Definitions of Total Float and Free Float
The two types of float most commonly monitored are total float and free float.27 Total float, the amount of time an activity can be delayed or extended before delay affects the program’s finish date, can be positive, negative, or zero. If positive, it indicates the amount of time that an activity can be delayed without delaying the program’s finish date.28 Negative total float indicates the time that must be recovered so as not to delay the program’s finish date beyond the constrained date. Negative total float arises when an activity’s completion date is constrained—that is, when the constraint date is earlier than an activity’s calculated late finish. In essence, the constraint states that an activity must finish before the date the activity may finish as calculated by network logic.
Negative float can also occur when activities are performed in a sequence that differs from the logic dictated in the network. Out-of-sequence logic is discussed in detail in Best Practice 9. Zero total float means that any amount of activity delay will delay the program’s finish date by an equal amount. An activity with negative or zero total float is considered to be critical.
Free float is the portion of an activity’s total float that is available before the activity’s delay affects its immediate successor. Depending on the sequence of events in the network, an activity with total float may or may not have free float. For example, it may be possible that an activity slips 2 days without affecting the finish date (2 days of total float), but this delay will cause a 2-day slip in the start date of its immediate successor activity (zero free float).
Total float and free float are therefore indicators of a schedule’s flexibility. Some activities in the schedule network can slip without affecting their immediate successors, and some may affect their immediate successors but not the program finish date. Knowing this allows management to reassign resources from activities that can slip to activities that cannot slip. Knowing the length of time an activity can or cannot be delayed is essential to successfully allocating resources and to completing the program on time.
Nonworking periods are not float. Nonworking periods are defined in project and resource calendars and dictate the availability of resources to work, not the flexibility of an activity’s start or finish dates. In addition, float should not be treated as schedule contingency. Because float is shared along a sequence of activities, it is available for use by any activity along that sequence.
Float, calculated from an activity’s early and late dates, is the length of time the activity can be delayed before delaying the early start of its successor or the project finish date. A schedule network may also mathematically calculate independent float and interfering float. Independent float is the amount of time the activity can be delayed without delaying successor activities, given preceding activities have started late. Interfering float is the difference between an activity’s total float and free float.↩︎
Typically, total float is calculated by the scheduling software to the last activity in the schedule file, but other activities or interim milestones may be monitored for total float, using constraints on key interim milestones. With these constraints, using total float to identify the critical path to the finish milestone is a flawed method.↩︎