Free Float VS Total Float

The difference between free float and total float comes down to scope. Total float is how long an activity can be delayed before it pushes the project’s completion date. Free float is how long the same activity can be delayed before it delays the early start of its immediate successor. Total float is always greater than or equal to free float, and the gap between the two is interfering float.

Both values come from the schedule’s logic, and both tell you where a schedule has room to move. This article covers how float is calculated, how free float and total float differ, and how to read them in a Primavera P6 schedule.

Understanding float begins with understanding how schedules are built and how relationships and constraints drive the calculations behind the scenes. A planner first develops a baseline schedule with activities and logical relationships that determine the sequence of work.

These logical relationships can be mandatory or discretionary dependencies. Mandatory, or hard logic, dependencies are typically tied to physical or contractual constraints, such as completing foundation work before starting structural framing. Discretionary, or soft logic, dependencies are based on planning decisions, resource availability, or preferred execution strategies.

That sequencing determines how work progresses from start to finish. Once the logical relationships are applied to the schedule, float emerges. How much float an activity has depends on its relationship types, the presence of lags or leads, and whether it has multiple predecessors or successors.

What Is Float in Project Scheduling?

Float is the amount of time an activity can be delayed without negatively impacting the project timeline. It results directly from how activities are linked, and it is influenced by logic, durations, and constraints.

Because float reflects schedule flexibility, it often becomes the most contested element in delay analysis and claims management.

Several float types exist, such as total float, free float, and negative float.

Float (most commonly total and free float) is calculated using four fundamental schedule parameters:

• Earliest Start (ES): The earliest an activity can begin.
• Earliest Finish (EF): The earliest an activity can finish. (EF = ES + Duration)
• Latest Finish (LF): The latest an activity can finish.
• Latest Start (LS): The latest an activity can start without delaying the project. (LS = LF − Duration)

What Is Total Float (Slack)?

Total float, also called slack, represents the amount of time an activity can be delayed without pushing back the project’s completion date.

Let’s look at an example where we have Activity A that runs for 5 days, followed by Activity B (4 days). Activity C starts in parallel with Activity A and runs for 12 days. The project finishes at Day 12 with Activity C ending.

Calculation: Total float = LF (late finish) − EF (early finish), or LS (late start) − ES (early start).

In our example we can delay the non-critical activities A and B by up to 3 days (their total float) without delaying the project end date.

Think of it like this: on a tight-deadline build, total float gives you breathing room across the entire sequence. Activities with zero total float sit on the critical path, meaning any slip here will cause the project to delay.

In P6, high total float is an alarm for potential issues. DCMA 14-point checks are commonly used to identify high float and negative float activities in the schedule.

What Is Free Float?

Free float measures how much an activity can slip before delaying its immediate successor’s early start date.

Calculation: Early Start of successor − Early Finish of activity.

For instance: we have two activities, the first pouring concrete (Activity A, 5 days) and the second framing (Activity B, 10 days). Activity B follows A.

If B has another predecessor finishing later, A might have 3 days of free float. Delay A by 2 days and B still starts on time. But exceed 3 days, and B shifts.

These distinctions matter in Primavera P6 free float vs total float views. When looking at what is critical, free float and total float should be checked together, as zero free float alone doesn’t always mean an activity is critical.

How Free Float and Total Float Relate

The two values are not independent, and a few rules connecting them answer most everyday questions:

• Total float is always greater than or equal to free float, so free float can never exceed total float.
• The difference between them is interfering float: Interfering float = Total float − Free float. It is the delay an activity can absorb that would push its successors but still not delay the project.
• On the critical path, both free float and total float are zero.
• An activity can have free float of zero and still have total float above zero. It would delay its successor if it slips, but the successor carries its own float, so the project finish stays safe.

	Free float	Total float
Measured against	The immediate successor’s early start	The project completion date
Formula	ES of successor − EF of activity	LS − ES, or LF − EF
Range	0 up to total float	Negative (project behind) or above
Reaches zero when	The next activity starts to slip	The project finish date starts to slip
On the critical path	0	0

A related measure is project float: the time a project can be delayed beyond its planned finish without breaching an imposed deadline. Unlike free and total float, it applies to the project rather than a single activity.

Critical Calculation Adjustments

Note: The simplified formulas presented above apply only to Finish-to-Start (FS) relationships with a single successor (a 1:1 relationship). In real-world schedules, this is rarely the case.

If you have Start-to-Start (SS) or Finish-to-Finish (FF) relationships, lags or leads applied, or multiple predecessors or successors, then the formulas must be adjusted accordingly. Ignoring this can lead to incorrect interpretation of float values.

Here’s where things get more realistic and more complex.

Different Relationship Types (SS, FF, SF)

The basic free float formula changes depending on relationship type:

• For Start-to-Start (SS): float is based on successor ES vs predecessor ES
• For Finish-to-Finish (FF): float is based on successor EF vs predecessor EF
• For Start-to-Finish (SF): rarely used, but again requires a different basis

Lags and Leads Must Be Included

If a lag or lead exists, it must be included in the calculation.

For example (FS with lag): Free Float = ES(successor) − EF(activity) − Lag

Ignoring a lag can significantly overstate available float. This is a common mistake in manual reviews.

Multiple Successors (Free Float Rule)

When an activity has multiple successors, free float is not calculated against just one.

Instead: Free Float = the minimum value across all successor relationships.

In other words, you include the most restrictive successor, the one that allows the least delay, so none of the successors are delayed.

Multiple Predecessors (Impact on Successors)

Similarly, when analyzing successor activities, multiple predecessors influence their early start. The successor’s ES is controlled by the latest-finishing predecessor, which indirectly affects float calculations upstream.

Why Free Float and Total Float Matter

Float is more than just a number. It’s a decision-making tool.

In construction project management, float is your buffer against weather, supply snags, or labor shortages. Total float keeps the owner happy by protecting milestones, while free float lets foremen flex without scheduler alerts.

Poor float management leads to negative values, which are red flags in P6, so low float paths often need priority resources. High float, on the other hand, might signal dangling logic or over-optimism.

Float also helps in lookahead schedules, allowing planners to spot successor risks.

Analyzing Float in Primavera P6 Schedules

ScheduleReader provides an efficient way to analyze float in Primavera P6 schedules. The software visualizes the information from project files exported in the .xer and .xml file formats, making float information available to the user right away.

There are several ways to audit float in ScheduleReader, including table columns, the Gantt chart, filters, and the project reports that include a DCMA 14 schedule analysis.

Audit Float Using Table Columns

The most direct way to review float is through the activity table view. Simply add the columns for Total Float and Free Float.

Zero float, negative float and high float values become instantly and clearly detectable, allowing you to spot delayed and critical activities and possible logic issues.

Visualize Float in the Gantt Chart

A visual review provides more context for your audit, and sometimes even allows you to get insights that are hard to detect in tables. The Gantt chart is instantly available in the Activities View on your right side of the application. To see the float, you can enable the float bar in the Bars settings of the Gantt chart.

In addition to the Gantt chart, the Bottom view and its Status window can additionally help you see the total and free float of a selected activity.

Filters

Filters help you focus on specific float conditions. You can use predefined filters (negative float and total float) or create custom ones to cut through the noise and quickly find the needed information.

Run a DCMA 14-Point Analysis – Project Reports

For a more formal and in-depth float audit, you can use the built-in project reports, available in the PRO version of ScheduleReader. The DCMA 14 analysis provides an industry-recognized benchmark for schedule quality, making it especially useful for reporting and internal quality assurance.

The report is instantly available with a click of a button and provides a comprehensive overview of high float (> 44 days) and negative float activities within a schedule.

Data about specific activities can be copied to Excel, and additionally, users can modify the reporting audit threshold and insert their own project-specific criteria limits. Using the formatting functionality, they can change the range criteria for all fourteen metrics.

Conclusion

Grasping free float vs total float transforms P6 scheduling from reactive to proactive. Total float guards the big picture while free float fine-tunes daily execution.

Together, they help teams understand where there is room to move, and where there isn’t.

Frequently asked questions

What is the difference between free float and total float?

Total float is how long an activity can be delayed before the project’s completion date moves. Free float is how long it can be delayed before its immediate successor’s early start moves. Total float measures against the project finish; free float measures against the next activity.

Can free float be greater than total float?

No. Total float is always greater than or equal to free float, so free float can never exceed total float on the same activity.

What is interfering float? Interfering float is the difference between total float and free float. It is the delay an activity can absorb that would push its successors but still not delay the project finish.

Can total float be negative?

Yes. Negative total float means the project is already behind its deadline under the current logic and durations. It is a red flag in P6 and usually calls for adjusting durations, dependencies, or resources.

Do critical path activities have float?

No. Activities on the critical path have both free float and total float of zero, which is why any delay to them delays the project.