Total and Free Float are handy metrics to use to track project progress.

What is Float?

Total Float, often referred to as ‘Total Slack’, is the amount of time a task can be delayed without impacting the overall project completion date.

Total Float is often confused with “Free Float”, which is the amount of time a task can be delayed before it impacts its successor activities, but not necessarily the overall project completion date.

The calculation of float stems from the basic scheduling algorithm, the Forward and Backward Pass. When the forward and backward pass is executed during the re-scheduling process, each activity in the plan will have a set of early and late dates calculated.

By taking the difference of a tasks Late Finish and Early Finish or Late Start and Early Start, a tasks total float can be calculated. Typically, if this value is less than or equal to zero, the task is deemed critical.

The calculation of free float however is based on subtracting the Early Finish of the current task from the Early Start of the next successor task.

Why is Float Useful?

The interrogation of Float is an important part of scheduling as it allows a project team to ascertain what the critical and near-critical paths are for a project. The total float calculation enables projects to prioritise work to enable timely delivery of the project.  

By tracking the near-critical path using total float, teams can monitor progress and trends to determine if tasks (which are not critical) are at risk of becoming critical.

Free float is also helpful as it can provide teams with a method by which to determine what tasks can be pushed back without impacting any successor tasks directly.

Example

To demonstrate how float can be of benefit to a project, take the following simple schedule example below:

Float 1

The plan shows the critical path runs through tasks A, D and E. Excluding milestones, all other tasks show as not-critical with total float and free float between 1 and 3 days.

Tasks B and C have the same total float as they do free float. That is because in this instance they are tied to the same successor (D which is critical). Should they be delayed by the same amount of free float as their total float, they will become critical. In essence, both tasks B and C use the same Early Start date from task D to form the calculation of their Free Float.

But if we examine tasks F, G and H, we see that the free float does not necessarily equal the total float value. Putting it simply, if task F is delayed it will impact task G. In turn if task G is delayed it will impact task H, there by making these two tasks have a total float of 0 days. 

The 2 days total float, derived by the logic from tasks H to the Finish milestone therefore dictate the total float of tasks F and G.

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