Free Float vs Total Float
Until Project Managers can control time itself, they can benefit from the use of “float” in project schedules to better manage how activity duration impacts the project completion date. Within project management, there are different types of float: free float, total float, and project float. It’s important to understand the similarities and differences between free float vs total float for the PMP certification exam, which we will outline later in the article.
The Project Management Institute (PMI)’s Project Management Professional (PMP®) certification exam may include questions that require the use of a float formula or interpreting scheduling diagrams to calculate float. Project Managers need to understand the concept and application of float to adjust to changing project needs or influence a project finishing on schedule.
PMP® Exam Formula Cheat Sheet
Learn how to successfully use project management formulas after reading this cheat sheet.
Know Your Float
In project management, float is the number of days you can delay a task without causing problems with the following tasks or the total completion date of the project. Sometimes referred to as “slack,” float connects with project scheduling, schedule network diagrams, and the critical path of a project. Each has a corresponding formula, and the PMP® credential holder must understand how each works in support of scheduling work.
In preparing for the PMP certification exam, you should be sure to include the definition of float types, the formulas for float types, the scheduling concepts connected to float, and the concept of “free float vs total float PMP” in your study materials. Project schedule work, including free float and total float, requires knowledge of the schedule diagram and the terms: ES/early start, LS/late start, AD/activity duration, EF/early finish, and LF/late finish.
Understanding and Using Free Float
Free float is how much time is available in the schedule without risk before impacting other project activities. In laymen’s terms, how long can there be a delay before there is a problem (including costs or delays on the overall effort). Free float is only present when two or more activities share a common successor, which on a network diagram appears as a convergence.
Free Float Defined
Free float only applies to activities not on the critical path; in other words, the delay is “free” of impact on other activities.
The amount of time a schedule activity can be delayed without delaying the early start date of any successor or violating a schedule constraint
– Source PMI.org Lexicon https://www.pmi.org/pmbok-guide-standards/lexicon
Free float is about flexibility. Project Managers and those working to earn a PMP® certification know one core responsibility is knowing when the work will be done. Float is part of the network diagram used to map out activity duration.
Free Float Formula
Fortunately, there is not a complicated sequence of mathematical computations needed to determine free float. A simple subtraction of two key figures determines free float:
Free float is part of critical path methodology work and as such, only occurs with activities not critical to reaching the final date.
Free Float Example
Consider this example from the PMI.org white paper Understanding the basics of CPM calculations:
The schedule can be calculated manually or using software. Regardless, the project manager needs to know how float is determined so any errors can be identified, and stakeholder questions can be accurately addressed. Note that after calculation of activity float values, the critical activities are identified as the Critical Path. Critical activities are those without float.
Understanding and Using Total Float
Informally called “total float PMP,” the calculation for the overall path duration happens when within a network diagram there are multiple paths to completion. The PMI’s A Guide to Project Management Body of Knowledge (PMBOK® Guide) uses day one of the project as one. As with free float, total float is within the critical path scheduling technique including network diagrams.
Total Float Defined
Those with a PMP® certification, know total float is looking at the impact a delayed activity can have on the overall project duration. It is shared among all activities in a sequence.
The amount of time a schedule activity can be delayed or extended from its early start date without delaying the project finish date or violating a schedule constraint.
– Source PMI.org Lexicon https://www.pmi.org/pmbok-guide-standards/lexicon
It can be a little confusing in that total float may be referred to as simply “float” or “slack.” For the PMP® certification exam, use the official PMI® definition and formula for total float when asked about float.
Total Float Formula
There are two formulas to choose to calculate total slack. For both, it is a matter of a simple subtraction using the data already within the network diagram.
The total float for a project is important for multiple reasons as it shows the flexibility within a schedule. Note these considerations for total float:
- an activity’s Total Float can either be greater than or equal to Free Float
- Free Float can never be larger than its value for Total Float
- Each activity has its own total float value
- Depending on the logic assigned to each activity, activities in a sequential string of logic have the same amount of total float
- If the total float is negative, the project cannot be finished at the desired time without changing the logic or durations
Total Float Example
Here is a network diagram with multiple paths to completion provided by izenbridge.com:
From the provided diagram, the project’s total float can be determined in addition to information about free float:
- There are 3 paths to completion: ACE, BCE and BDE.
- ACE will be the critical path with total float 0 and the critical path length 18
- Activities A, C and E have free float of 0 (no kind of float/flexibility for critical path activities)
- Total float for B is 1 (LF-EF OR LS-ES) and activity D is 6
Always note the different ways activities can be sequenced. In the provided example, a quick glance may indicate only two paths to completion. That would ignore the third possible path at activity B in which rather than moving to D it moves up to C. Review the diagram carefully to ensure a complete understanding of all potential paths.
Studying for the PMP Exam?
Understanding and Using Project Float
Within project schedule work, there is a third float concept known as “project float” which looks at the overall project impact of delays. Currently, project float is not included in the PMI’s online lexicon yet can be found in other project reference sources.
Project Float Defined
Interestingly, project float connects to the commitment of the project manager for a specific completion target. Whereas free and total float are about the time an activity can be delayed, project float is the time of delay based on imposed deadlines.
Amount of time a project can be delayed without impacting an imposed deadline or commitment by the project manager
Depending on the work environment, a project may have an imposed deadline to be complete before a holiday or to align with the sales team’s new marketing campaign.
Project Float Formula
Project float is determined by subtracting the planned finish date from the imposed deadline. And with it being an imposed deadline, it may be something that changes outside of the project.
For example, a CEO may say the project deadline is December 15th, to occur before employees leave for a year-end holiday. However, the CFO needs to close out the financials for the year and thus imposes a deadline of December 1st, which the CEO approves. This imposed deadline is not determined within the project itself but is imposed on the project by outside forces.
Comparing Float: Free Float vs Total Float
No discussion of float is complete without a comparison of free float vs total float for the PMP exam. Keep these in mind from the PMI white paper Critical Path Method Calculations and Deep Fried Brain project blog:
In preparing for the PMP certification exam, understand the use, connection, and differences of free float vs total float.
PMP® Certification Exam Float Question Examples
|You are in the process of executing your project and need to know when it will be completed. You have the following activity durations on your project: A=8, B=14, C=6, D=6, E=12, F=6. You also know that D is dependent on A and B. Task F is a successor of D and E. Task C is a predecessor of task E. Based on this information what is the critical path and what is the float of task A?||Critical path is BDF and the float of task A is 6||Critical path is ADF and the float of task A is 0||Critical path is CEF and the float of task A is 0||Critical path is BDF and the float of task A is 2|
|An externally imposed project completion time sets the project duration at 75 days. The critical path of the project is established as 75 days, with a standard deviation of 3 days. What could be the maximum project float available?||Six days||Zero days||Three days||Cannot be established|
|The sponsor wants the project completed in 40 days. The CPI is 1.1 and the project critical path duration is 38 days with a standard deviation of two days. What is the maximum project float?||Eight days||Two days||Zero days||Four days|
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- A. For this question you need to draw out the schedule network diagram based on the information provided. You would have the following paths: A to D, B to D, C to E, D to F and E to F. You would then need to use the durations provided to do a forward and backwards path analysis. The end result is that the critical path is BDF and the float of task A is 6.
- C. Project float is the difference between an externally-imposed completion period and estimated project duration, which is equal to the length of the critical path. The difference is zero. Add to this the standard deviation of 3 days (implying that the critical path could be +/- 3 days from the estimated one). The maximum float could be 0+3 = 3 days.
- D. Project float is defined as the variance between the critical path duration (38 days) and the sponsor imposed duration (40 days). However, in this case we have standard deviation as an additional consideration. Since the standard deviation is +/- 2 days our critical path duration can actually be between 36 and 40 days (i.e. 38 +/- 2). Knowing we could have a critical path of 36 days against the imposed sponsor constraint date (40 days), we therefore have a maximum project float = 4 days.
Project schedules vary in complexity and in what can impact the final duration. Project managers need to understand float within the Critical Path Methodology for the PMP® certification exam and as a means to better manage a schedule.