Critical path vs longest path

Every owner asks the same question of a contractor's schedule: what is driving completion? Oracle Primavera P6 gives two answers, and they are not always the same. One flags every activity whose total float is at or below a threshold. The other traces the chain of driving relationships back from the project's last finish. In a simple schedule they agree. In a real one, with constraints, several calendars and a few open ends, they can disagree on dozens of activities. This lesson explains both methods, why they differ, and which one an owner should ask for.

Learning outcomes

• Explain the two ways P6 defines critical activities: a total float threshold and the longest path.
• Describe four situations where the two methods give different answers.
• Display the Critical and Longest Path columns, the longest path filter and multiple float paths in P6 Professional.
• Write a critical path requirement into an owner's schedule specification and check it.

The lesson

Critical is a flag, and you choose the rule

In P6, "critical" is a checkbox on each activity that P6 sets every time it schedules. Oracle's column description says an activity is critical when its total float is below the critical duration set for the project, and that delaying critical activities delays the project finish [1]. Which rule P6 uses is set in Tools, Schedule, Options, General tab, under Define critical activities as [2]:

• Total Float less than or equal to a value you type, such as 0d. P6 converts the value to hours using the hours-per-period settings or the default project calendar, depending on an administrator preference. A number typed without a unit is read as hours [2].
• Longest Path. P6 finds the activities whose early finish equals the project's latest calculated early finish, then traces all driving relationships back to the project start [2].

With several projects open, the default project's setting is used [2]. In P6 EPPM the same choice is on the Settings tab of the scheduling options, reached from the Schedule button on the Activities page [3]. Critical activities are drawn as red bars in the Gantt chart by default [3].

Why the two answers differ

Oracle is candid about this: either method can flag activities that are not truly critical to the project end date, such as open-ended or constrained activities [2]. In practice, four things pull them apart (Figure 1):

• Constraints. A finish constraint set tight to an activity's early finish gives that activity and its predecessors zero, or negative, total float. Under the total float rule they turn red, even if they have nothing to do with the project finish [4][8]. The longest path ignores them unless they actually drive the end.
• Several calendars. Float is counted on each activity's own calendar [5]. An activity on a seven-day calendar that finishes on a Friday and drives a Monday start on a five-day calendar has two days of total float, Saturday and Sunday, yet it is driving. The total float rule misses it; the longest path does not.
• Open ends. An activity with no successor takes its late dates from the project finish, so its float is meaningless. If Make open-ended activities critical is marked in Schedule Options, P6 flags it critical anyway [2].
• Level of Effort. An LOE activity can sit on the longest path without being critical, because its dates come from the work it spans [4].

View the data

Activity	Total float	TF ≤ 0 rule	Longest path	Drives M9?
Roof deck & membrane	0d	Critical	On path	Yes
Panel cure, 7-day calendar	2d	Not critical	On path	Yes
Hydro service to M7 (tight constraint)	0d	Critical	Not on path	No
Dock leveller submittal (open end)	Large	Critical only if option marked	Not on path	No
Site supervision (LOE)	n/a	Not critical	Can be on path	No

Showing the longest path in P6 Professional

Add the Critical and Longest Path columns to the Activity Table; both are in the General category. Oracle describes the Longest Path field as identifying activities in the sequence of driving activities that determines the project end date [1]. P6 also ships a Longest Path filter, which shows the incomplete activities on the longest path [4]. Seeing both columns side by side is the quickest way to find the disagreements in Figure 1.

Multiple float paths

The longest path shows one chain. Owners also want to know what comes next. On the Advanced tab of Schedule Options, mark Calculate multiple float paths, choose Total Float or Free Float, pick the activity the paths should end on (usually a contract milestone) and set the number of paths. Oracle suggests 10 or fewer for performance [6]. The Free Float method gives a first path that matches the longest path; the Total Float method follows the relationships with the most critical total float [6][7]. P6 numbers the most critical path 1 and the rest 2, 3 and so on. Group by Float Path and sort by Float Path Order to read them [3][7].

Screenshot to come

Northgate worked example

Northgate Distribution Centre (fictional) is a 12,000 m² tilt-up warehouse in Vaughan, Ontario, built by Halvorsen Builders Ltd. (fictional) for Northgate Logistics Inc. (fictional) under CCDC 2-2020 (see the Northgate fact sheet). The contractual completion target is M9, Ready-for-Takeover and Substantial Performance, on Fri Jan 14, 2028. Northgate's schedule specification asks Halvorsen to define critical activities by longest path. As we saw in Lesson 10, the owner's scheduler confirms that setting in the schedule log before looking at the path itself.

Two lists that should match, but don't

To see what a total float rule would have shown, the scheduler schedules a copy of NDC-BL00 twice: once with Total Float less than or equal to 0d and once with Longest Path, with the Critical and Longest Path columns displayed. The illustrative results:

• Both rules agree on the main chain: footings, slab, panel casting and erection, roof deck and membrane, M6 roof complete (Fri Sep 3, 2027), office mezzanine interiors, commissioning and M9.
• Total float flags extra work. Halvorsen put a Finish On or Before constraint on M7 permanent power at Fri Oct 1, 2027, equal to its early finish. The hydro service activities leading to M7 show zero float and turn red. They are not on the longest path: commissioning starts are driven by the interiors.
• Total float misses one driver. In this version of the file, Halvorsen had modelled the panel cure as an activity on a seven-day calendar instead of the lag used in Lesson 9. It finishes on a Friday and drives Monday's erection start, so it shows two days of float and is not flagged. The longest path includes it.

Figure 2 shows the shape of the network. The owner's conclusion is not that one list is wrong. It is that a critical path defined by a float threshold, in a file with constraints and mixed calendars, needs a second look before anyone relies on it for a delay discussion.

Ranking the near-critical paths

Next, the scheduler turns on multiple float paths using Free Float, ending on M9, and asks for five paths. Grouped by Float Path, the result reads like a risk register: path 1 is the longest path above, path 2 runs through the office mechanical rough-in with a few days of float, path 3 through the dock doors and levellers with more (Figure 3). Those are the paths the owner watches at every update, because a slip on path 2 can become path 1 within a month.

View the data

Float path	Main work	Smallest total float
1	Structure, roof, interiors, commissioning	0d
2	Office mechanical rough-in	4d
3	Dock doors and levellers	9d
4	Site paving	15d
5	Exterior lighting	22d

Screenshot to come

Screenshot to come

Owner's move

• Specify longest path, and say why. Ask for critical activities to be defined by longest path, with a float path report to the substantial performance milestone at every update.
• Ask for both columns. Critical and Longest Path side by side make disagreements visible in seconds.
• Treat red bars from constraints as a question. If an activity is red only because of a constraint, ask what the constraint protects.
• Keep Make open-ended activities critical off, and fix open ends instead. The option hides the defect behind a red bar.
• Watch path 2. Track the float on the first two or three float paths over time. The critical path changes when near-critical paths catch up.

Common mistakes

1. Calling every red bar "the critical path". Under a float threshold, red means low float, which can come from constraints or calendars.
2. Using a positive threshold to "show more". Total float less than or equal to 10d is a near-critical filter, not a critical path.
3. Typing a threshold without a unit. A bare number is read as hours, so 5 means five hours, not five days [2].
4. Ignoring calendar effects. A driving activity on a seven-day calendar can show float. Check the Longest Path column, not only Total Float.
5. Taking the longest path on trust. It follows driving relationships, so bad logic gives a bad path. Check the logic first (Lesson 9).

Quick quiz

1. With Longest Path selected, P6 starts from:

   1. The activity with the least total float
   2. Activities whose early finish equals the project's latest early finish
   3. The project start date
   4. The data date

2. Typing 5 without a unit in the Total Float threshold means:

   1. 5 days
   2. 5 weeks
   3. 5 hours
   4. 5 per cent

3. An activity with a tight Finish On or Before constraint shows 0d total float but does not drive M9. Under a TF ≤ 0 rule it is:

   1. Critical
   2. Not critical
   3. On the longest path
   4. Ignored

4. Using the Free Float method for multiple float paths, path 1 matches:

   1. The baseline critical path
   2. The longest path
   3. All activities with zero free float
   4. The WBS order

5. Northgate's panel cure on a seven-day calendar drives a Monday erection start and shows 2d of total float. Which rule flags it?

   1. Total float ≤ 0 only
   2. Longest path only
   3. Both
   4. Neither

Show answers and explanations

1. B. P6 finds activities whose early finish equals the project's latest calculated early finish, then traces driving relationships back [2].
2. C. A value without a unit is displayed in hours [2].
3. A. The rule only looks at float, so a constraint-driven zero is flagged critical [2][4].
4. B. The Free Float method gives a most critical path matching the longest path [6].
5. B. It is on the driving chain but its float, counted on its own calendar, is above zero.

Key terms

Critical activity

An activity P6 flags as critical under the rule chosen in Schedule Options.

Longest path

The chain of driving relationships from the project start to the activity with the latest early finish.

Total float threshold

The float value at or below which P6 flags an activity critical when the total float rule is used.

Open-ended activity

An activity with no predecessor or no successor.

Float path

A numbered chain of activities from the multiple float path calculation; path 1 is the most critical.

Near-critical path

A path with small positive float that could become critical if it slips.

References

1. Oracle. General Columns of the Activity Table. Primavera P6 EPPM Help, Version 26.
2. Oracle. General tab - Schedule Options dialog box. Primavera P6 Professional Help, Version 26.
3. Oracle. Working with Critical Activities. Primavera P6 EPPM Help, Version 26.
4. Ten Six Consulting. Identifying the longest path in Primavera P6. September 10, 2018.
5. Oracle. Scheduling projects. Primavera P6 Professional User Guide, Version 26.
6. Oracle. Calculate multiple float paths. Primavera P6 Professional User Guide, Version 26.
7. Oracle. Advanced tab - Schedule Options dialog box. Primavera P6 Professional Help, Version 26.
8. Ten Six Consulting. What's the difference between total float and the longest path? August 28, 2019.

Verified on November 6, 2026.

Next lesson in this trackTotal float and free float