Total Slack, Free Slack, and the Critical Path, Explained

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Total Slack, Free Slack, and the Critical Path

Project Management · Chapter 9: Project Monitoring, Control, and Performance ManagementSection 9.6 · lesson 60 of 7312 min · free with a free account

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In this lesson

Use total, free and project float to judge which delays matter.

  • Calculate total float and free float from early and late dates
  • Identify the critical path and near-critical paths by comparing path lengths
  • Explain project float and what negative float means for a commitment
  • Explain why float is shared along a path and how the critical path can shift

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Transcript

Everything said in the video, section by section (1,656 words). Read along, or search it.

Opening Question

Will a Two-Day Overrun Move Handover

Back on the Riverside clinic build. On the Tuesday call, Sam reports that setting the wall frames will take 4 working days, not the planned 2. The figures are made up. Helen, the sponsor, has been told the building is handed over on day 18. She asks one question: does this move the handover? Priya won't answer from the overrun. She'll answer from the float.

Key Terms

Early Dates, Late Dates and Float

From the forward and backward passes, every activity has four dates. Early start and early finish: the earliest it can begin and end. Late start and late finish: the latest, without pushing back the finish. The project starts at day 0. Early finish is early start plus duration, and late start is late finish minus duration. Every link here is finish-to-start. Float, also called slack, is how long an activity or a path can slip before a stated consequence. Total float protects the finish. Free float protects the next activities. Project float compares the forecast with the required finish.

Example Network

The Riverside Site Network

Here's the site network, with its dates already worked out. Foundations, A, take 6 days, then three branches start on day 6. Wall frames, B, 2 days, then drywall, C, 2 days. Roof steel, D, 7 days. Drainage, E, 3 days, then the parking lot surface, F, 3 days. All three branches must finish before inspection and handover, G, 5 days, which ends on day 18.

Total Float

Late Start Minus Early Start

Total float is how long an activity can slip, or overrun, without delaying the project finish. It's late start minus early start, or, the same thing, late finish minus early finish. Drywall, C: 11 - 8 = 3, and 13 - 10 = 3. The others: A, 0. B, 3. D, 0. E, 1. F, 1. And G, 0. An activity with zero total float is critical: any delay delays the finish. And total float isn't the activity's own. It belongs to the chain through it.

Free Float

Protecting the Next Activity's Start

Free float is how long an activity can slip, without delaying the early start of anything that follows. It's the earliest early start among its successors, minus its own early finish. Wall frames, B, finish early on day 8, and drywall starts early on day 8. So B's free float is 8 - 8 = 0, though its total float is 3. C: 13 - 10 = 3. F: 13 - 12 = 1. E: 9 - 9 = 0. A, D and G: 0. For the last activity, compare with the project finish. Free float can never exceed total float. And it says who must be told: within free float, the owning team absorbs it. Beyond it, other teams' starts move. Beyond total float, the finish moves: that's for the sponsor.

Critical Path

Checking Every Complete Path Length

The critical activities, A, D and G, join end to end: the critical path, 6 + 7 + 5 = 18 days, the forecast finish. Check it with every complete path. A, E, F, G is 6 + 3 + 3 + 5 = 17. A, B, C, G is 6 + 2 + 2 + 5 = 15. Each path's gap to 18 is its activities' total float: 1 for E and F, and 3 for B and C.

Quick Check

Total and Free Float for Cabling

Quick check, from a ticket-office fit-out. Running the cabling, R, has early dates 4 and 6, and late dates 8 and 10. Its only successor starts early on day 8. What are R's total float and free float? Pause and predict. Total float 4, free float 2. Total float is 8 - 4 = 4. Free float is the successor's early start, 8, minus R's early finish, 6: two. So R can slip 4 days without moving the finish, but past 2 days it moves the next crew's start. Assuming free float equals total float is the trap.

Project Float

Required Finish Versus Forecast Finish

Now the outside world. A required finish is fixed from outside the schedule, by a contract, a sponsor, or an event. The forecast finish is what the schedule says is achievable. Project float is required finish minus forecast finish. Positive is margin; zero is none; and when the required date is earlier, it's negative float. A required date shifts every total float by the project float. Required day 21: project float +3, so critical activities carry 3, E and F 4, B and C 6. Required day 16: 16 - 18 = -2. The critical activities carry -2, E and F -1, and B and C still +1. Negative float isn't lateness already incurred. It means that even if everything goes to plan, the project lands two days after the commitment. It marks the paths that are too long. Positive float is a margin someone should own. Zero means any slip misses the date. Negative means plan and commitment disagree: compress the work, or change the commitment through the change process.

Near-Critical Paths

Watching Paths With Thin Float

A near-critical path has float small enough that a plausible overrun would make it the longest. Organizations set the threshold, in days or as a share of duration. With a threshold of 2 days, A, E, F, G, at 1 day of float, is near-critical. A, B, C, G, at 3, isn't yet. The critical path demands action on any slip. A near-critical path demands its float be watched, with a response agreed in advance. And merge points add risk. G waits for C, D and F, so several low-float paths give several chances of delay. Count near-critical paths, not just the critical one.

Path Shifts

How a Drainage Overrun Moves the Path

The critical path moves as durations change. Suppose drainage, E, takes longer, with everything else unchanged. As planned, 3 days: the paths are 18, 17 and 15, and A, D, G is critical. At 4 days, an overrun equal to its float: A, E, F, G becomes 6 + 4 + 3 + 5 = 18. The finish stays at 18, but now there are two critical paths. At 6 days: A, E, F, G is 20. The finish moves to day 20, and A, E, F, G is critical. The old critical path now has 2 days of float. The finish moved 2 days: the overrun of 3, minus the 1 day of float. The middle case is the one most likely to go unnoticed: the finish hasn't moved, but the margin is gone.

Scheduling Software

Baselines, Tracking and What-If Analysis

Scheduling software recalculates dates and floats after each update. Four kinds of feature support control. A baseline: a saved copy of the approved dates, the reference for asking whether the critical path has shifted. Progress tracking: actual dates and remaining durations at a status date. What-if analysis: testing a change, like E overrunning by 3 days, on a copy, before anything is committed. And variance views: comparing the current schedule with the baseline, often filtered by float. These features compute; they don't judge. Float is only right if the logic is complete and progress is current, and tools define critical differently, so check the definition in use.

Key Limitation

Float Belongs to the Path

Float ranks where to look: zero or negative float first, then near-critical paths with their float reported at each update, and comfortable float less often. But the biggest limit: float belongs to a path, not an activity. B and C each show 3 days. That's not 6 days in all. It's the same 3 days, shared. If B takes 1 day longer, C starts on day 9, and both show 2. If B takes 3 days longer, both show 0, and the drywall team has no float left, through no action of its own. If B takes 4 days longer, the path is 19 days, and the finish moves to day 19. Float is also a snapshot, only as good as the network, since a missing link creates float that doesn't exist. And it knows nothing of crews, cost or safety.

Quick Check

A Longer Parking Lot Surface

Quick check, on the same network. Drainage, E, finishes on time. But the parking lot surface, F, is now forecast to take 5 days, not 3. When is the project finish: still day 18, day 19, or day 20? Pause and predict. Day 19. A, E, F, G becomes 6 + 3 + 5 + 5 = 19, now the longest path. The 2-day overrun used up the path's 1 day of float, and the other day moved the finish. Day 20 would ignore the float. And day 18 forgets that E and F's float is one shared day, not one day each.

Debate

How Far Float Should Rank Attention

How far should float rank attention? One view: attention is the scarcest resource, only critical-path delays move the finish, so concentrate on zero-float work. The other: estimates are uncertain and paths converge, so watch every path whose float is small compared with its uncertainty. How often non-critical paths overrun is an empirical question, answered only by an organization's own records. How much attention to trade is about values. No side is taken here.

Case Resolved

Handover Holds but Drywall Moves

Back to Sam's wall frames: 4 days instead of 2. Priya reads the float, not the overrun. B's total float is 3 and the overrun is 2, so the finish holds. A, B, C, G becomes 6 + 4 + 2 + 5 = 17, against 18. But B's free float was 0, so drywall's early start moves from day 8 to day 10, and that booked crew must move today. B and C now share 1 day of float, and two paths sit at 1. Had the same overrun hit the roof steel, the finish would be day 20, and she'd escalate at once. So she tells Helen day 18 holds, rebooks the drywall crew, and adds the wall-frame path to the near-critical watch list. The cost: two thin paths to report at every update.

Key Takeaways

Float Types and Path Behavior in Brief

To recap. Total float is late start minus early start, and protects the finish. Free float is the successors' earliest start minus the early finish, and protects the next team. Project float is required minus forecast finish. Negative float means plan and commitment disagree. Float is shared along a path, and the critical path can shift before the finish does.

About Project Management at UPI Study

Project Management (Business 230) is a self-paced online course of 73 video lessons in 12 chapters, from starting and defining a project to planning, budgets, scheduling tools, proposals, contracts, execution, monitoring, closure, teams and certification. It carries a NCCRS credit recommendation for 3 semester credits.

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Why students take Project Management with UPI Study

How projects are planned, run and closedProject Management teaches how to deliver work on time and on budget: scope, schedules, the critical path, budgets and earned value, risk, contracts, monitoring and closing a project, and leading the team. It is a common course in business, management and engineering degrees.
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130,000+students helped with college creditsACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

Where the credit goes

Universities that work with NCCRS

Project Management carries a NCCRS credit recommendation. These universities work with NCCRS; some are UPI Study's signed partners. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

Thomas Edison State UniversityThomas Edison State UniversitySigned partnerUniversity of the PeopleUniversity of the PeopleSigned partnerCharter Oak State CollegeCharter Oak State CollegeSigned partnerStrayer UniversityStrayer UniversitySigned partnerColumbia Southern UniversityColumbia Southern UniversitySigned partnerIndiana TechIndiana TechSigned partnerPenn StatePenn StateWorks with NCCRSSouthern New Hampshire UniversitySouthern New Hampshire UniversityWorks with NCCRSSUNYSUNYWorks with NCCRS +many more cooperating universitiesFind my college →

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FAQ

Questions students ask us

About Total Slack, Free Slack, and the Critical Path

What is the difference between total float and free float?
Total float is how long an activity can slip without delaying the project finish, while free float is how long it can slip without delaying the early start of any successor. An activity with an early finish on day 6, a late finish on day 10 and a successor starting early on day 8 has total float of 4 but free float of 2. Free float cannot exceed total float.
What is a near-critical path and why should it be monitored?
A near-critical path has float small enough that a plausible overrun would make it the longest path. Organizations set the threshold, in days or as a share of duration. With a 2-day threshold, a path with 1 day of float is near-critical. Its float should be reported at each update with a response agreed in advance, especially where several low-float paths merge into one activity.
What does negative float mean on a project schedule?
Negative float means the required finish date is earlier than the forecast finish, so even if everything goes to plan, the project misses its commitment. With a forecast finish of day 18 and a required finish of day 16, critical activities carry float of -2. It marks the paths that are too long. The response is to compress the work or change the commitment through the change process.
Why is float shared along a path instead of belonging to one activity?
Float belongs to the chain of activities, so activities on the same path draw on the same spare days. If wall frames and drywall each show 3 days of total float, the path has 3 days in all, not 6. When the wall frames overrun by 1 day, both show 2 days, and the drywall team loses float through no action of its own.

About the Project Management course

How many college credits is Project Management?
Project Management (Business 230) carries a NCCRS credit recommendation for 3 semester credits. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.
How much does Project Management cost and what is included?
$250, paid once, with lifetime access: all 73 video lessons, a practice quiz after each, the graded work and final exam, and an official transcript when you finish. Students who want several courses choose UPI Study PRO instead.
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UPI Study has signed partner agreements with universities such as Thomas Edison State University, University of the People and Charter Oak State College, and works with universities such as Penn State, SNHU and SUNY. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities. Use "Find my college" on this page to check yours.
How long does Project Management take?
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