The critical path is the longest chain of dependent activities through a project's schedule, which sets the earliest possible finish date and on which any delay delays the whole project.
Our take. Most project managers can name their critical path and few recalculate it after the first week, which is when it starts to move. The path is a property of the current network, not of the original plan, and the activity that is critical in month three was often comfortably off the path in the plan everyone is still looking at.
Lay out the activities, draw the dependencies between them, estimate each one's duration, and there will be many chains from start to finish. The longest of them is the critical path, and its length is the shortest time the project can take: no amount of effort on activities off the path shortens the project, and any slip on an activity on it lengthens the project by the same amount. Every other activity has float, the amount it can slip before it joins the path. The method is arithmetic on a network and any scheduling tool performs it; the judgement is in what a manager does with the answer.
Protect the path. The activities on it get the best people, the earliest attention and the most frequent checks, because their delay is the project's delay.
Spend float deliberately. Activities with float can lend resources to the path, absorb a late start, or be scheduled where they cause the least disruption. Float is a resource; spent by accident, it is gone.
Recalculate. Every progress update changes durations and every change request alters the network, and the path moves. A near-critical chain with two days of float becomes the path when one activity slips three.
Shorten it only in two ways. Crash it by adding resources to path activities, at a cost; or fast-track it by overlapping activities that were planned in sequence, at a risk. There is no third way that is not wishful.
Watch the near-critical paths. The chain with the least float after the critical one is the one most likely to become it, and the one most managers ignore.
The method dates from the late nineteen-fifties and is the core of every scheduling tool since. The professional certifications examine it directly, asking for the path, the float on a given activity and the effect of a delay; the entry-level ones ask what the terms mean. Agile methods have less use for it, because a fixed-length iteration with a chosen scope has no long dependent chain to protect, but any project with a hard external date and real dependencies between pieces of work has a critical path whether anyone has calculated it or not.
In practice
An office move has a network of activities: sign the lease, fit out the space, install the network, move the furniture, move the people. The fit-out is eight weeks and depends on the lease; the network installation is three weeks and depends on the fit-out; the move depends on both. The path runs lease, fit-out, network, move, and the furniture, which needs one week and can happen any time after the fit-out, has float. The manager, seeing the path, books the network installer for the day the fit-out ends and treats the fit-out contractor's weekly report as the most important document in the project. When the fit-out slips a week, the move date slips a week, and everyone knew on the day it happened rather than the day it mattered.
On the path: lease, fit-out, network, move. Any slip is the project's slip.
Off the path: furniture, with weeks of float. Schedule it where it disturbs least.
Watch: the fit-out contractor's weekly report, the most important document in the project.
A Gantt chart displays activities against dates and can show the critical path highlighted. The path is calculated from the network of dependencies; the chart is one way of drawing the result.
The breakdown structure lists the pieces of work with no dates or dependencies. Adding dependencies and durations turns it into the network the critical path is calculated from.
The critical path shows which activities cannot slip without consequence. The risk register records what might make them slip. The two are read together: a risk against a path activity matters more than the same risk against one with float.
Key takeaways
→The longest dependent chain sets the finish; delays on it are project delays.
→Float is a resource to spend deliberately, not slack to be surprised by.
→The path moves with every update; recalculate, and watch the chain with the least float.
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FAQ
Can a project have more than one critical path?
Yes, when two chains are equally long, and it is worse rather than better: two paths means twice as many activities whose slip is the project's slip. Near-equal chains are common, and the manager's attention has to cover all of them.
What is the difference between crashing and fast-tracking?
Crashing adds resources to critical activities to shorten them and costs money. Fast-tracking runs activities in parallel that were planned in sequence and costs risk, because the second may have to be redone if the first changes. Both shorten the path; neither is free.
Does the critical path matter in agile delivery?
Within an iteration, rarely, because the scope is chosen to fit the time. Across a programme with a fixed external date and dependencies between teams, yes, and pretending otherwise is how agile programmes are late by a whole release rather than a sprint.
Sources
The primary text this definition rests on. Read it before you trust ours.
Kelley, J. E. and Walker, M. R., Critical-Path Planning and Scheduling (1959)
Project Management Institute, Practice Standard for Scheduling, Third Edition (2019)
