Critical path and total slack

The critical path is the chain of tasks that sets the finish date. Delay any of them and the project finishes late. Total slack, which Microsoft Project uses where other tools say total float, is the measure behind it: how far a task can slip before it moves the finish. Owners use both every month, to see which contractor delays matter and which ones the schedule can absorb. This lesson explains how Project calculates slack, how to show the critical path in Project desktop, and how to read slack on Northgate's foundations work without being misled by it.

Learning outcomes

• Explain early and late dates and how Project derives total slack and free slack from them.
• Show, filter and group critical tasks in Microsoft Project desktop, and set the critical-slack threshold.
• Calculate the effect of a delay on a task with positive slack.
• List the settings and modelling choices that make a Project critical path untrustworthy.

The lesson

Early dates, late dates

Project schedules every auto-scheduled task twice. The forward pass starts at the project start and pushes through the links to find each task's Early Start and Early Finish, the soonest the logic allows. The backward pass starts at the project finish and works back to find each task's Late Start and Late Finish, the latest it can happen without moving that finish. The Start and Finish columns you see are normally the early dates.

The gap between the two passes is slack. Microsoft defines Total Slack as the smaller of Late Finish minus Early Finish and Late Start minus Early Start [1]. Positive total slack is time a task can slip without delaying the project finish; negative slack means there is not enough time scheduled for the task, and Microsoft notes it is usually caused by constraint dates [1]. Project can also draw it as a bar through the Negative Slack field in Bar Styles [5].

Free Slack is narrower: the time a task can be delayed without delaying any successor task [2]. For a task with no successors, free slack equals total slack [2]. Total slack can be shared along a chain; free slack belongs to one task. If two tasks in a row share five days of total slack and the first one uses them, the second has none left.

Which tasks Project calls critical

A task is critical when its total slack is at or below a threshold. Microsoft's slack article puts it simply: a task with zero slack is critical [3]. The threshold is set at File > Options > Advanced, in the "Calculation options for this project" area, in the box Tasks are critical if slack is less than or equal to [4]. It is 0 days by default. Raising it to, say, 5 days marks near-critical tasks as critical too, which is useful for a risk view but must be disclosed, or readers will think the path is longer than it is.

On the same Advanced page, Calculate multiple critical paths changes the backward pass so that each chain is measured against its own end rather than the project finish [4]. Leave it off for the owner's master unless you have a reason; it produces several "critical" chains, and only one of them sets the finish.

Showing it on screen

• Critical bars. In the Gantt Chart, on the Gantt Chart Format tab, check Critical Tasks; critical bars turn red [4].
• Filter or group. View > Filter > Critical shows only critical tasks; View > Group By > Critical splits the list into critical and non-critical [4]. In other task views, View > Highlight > Critical highlights them [4].
• Slack numbers. Choose View > Other Views > More Views > Detail Gantt, then View > Tables > Schedule; the Free Slack and Total Slack columns are at the right of the table [3]. The Detail Gantt also draws slack as a thin line after each bar.
• Master projects. With subprojects inserted, File > Options > Schedule, Inserted projects are calculated like summary tasks, lets you see whether whole subprojects fall on the critical path [4].

In the Network Diagram, critical tasks are red automatically [4]. The figures in this lesson use orange for critical tasks to match BRC's palette; on screen they will be red.

Slack is a total-slack test, not a guarantee

Project marks a task critical because of a number, not because it traced the longest chain. Anything that distorts the late dates distorts the critical path: manual tasks whose dates do not respond to logic (Lesson 10), tasks with no successor, which get their late finish from the project finish and look as if they have huge slack, and constraints or deadlines that pull late dates in (Lesson 12). Before you trust the red bars, check that every task has a predecessor and a successor and that the logic types from Lesson 7 make sense.

Northgate worked example

The owner's master for Northgate Distribution Centre (fictional), NDC_OwnerMaster_2027-01-11.mpp, carries Halvorsen Builders Ltd.'s (fictional) foundations sequence at summary level, between M3 site mobilization (Mon Mar 1, 2027) and M4 foundations and slab complete (Fri May 14, 2027), from the Northgate fact sheet. The durations are illustrative. To keep the arithmetic visible, treat M4 as the end of the network for this example; in the full file the chain carries on to M9.

• Site preparation and excavation, 10 days, after M3.
• Footings and piers, 19 days, after site preparation.
• Underground services below slab, 14 days, after site preparation.
• Slab-on-grade, 25 days, after both footings and underground services.

The passes

Forward: site preparation runs Mon Mar 1 to Fri Mar 12. Footings and underground services both start Mon Mar 15. Footings finish Fri Apr 9, after losing Good Friday (Mar 26) to the Ontario calendar from Lesson 4. Underground services finish Fri Apr 2. The slab waits for the later of the two, so it starts Mon Apr 12 and finishes Fri May 14: M4.

Backward: the slab must start by Apr 12, so both footings and underground services must finish by Fri Apr 9. Underground services can start as late as Mon Mar 22 and still finish by Apr 9. Its total slack is five working days, and so is its free slack, because its only successor is the slab. Everything else has zero slack and is critical (Figure 1).

View the data

Task	Dur.	Early start	Early finish	Late start	Late finish	Total slack	Free slack
Site preparation and excavation	10d	Mon Mar 1	Fri Mar 12	Mon Mar 1	Fri Mar 12	0d	0d
Footings and piers	19d	Mon Mar 15	Fri Apr 9	Mon Mar 15	Fri Apr 9	0d	0d
Underground services below slab	14d	Mon Mar 15	Fri Apr 2	Mon Mar 22	Fri Apr 9	5d	5d
Slab-on-grade	25d	Mon Apr 12	Fri May 14	Mon Apr 12	Fri May 14	0d	0d
M4 Foundations and slab complete	0d	Fri May 14	Fri May 14	Fri May 14	Fri May 14	0d	0d

What the owner sees in the Detail Gantt

Figure 2 is the same network as the Detail Gantt draws it: critical bars highlighted, and a thin slack line after underground services running to Apr 9. That line is the room the schedule has on that branch. Who is entitled to use it is a contract question, often addressed in an owner's Supplementary Conditions, and outside the scope of this lesson.

Screenshot to come

Spending the slack

Suppose Halvorsen reports that underground services will take longer (an illustrative what-if, not a fact-sheet event). Figure 3 shows three cases. Three extra days are absorbed: the branch finishes Wed Apr 7, still has two days of slack, and M4 holds. Five extra days use all the slack: the branch finishes Fri Apr 9, becomes critical, and M4 still holds. Seven extra days go beyond it: the branch finishes Tue Apr 13, the slab starts Wed Apr 14, and M4 moves two working days to Tue May 18.

View the data

Extra days on underground services	Branch finishes	Slack used	Slab starts	M4
3	Wed Apr 7, 2027	3 of 5	Mon Apr 12	Fri May 14 (holds)
5	Fri Apr 9, 2027	5 of 5	Mon Apr 12	Fri May 14 (holds)
7	Tue Apr 13, 2027	5 of 5, plus 2	Wed Apr 14	Tue May 18 (2 days late)

Screenshot to come

Owner's move

• Ask for the Total Slack column in every update. A schedule submitted without slack values asks you to trust the red bars.
• Track the near-critical band. Tasks with up to ten days of slack are where next month's critical path comes from. Filter for them, or raise the threshold in a copy of the file and say so.
• Check the inputs before the output. No manual tasks, no open ends, no unexplained constraints, the Ontario calendar applied. Then read the critical path.
• Watch slack erosion, not just the finish date. A branch going from 5 days to 0 has not delayed M4, but it has used up the project's cushion.
• Address float ownership in the contract. Get advice on how your Supplementary Conditions treat shared float before the baseline is accepted.

Common mistakes

1. Trusting slack on open-ended tasks. A task with no successor takes its late finish from the project finish, so it shows months of slack it does not really have.
2. Treating free slack as total slack. Free slack is one task's room before its successor moves; total slack is room before the finish moves. They differ whenever slack is shared along a chain.
3. Changing the threshold silently. A 5-day threshold makes more tasks red. If the report does not say so, readers assume a longer critical path.
4. Turning on multiple critical paths for reporting. Each chain is measured to its own end, so several chains look critical that do not set the finish date.
5. Reading the critical path from a file with manual tasks. Their dates do not respond to logic, and neither does their slack.

Quick quiz

1. Microsoft calculates Total Slack as:

   1. Late Finish minus Early Start
   2. The smaller of Late Finish minus Early Finish and Late Start minus Early Start
   3. Finish minus Baseline Finish
   4. Duration minus Actual Duration

2. Free Slack is the time a task can be delayed without delaying:

   1. The project finish date
   2. Any successor task
   3. Its own deadline
   4. Its summary task

3. Where is "Tasks are critical if slack is less than or equal to"?

   1. File > Options > Advanced
   2. File > Options > Schedule
   3. Project > Project Information
   4. Gantt Chart Format > Bar Styles

4. In the Northgate example, underground services has 5 days of total slack. It slips 7 days. M4 moves:

   1. 0 days
   2. 2 working days
   3. 5 working days
   4. 7 working days

5. Which shows Free Slack and Total Slack as columns?

   1. Detail Gantt with the Schedule table
   2. Timeline view
   3. Resource Sheet
   4. Calendar view

Show answers and explanations

1. B. That is Microsoft's definition [1].
2. B. Free slack protects the successors; total slack protects the project finish [2].
3. A. It is in the "Calculation options for this project" area of the Advanced page [4].
4. B. The first 5 days are absorbed by slack; the remaining 2 move the slab and M4, from Fri May 14 to Tue May 18.
5. A. Microsoft's steps are Detail Gantt, then View > Tables > Schedule [3].

Key terms

Critical path

The chain of linked tasks that sets the project finish date; a delay on any of them delays the finish.

Total slack

How far a task can slip without delaying the project finish. Called total float in Oracle Primavera P6 and most scheduling texts.

Free slack

How far a task can slip without delaying any successor.

Negative slack

Total slack below zero: the schedule does not have enough time for the task, usually because of a constraint or deadline.

Early and late dates

The soonest and latest a task can start or finish, from the forward and backward passes.

Critical-slack threshold

The value at or below which Project marks a task critical; 0 days by default.

References

1. Microsoft. Total Slack (task field). Microsoft Support.
2. Microsoft. Free Slack (task field). Microsoft Support.
3. Microsoft. Show slack in your project in Project desktop. Microsoft Support.
4. Microsoft. Show the critical path of your project in Project. Microsoft Support.
5. Microsoft. Negative Slack (task field). Microsoft Support.

Verified on November 6, 2026.

Next lesson in this trackConstraints and deadlines