project management?

It’s fair to say that a project is an amalgamation of tasks.

Once you’ve got your work breakdown structure (WBS) ready and you’ve charted project task dependencies, you just need to perform all the tasks, and voila — your project is completed.

It sounds simple, and it would be simple if you didn’t have additional project constraints to worry about — project risks, resources, costs, and project scope, just to name a few.

How do you prioritize tasks and stay on top of the schedule with all these moving parts and constraints weighing you down?

One way to accomplish this is through effective use of float.

In this guide, we will:

  • Define what float is,
  • Explain the different types of float,
  • Showcase the benefits of using float in project management, and
  • Provide examples of float in projects.
Float in project management - cover

What is float in project management?

In project management, float is used to denote the extra time that a project task, a branch of project tasks, or the entire project can be extended by.

If a task has float, you can spend more time on it than is allotted, without disrupting the rest of the project schedule.

For example, if a task is scheduled to take 5 days to complete and it also has 5 days of float, then you can spend 9 or even 10 days total doing that task without affecting the project schedule in any negative way.

Conversely, if a task has 0 float, any delays on it will inevitably result in delays to all subsequent tasks, pushing the project past its deadline, unless you perform some course correcting.

The amount of float a task has dictates its urgency — less float equals more urgent.

Float versus slack in project management

Another term that often gets used in the same context as float is slack, with its variants — total slack and free slack.

However, the difference between float and slack really couldn’t be any simpler — there is no difference.

Float and slack are completely synonymous.

Float is the term used in the Project Management Book of Knowledge (PMBOK), but you’ll hear project managers using both.

Float and the Critical Path Method (CPM)

Float is closely connected to the concept of the Critical Path.

The Critical Path is defined as the longest sequence of dependent tasks in a project. By adding up the duration of all tasks within the Critical Path, we can calculate how long it will take to complete the project.

Activities that are on the Critical Path don’t have any float, but all tasks that aren’t on the Critical Path have either total float or free float. However, when entering data into activity nodes, most project managers will only enter the value of total float (TF) for reasons that will soon become clear.

Critical Path Methods (CPM) values important for calculating float

Map of a CPM activity node
Map of a CPM activity node

Float is the last value shown in the activity node above that you will derive when calculating a project’s Critical Path.

In order to calculate float for a task, you’ll first have to calculate the following values for that task:

  • Early start (ES) — the earliest point at which you can start an activity,
  • Duration (Dur) — the estimated duration of the task used as the starting point for all calculations,
  • Early finish (EF) — the earliest point at which you can finish a task,
  • Late finish (LF) — the latest point at which you can finish a task without affecting the project deadline,
  • Late start (LS) — the latest point at which you can start a task without affecting the project deadline, and
  • Total float (TF) — how much extra time you can spend working on a task — beyond its estimated duration — without affecting the project deadline.

💡 Plaky pro tip

In this guide, we’ll only show you how to calculate float, but calculating float is the last step performed when using the Critical Path Method. 

To learn how to calculate the early start (ES), late start (LS), early finish (EF), and late finish (LF) — metrics which are needed for calculating float for each project task — we recommend reading out Critical Path Method guide:

4 Types of float in project management

Tasks can have different types of floats depending on their context within the project schedule and dependency relations with other tasks.

The 2 main types of float in project management are:

  • Total float, and
  • Free float.
The Critical Path Method depicted as a cooking recipe
The Critical Path Method depicted as a cooking recipe

However, we’ll also explain some lesser known or unofficial float subtypes that you can hear about in project management discourse, such as:

  • Negative float, and
  • Project float.

Type #1: Total float

Total float shows how much extra time you have to complete tasks within non-Critical Path branches of your project schedule diagram.

The important thing to note about total float is that it doesn’t belong to any single task. Rather, dependent activities inside a non-Critical Path branch share one total float — that’s why it’s total.

If a branch has a total float of 5 days, and you expend 3 on the first task, the remaining total float for all subsequent tasks within that branch will be 2 days. This also means that those subsequent tasks will get delayed by 3 days.

In the picture above, activities (i) and (f) both share a total float of 1 hour and 30 minutes.

To calculate total float (TF), simply subtract a task’s late finish (LF) value from its early finish (EF) value.

TF = LF – EF

Alternatively, you can subtract a task’s late start (LS) from its early start (ES). You’ll get the same result either way, since the task’s duration doesn’t change.

TF = LS – ES

Type #2: Free float

Free float shows how much extra time you have to complete a non-Critical Path task without delaying any other task.

Interestingly enough, the formulas for calculating free float are identical to the formulas for calculating total float:

FF = LF – EF

FF = LS – ES

The difference between free float and total float is purely contextual.

We can only have free float in specific situations where the task in question isn’t part of a longer non-Critical Path branch.

In the culinary example of CPM shown on the image above, tasks (c) and (f) both serve as good examples of tasks with free float. Even if they use up their float, it won’t affect any subsequent task.

Type #3: Negative float

The tasks inside the Critical Path are characterized by having 0 float.

This means you can’t delay any activities inside the Critical Path without delaying the project timeline.

However, unexpected delays do sometimes happen.

If an activity inside the Critical Path runs 2 days late, the rest of the Critical Path will be left with -2 days of total float — negative float.

When this happens, you have to look for ways to fix the project timeline or risk missing your deadline.

Therefore, negative float isn’t defined by any formula.

It uses the same formulas as total float, but is defined by negative results.

Type #4: Project float

A term that you won’t find in PMBOK but will still hear a lot is project float.

Project float denotes how much you can delay the entire project without crossing the project deadline.

This happens when the internal deadline that the team operates off of is earlier than the actual deadline that the client is aware of.

A version of project float is also found in Critical Chain Project Management (CCPM), although under a different name — project buffer.

Critical Chain Project Management (CCPM) project buffers

Critical Chain Project Management (CCPM) assumes that each task has a built-in buffer — the extra time allotted to the base task duration with the goal of preventing every bump on the road from having project deadline altering consequences.

So, if a task would require 3 days to complete in ideal conditions, we give it 4 or 5 days, just to be safe.

But, when every task in the project has this just to be safe buffer, the wasted time starts to pile up.

CCPM takes the buffers away from the tasks and sandwiches their total sum between the final project task and project deadline.

This way, even tasks inside the Critical Path end up with some extra wiggle room.

Example of a project buffer in CCPM
Example of a project buffer in CCPM

💡 Plaky pro tip

There’s more to CCPM than just using project buffers — including other kinds of buffers. Learn all about its advantages and disadvantages by reading our guide on Critical Chain Project Management:

Benefits of float in project management

The question remains — why know all of this?

What good does differentiating between total float, free float, and negative float actually do for your project?

The answer is quite simple — prioritization and flexibility.

By using float effectively, you can manage your projects in a way that will maximize resource utilization.

Imagine a scenario where three project tasks are being worked on at the same time and all three look like they’ll be running late.

In cases such as this, you can look at how much float each task has. Chances are that only one of these tasks — the one on the Critical Path — will have 0 float.

Given that the other two tasks can run late without any dire consequences, you can divert their resources — manpower, equipment, etc. — into the task that’s on the Critical Path so as to keep the overall project on schedule.

The same holds true when you encounter a task with negative float.

Managing projects with float in mind can also prevent burnout in team members and project managers by adding a degree of flexibility to otherwise strict and immutable deadlines.

This allows project managers to prioritize tasks and thereby keep the project on track.

An example of float in project management

An example of a Critical Path Method project schedule
An example of a Critical Path Method project schedule

In the picture above, we can see an example of a project outlined using CPM.

Tasks A-C-F-G-I constitute the Critical Path — the longest sequence of dependent tasks in the project. These tasks have 0 float. Any delays on them would result in the whole project being delayed.

Therefore, by adding up their durations, we can see that this project can be completed in 31 days:

3 (A duration) + 5 (C duration) + 8 (F duration) + 9 (G duration) + 6 (I  duration) = 31 (Critical Path duration)

Now, let’s see how much negative float, total float, and free float we have in this mock project.

Example of negative float

If task F were to run 2 days late, the rest of the tasks on the Critical Path — tasks G and I — would end up with 2 days of negative float.

This is where the project manager would need to divert resources away from tasks with float in order to get the Critical Path tasks back on track.

Negative float isn’t depicted in the illustration above, as this isn’t something you plan for.

If anything, noticing that your CPM diagram has negative float is a red flag — a signal to initiate risk management.

Example of total float

As soon as work on task A is completed, project team members can also start working on the B-D-H task branch. These three tasks have a combined duration of 11 days. 

Coincidentally, they also have 11 days of total float:

31 (project duration) – 11 (B-D-H duration) – 3 (A duration) – 6 (D duration) = 11

The earliest that task B can start is on day 3, which is why we’ve subtracted the duration of task A. The B-D-H branch of tasks has to be completed by day 25, just before work starts on task D, which is why we’ve subtracted the duration of task D from the equation.

Since this total float is shared throughout the entire branch, if you use up 4 extra days on task B, you’ll only have 7 days of total float remaining for tasks D and H. 

In other words, using total float doesn’t affect the project deadline, but it does push back the starting times for beginning work on subsequent tasks in the same branch — in this case D and H.

Example of free float

Task E, as depicted in the illustration above, is a perfect example of a task with free float — it’s not part of the Critical Path, and it’s not part of a non-Critical Path branch either. 

Work on both tasks E and F can commence only once task C is completed. Furthermore, work on task G can only start once both E and F are completed. F has a duration of 8 days, while E has a duration of 4 days.

This means that task E has 4 days of free float. You can stretch out work on this task for up to 4 days beyond its regular 4 day duration, and it wouldn’t affect any other project task in any way whatsoever.

Hopefully, these examples have washed away any remaining uncertainties you may have had about float in project management.

Conclusion: Understanding float helps you better understand your projects and how to management them

While the Critical Path Method is best known as a schedule algorithm for calculating a project’s earliest conceivable deadline based on task duration, its float system is arguably the best thing about it.

With float, project managers are given objective values by which they can prioritize tasks and stop the project from going off rails.

References

  • Plaky. (n.d.). What is Critical Path Method in project management? Retrieved November 9, 2022, from https://plaky.com/learn/project-management/critical-path-method/
  • Plaky. (n.d.). What is Critical Chain Project Management (CCPM)? Retrieved November 9, 2022, from https://plaky.com/learn/project-management/critical-chain-project-management/
  • PM Vidya. (n.d.). Types of Float – Free Float, Total Float, Project Float. Retrieved November 9, 2022, from https://pmvidya.com/blog/float-types-free-float-total-float-project-float
  • Project Management Academy. (n.d.). Free Float vs Total Float. Retrieved November 9, 2022, from https://projectmanagementacademy.net/resources/blog/free-float-vs-total-float/#pmp-example-questions

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