In project management, total float is the amount of time you can delay an activity before it affects the project’s completion. Total float determines whether an activity is non-critical or critical. Non-critical activities are those with float – meaning you can delay them if necessary. Critical activities are those without float – these activities must start and finish directly on time to complete the project on time.

Activities can also incur negative float. Negative float will cause a delay to the overall completion of the project. It indicates that your project is not on track to finish on time. For example, an activity with -2 days of float shows that it is 2 days behind schedule. If an activity has negative float, you will want to resolve it to ensure that your project completes on time.

In this article, we will look at what negative float is, what causes it, and what you can do to resolve it. The following examples are all from Primavera P6 Professional. However, these concepts are relevant for all projects, regardless of software.

How Do You Calculate Negative Float?

When using the Critical Path Method of scheduling, you can calculate total float using the following formula: Late Date – Early Date = Total Float. You can either use Late Finish date – Early Finish date or Late Start date – Early Start date. If your activity has negative float, its early dates are later than its late dates.

Negative float usually occurs when there’s a change in the original baseline schedule’s logic. Commonly, this will come up if you use constraints or change the project’s finish date. Beyond this, however, there are many scenarios that can cause an activity to incur negative float. In the following examples, we will take a look at a couple situations that may lead to negative float and solutions you can apply to resolve it.

Causes and Solutions

Negative Float from a Constraint on the Project’s Finish Date

Negative float can occur if you place a constraint on the project’s finish date, otherwise known as the Must Finish By date. If your project’s Must Finish By date is earlier than the finish date calculated by its logic, it will incur negative float. Basically, you’re telling the program that you need the project to finish earlier than expected. In return, the program shows you how many days behind you are with negative float.

In the following example, I have a project that has a calculated Finish date of September 29th. Currently, the project and its activities have no negative float.

Let’s say that I received word from our contract that the project requirements have changed. I now need to finish this project by August 27th. In P6, I can make this adjustment on the Projects page, using the Dates tab in the details section. Here, I can use the Must Finish By field to adjust the contractual finish date.

After rescheduling the project, I now have several activities with negative float – as seen from the Total Float column. Additionally, the project as a whole now has -24 days of float. This tells me that this project is 24 days behind hitting the new Must Finish By date. If I don’t make some adjustments, this project will not finish on time.

Solution

With a situation like this, you could remove negative float by compressing the project schedule. Schedule compression is a method of shortening the project’s duration while maintaining its scope. A few common schedule compression techniques include fast tracking and crashing.

Fast Tracking

Fast tracking involves adjusting activity relationships to allow activities to work simultaneously. Focusing on activities currently performing sequentially, you would evaluate whether they can be done in parallel, instead. Making this relationship adjustment will allow activities to start and finish faster. Overall, this would shorten the project’s duration.

In this example, I’ve found a few activities with Finish to Start relationships that can be performed simultaneously. I’ll fast track these activities by adjusting their relationship types to Start to Start. This will allow both activities to start at the same time.

After rescheduling the project, the negative float has dropped down to 19 days from its original 24.

Because the project still has some negative float, I could continue to analyze and fast track activities. I could also try using the crashing method. 

Crashing

Crashing is a schedule compression technique involving adding resources to the project. You could either increase the work durations for existing resources, or add new ones. With more hands working on the task, it might get done faster.

To crash the activities in this project, I’ll add an additional resource to each of them. In order to do this in Primavera P6, I will first need to adjust my Resource Assignment calculation settings. To do this, navigate to the User Preferences window under the Edit menu. In the Calculations tab, select Recalculate the Units, Duration and Units/Time for additional resources to an activity. This will allow the program to adjust activity values as you add additional resources.

I also need to adjust the activities’ Duration Type to Fixed Units/Time. This can be done using the field in the General tab of the Activity Details section.

Now, I’ll add a second resource to several of my activities. After scheduling, the project’s negative float has decreased from 19 days to 12 days. I could now continue using both fast tracking and crashing to mitigate the rest.

Negative Float From Activity Constraints

Similarly, negative float can arise when activities are using constraints. Constraints will set a date that the activity can start or finish, and they may override the project’s logic.

In the following example, I need to finish the Install SWPPP activity no later than January 19th. To show this in Primavera P6, I’ll add a constraint to the activity using the Status tab in the details section. I’ll give this activity a primary constraint of Finish On or Before with a constraint date of January 19th.

After rescheduling the project, this activity now has a 4 days of negative float. This shows that the activity is 4 days behind hitting the set constraint date. Unless I remove this negative float, this activity – and the project as a whole – will not finish on time.

Solution

With a constrained activity, you could try to rearrange the project’s sequence to finish the activity earlier.

In this case, the Install SWPPP activity is set to occur after the Construction Area Signs activity. We could try to fast track these activities – but in this case, they both use the same resource crew. However, it is not imperative to have Install SWPPP start after Construction Area Signs. We will be installing the SWPPP away from the main road, so we don’t necessarily need the signs in place prior. Instead, the construction area signs will just need to be up before construction begins. To resolve this negative float, I could switch the sequence of these two activities. Now, Install SWPPP will act as the predecessor with Construction Area Signs as its successor.

With Install SWPPP starting first, it can now finish by its constrained finish date. This will remove all negative float from the activity.

Negative Float on Summary Activities

In rare circumstances, you might gain negative float on Level of Effort or WBS Summary activities. This can occur even if you haven’t used any constraints or adjusted the Must Finish By date.

Rather than the result of your actions, this negative float can arise from how P6 is calculating Total Float. You’ll only see the effect of this on summary activities, such as Level of Effort and WBS Summary.

Solution

To remove negative float from summary activities, you’ll need to adjust the P6 Schedule Options. You can do this by selecting Tools > Schedule > Schedule Options.

In the Schedule Options window, you can adjust how the program calculates Total Float. There are three options available:

• Start Float, which is the difference between the early and late start dates (Start Float = Late Start – Early Start)  
• Finish Float, which is the difference between the early and late finish dates (Finish Float = Late Finish – Early Finish)  
• Smallest of Start Float and Finish Float, which is the most critical float value

By default, Primavera P6 will calculate float using Finish Float = Late Finish – Early Finish. However, if this option is ever adjusted, your summary activities might gain negative float. If you are using Level of Effort or WBS Summary activities, you should make sure that the P6 is calculating Total Float with the Finish Float option. This will resolve any negative float that may occur with these activities.

Negative Float from External Relationships

Negative float may also occur if you use external relationships. This is when you have an activity in one project linked to another activity within a separate project. External relationships create logical ties between two projects, which can lead to issues.

In the following example, I have tied my milestone activity, Notice to Proceed, to an activity within another project.

The predecessor activity from the other project has a constraint of Finish On or Before January 7th. Because Notice to Proceed will follow this constrained activity, it has incurred -7 days of float.

However, because the constrained activity is in another project, this issue may not be immediately apparent. Schedulers might be confused about the cause of this negative float, unless they have both projects open.

Solution

To fix this issue, you should remove or rearrange the external relationship. Luckily, you can quickly do this by adjusting the schedule options.

Once again, you can open the Schedule Options window by selecting Tools > Schedule > Options.

At the top of the window, there is an option to Ignore relationships to and from other projects. This option only works when the other project is not open. Ticking this box will remove the external relationship between the two activities.

After rescheduling the project, Notice to Proceed no longer has negative float.

In most situations, however, this would be a temporary solution at best. Most likely, the external relationship was there for a reason. You may need to come back and address why this relationship was there in the first place, instead of simply removing it.

Negative Float from Custom Work Calendars

Lastly, your project could incur negative float when using specific custom work calendars.

In the following example, I have a milestone activity called Board Meeting. This activity represents one of several board meetings set to occur every 4th Monday of the month. To model this in the schedule, I’ve created a calendar that only has workdays on every 4th Monday and set it to the Board Meeting activity.

Board Meeting is a Finish Milestone – due to this and its assigned calendar, the predecessor activities have been affected. The predecessors, Temp Utilities and Furnish Trailers, now have late dates that are earlier than their early dates – leading to negative float.

Solution

This situation has an easy solution. In this case, we can change Board Meeting’s activity type to Start Milestone. This will keep the activity’s duration the same, but adjust its date to a start date, rather than a finish date.

After rescheduling the project, the late dates of the predecessor activities are now later than the early dates. As one would expect, the negative float disappears!

Summary

In project management, negative float indicates that the project will not finish on time. This is a schedule issue that can arise from a variety of different reasons. As seen from the examples above, negative float can come from the use of constraints, external relationships, or even specific schedule settings. If your project has negative float, you will most likely want to resolve it to ensure that your project finishes on time.

There are a variety of methods to remove negative float. In most cases, you can remove it by compressing, or shortening, the schedule. Sometimes, adjusting the schedule options or changing the activity types can help as well. Regardless of the situation, you’ll want to try and remove all negative float to ensure that your project can finish on time.

If you have any comments, questions or suggestions, please use the comment section on the bottom of this page. Don’t forget to subscribe to our blog to get more Primavera P6 tips & tricks sent directly to your inbox!

Lauren Hecker is the instructor for our Primavera P6 Professional training courses. To see her next open enrollment course, please visit our calendar. To schedule an onsite or custom course, please contact us!