What is Free Float, Total Float, and Lag in CPM

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Float is a term that often appears in Critical Path Method (CPM) calculations in project scheduling.

Simply, a Float can be said as a number of duration between activities that constitute a tolerance limit so that another activity schedule or project schedule is not affected (delay).

Total Float

Total Float is a time delay that can still be used or occur in an activity without causing delays in the overall project.

For activities that are on the critical path, it should have a total float of zero.

What about the total Float on the project?

The total Float on a project is marked by the activity that ends a project.

This activity should be an activity that is on the critical path, so it can be said the total Float on a project should be zero.

Free Float

Free Float is a time delay that can still occur or be used in an activity without causing delays in the next activity.

What is the ideal amount of free Float and Total Float in a schedule?

According to DoD rules, the maximum number of floats should be no more than 44 working days.

What is Negative Float?

A project that has a negative float can be said to have a project schedule that is not realistic.

When your project has a constraint, for example, the project must be completed on a certain schedule and your project turns out behind schedule (delay occurs), then it is not possible to be on time, this can cause the float to become negative.

The negative float amount states the amount of time needed to catch up with the schedule to finish on time. Negative Float is a sign that your schedule is not realistic.

Also, read Project scheduling terms in Primavera, What is Project Management.

What’s the difference between Free Float and Lag?

Before reviewing the differences, let us first understand the respective meanings in their use in project scheduling.

Lag is a certain amount of time between two different activities that indicates the time difference between the start or end of activity with the start or completion of another activity.

Example:

Activity B will begin after the completion of activity A plus ten days (for some reason). So this ten day time is a lag that must be met before the start of activity B.

From the above understanding between Lag and Float, we certainly can understand the difference.

For example, there is a free float of 10 days between A and B; this means activity A has a tolerance of delay of 10 days, so activity B is not late.

Whereas in the ten-day Lag above, if activity A is late, then the start of activity B is still calculated from the time of completion plus ten days of Lag between the two activities.

In the project schedule, it is best to use this Lag carefully. Don’t use Lag to replace activities. According to the rules adopted from the DoD (Department of Defense), the use of this Lag should be no more than five working days.

For example, you make a work schedule between two activities as follows:

1. ABC Design.

2. Built ABC.

Between the two activities above, you give a Lag time of 15 days for approval. If you see the DoD rules, this schedule has been more than five days, so it can be said to be not good. You can replace the lag time with the Review and Approve Design activity, so the sequence becomes:

1. ABC Design.

2. Review and Approve Design ABC.

3. Built ABC.

In addition to the above, the use of Lag is recommended by DoD of no more than 5% of all activities.

On Oracle Primavera P6 you can see the number of floats by doing the following steps:

Free-Total-Float

You can also see the amount of flag duration by following these steps:

Activity-Lag
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