Class 105: Float

Float, also known as “total float,” is defined as the amount of time an activity can be delayed without impacting the overall project. It can also be defined as the “difference between the pairs of start and finish dates for each task.”¹

If an activity is delayed and directly causes delay to the overall completion of the project, this activity is on the critical path and has zero or negative float. However, if an activity can be delayed 15 days before causing delay to the overall project, this activity has 15 days of float.

Every day that an activity with zero float is delayed, the overall project duration is extended by one day. Conversely, activities with positive float can be delayed by the number of days equal to its float value. As float is absorbed, delays to an activity become closer to impacting the overall project duration. Activities with negative float, a common result of delayed activities tied to a constrained completion milestone, have been delayed beyond what is feasible to meet the finish date of the overall project. 

Example of Float in a Construction Schedule:

In the mixed-use skyscraper example, activities have a range of float values that can be used to determine which activities are most important to completing the project on time. Activity 11103, Install Pit Equipment, has 388 days of float, meaning is can experience delay up to 388 calendar days before impacting the overall project completion date. Activity 34110, Mechanical Rough, has -74 days of float, meaning this activity and others within its sequence of activities are scheduled to occur later than their target dates. Typically, project teams will work to mitigate delays to activities with negative float to minimize the impacts to the project completion date. As seen in this example, analyzing activity float values helps determine which activities have the most significant impact on achieving project completion. 

What is the difference between free float and total float?

Float is typically used interchangeably with the term total float. Total float is the type of float used in the example above. However, there is another important type of float called free float. Free float is defined as the “maximum amount by which an activity can be delayed beyond its early dates without delaying any successor activity beyond its early dates.”¹ Typically, critical activities, or activities with zero total float, will also have zero free float; however, free float and total float values can vary when non-critical activities are sequenced concurrently with other activities. While free float is important to understand, total float is more commonly used when reviewing construction schedules. 

Float Summary:

1. Float informs schedule reviewers of which activities have the most significant impact on achieving the project's intended completion date. 

2. Negative float is how far behind an activity is from its planned early start/finish date. 

Key Terms:

Float (Total Float) – amount of time an activity can be delayed without impacting the overall project

Activity – individual component or step of a construction schedule 

Free Float  – amount of time an activity can be delayed without delaying any successor activity beyond its early dates 

Resources:

1. AACE International Recommended Practice.  Cost Engineering Terminology. https://web.aacei.org/docs/default-source/rps/10s-90.pdf?sfvrsn=58