Critical Path Method (CPM): Steps & Example [2026]
A. Togay Koralturk, Best-Selling PMP Author
Last updated on September 17, 2026
12 min read
Not every late task delays your project — but some do, and the critical path method is how you tell which is which. It finds the one chain of activities that actually controls the finish date, so that instead of chasing every slipping task, you know exactly where a day lost is a day added to the whole project. It is the backbone of project scheduling and one of the most tested topics on the PMP. This guide explains the critical path method in full — what it is, how to calculate it step by step, a worked example, how float works, CPM versus PERT, and how it is tested on the PMP and CAPM exams.
What is the critical path method?
The critical path method (CPM) is a scheduling technique that identifies the critical path — the longest sequence of dependent activities through a project. Because that chain has no slack, its total length is the shortest possible time in which the project can finish. Every activity on the critical path has zero float, meaning any delay to it pushes the project's end date out by the same amount.
The insight that makes CPM so useful is that not all activities are equal. In any project, some activities have room to slip without affecting the finish, while others do not. The critical path is the set that does not — the tasks where a day lost is a day added to the entire project. Finding that path turns a schedule from a flat list of tasks into a map of where your attention actually matters. CPM was developed in the late 1950s and has been the foundation of project scheduling ever since.
Why use the critical path method?
The critical path method matters because it tells you which activities control the finish date — and therefore where to focus. On a project with hundreds of tasks, it identifies the handful that cannot slip, so you protect the right ones instead of spreading effort evenly.
What it gives you:
- The shortest project duration: the length of the critical path is the earliest the project can realistically finish.
- Focus: it pinpoints the activities that must stay on time, so monitoring and resources go where they matter.
- Float visibility: it reveals how much slack every non-critical activity has, so you know what can safely absorb a delay.
- Better decisions under pressure: when you need to shorten the schedule, it shows exactly which activities to compress for any effect at all.
Without it, a project manager treats every delay as equally urgent — and wastes effort expediting tasks that had slack to spare while missing the one that is quietly sinking the deadline.
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How to calculate the critical path
Calculating the critical path is a four-step routine over the project's network diagram. You work forward to find the earliest each activity can happen, backward to find the latest it can happen, and the difference reveals the path with no room to spare:
- Build the network. List the activities, their durations, and their dependencies, and lay them out as a network of connected activities.
- Run the forward pass. Moving left to right, calculate each activity's early start (ES) and early finish (EF = ES + duration).
- Run the backward pass. Moving right to left from the project end, calculate each activity's late finish (LF) and late start (LS = LF − duration).
- Compute float and find the path. For each activity, float = LS − ES (or LF − EF). The activities with zero float, connected start to finish, are the critical path.
The critical path is simply the chain of zero-float activities, and its length is the project duration. Modern scheduling software runs these passes automatically, but the exam expects you to do them by hand — so the worked example below walks through the full calculation.
The forward pass and backward pass, explained
The forward pass answers one question: how early can each activity happen? You start at the project's beginning and move left to right. The first activity starts at day 0, and its early finish is its duration. Every following activity starts the moment its predecessors are done. When two paths feed into the same activity, the later early finish wins — an activity cannot begin until every path arriving at it has finished. When the pass reaches the end of the network, it has produced the earliest possible finish date for the whole project.
The backward pass answers the mirror question: how late can each activity happen without missing that finish date? Now you start at the project's end and move right to left. The last activity's late finish is the project finish, and its late start is that minus its duration. Every earlier activity must finish in time for its successors to start on schedule. When one activity feeds two paths, the earlier late start wins — being late for either successor is still late.
If the two rules ever blur together on exam day, remember which direction each pass is being careful about: going forward, an activity waits for its slowest predecessor; going backward, it defers to its neediest successor. Later wins forward, earlier wins backward. The gap between the two schedules each activity ends up with is its float — and the worked example below runs both passes, number by number, on a full network.
Critical path method example
Consider a five-activity project. The critical path runs A → B → D and takes 10 days; a parallel path, A → C → E → D, is shorter, so C and E carry float. The network looks like this, with the critical path in red:
The forward pass, left to right: A starts at day 0 and finishes at day 3. B and C both start the moment A is done, so B runs days 3–7 and C runs days 3–5. E follows C and runs 5–6. D is the merge point: it waits for the later of its two predecessors — B finishes at day 7, E at day 6 — so D runs 7–10. The earliest the whole project can finish is day 10.
The backward pass, right to left from day 10: D's late window is 7–10. B must finish by day 7, so its late window is 3–7, identical to its early one. E must also finish by day 7, giving 6–7, and C must finish by E's late start, giving 4–6. A is the split point: it feeds both B (late start 3) and C (late start 4), and the earlier one wins, so A's late window is 0–3.
Putting the two passes side by side gives each activity's float:
| Activity | Duration | Early (ES–EF) | Late (LS–LF) | Float |
|---|---|---|---|---|
| A | 3 | 0–3 | 0–3 | 0 |
| B | 4 | 3–7 | 3–7 | 0 |
| C | 2 | 3–5 | 4–6 | 1 |
| E | 1 | 5–6 | 6–7 | 1 |
| D | 3 | 7–10 | 7–10 | 0 |
Activities A, B, and D all have zero float, so the critical path is A → B → D, and the project's duration is 10 days. Activities C and E each have one day of float — they can slip a day without moving the finish date, which is exactly the kind of slack the critical path does not have.
Float and the critical path
Float (also called slack) is the gap between an activity's early and late dates, and the two passes produce it automatically. It is the practical output of the whole calculation: it tells you which delays to worry about and how much slippage each activity can absorb. Float is also why the critical path can shift: delay a non-critical activity beyond its float and its path becomes the new longest one, making a different set of activities critical. There is more nuance here — total float versus free float — which we cover in depth in our guide to total float vs. free float.
Critical path method vs. PERT
CPM and PERT are closely related network techniques that are often confused. Both map activities and dependencies to analyze the schedule, but they differ in how they treat duration: CPM uses a single, deterministic estimate per activity, while PERT uses a three-point estimate to account for uncertainty. Here is the comparison:
| Critical path method (CPM) | PERT | |
|---|---|---|
| Duration estimate | Single, deterministic | Three-point (optimistic, likely, pessimistic) |
| Focus | Time and the critical path | Uncertainty in the schedule |
| Best for | Well-defined projects with known durations | Projects with uncertain activity times |
| Output | The critical path and float | An expected duration with a probability range |
In practice the two are complementary and often combined: you use PERT's three-point estimates to get more realistic durations, then run the critical path method over them to find the path that drives the schedule.
Keeping the critical path on track
Once you know the critical path, managing the schedule is largely about protecting it: critical activities are the ones to monitor most closely, and the only ones worth compressing when you need to recover time.
When a project must finish sooner, the critical path is where you apply schedule compression — crashing (adding resources) or fast-tracking (overlapping activities) — because shortening a non-critical activity changes nothing. And after any change, recalculate: compressing the critical path enough can make a parallel path the new critical path, at which point your focus has to move with it. Keeping the critical path current is what turns CPM from a one-time calculation into a live management tool. Our PMP Complete Study Guide, the most complete on the market, works through these calculations and decisions with exam-style scenarios.
The critical path method on the PMP® and CAPM® Exams
On the PMP exam, the critical path method is one of the most reliably tested scheduling topics, in two forms. The first is calculation: you are given a network or a table of durations and dependencies and asked to find the critical path, the project duration, or an activity's float — so the forward and backward pass must be automatic. The second is judgment: a scenario where you must decide which activity to expedite or protect, and the answer turns on whether an activity is on the critical path or has float to spare.
The single most common trap is treating a badly slipping non-critical activity as more urgent than a critical one — when float means the non-critical slip may cost nothing while the critical one costs the project directly. The CAPM tests the same ideas a little more directly, often the definition of the critical path or a straightforward float calculation, but its scenario format means you should still expect to apply the concept. Because these calculations recur throughout the exam, our study materials drill the forward and backward pass until they are second nature.
PMP Practice Question: Critical Path Method
At the last approved schedule update, a project had three parallel paths to completion: Path X at 24 days (the critical path), Path Y at 22 days, and Path Z at 20 days. This week's status report shows Path X has slipped 1 day, Path Y has slipped 4 days, and Path Z has slipped 5 days. The project manager has the resources to expedite only one path.
Which path should the project manager expedite?
a) Path X, since it is the critical path on the approved schedule and recovery priority always goes to the critical path.
b) Path Z, since a 5-day slip in one week is the largest overrun and marks the least controlled part of the project.
c) Path Y, since after this week's slips it totals 26 days against 25 for both X and Z, making it the path that now sets the finish date.
d) Split the effort between Path Y and Path Z, since both have consumed more than their float and both are now delaying the project.
Correct answer: C.
Rationale: The critical path is a calculation, not a label, so the first move is to redo it with this week's numbers: X is now 24 + 1 = 25, Y is 22 + 4 = 26, and Z is 20 + 5 = 25. The finish date is set by the longest path, and that is now Y — its 4-day slip overran its 2 days of float, while Z's larger 5-day slip merely consumed its larger cushion of 4. That asymmetry is the trap in choice b): the size of a slip says nothing by itself, because what matters is slip relative to float, and the "worst-performing" path can still be harmless. Choice a) applies last week's critical-path label to this week's network, protecting a path that no longer sets the date; choice d) halves the effort on the one path that determines the finish — Z at 25 days is not delaying a 26-day project, so effort spent there recovers nothing. Expedite Y, and after the recovery recompute again, because today's fix reshuffles tomorrow's longest path. To face more questions where every option requires doing the arithmetic first, work through our PMP practice exams or, at the entry level, our CAPM practice exams.
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Frequently asked questions
What is the critical path method?
The critical path method (CPM) is a scheduling technique that finds the critical path — the longest sequence of dependent activities through a project. That path's length is the shortest time the project can finish, and every activity on it has zero float, so any delay to a critical activity delays the whole project.
What is the critical path?
The critical path is the longest chain of dependent activities in a project, running from start to finish. Because it has no slack, it determines the project's minimum duration. Activities on the critical path have zero float, meaning they cannot be delayed without delaying the project's completion date.
How do you calculate the critical path?
Build the network of activities and dependencies, run a forward pass to find each activity's early start and finish, run a backward pass to find each late start and finish, then calculate float as late start minus early start. The activities with zero float, connected from start to finish, form the critical path, and its length is the project duration.
What is the difference between CPM and PERT?
CPM uses a single, deterministic duration for each activity and focuses on finding the critical path, so it suits projects with well-known task times. PERT uses a three-point estimate — optimistic, most likely, and pessimistic — to account for uncertainty and produce an expected duration with a probability range. The two are often combined.
Can the critical path change during a project?
Yes. If a non-critical activity is delayed beyond its float, its path can become the new longest path, making a different set of activities critical. This is why the critical path should be recalculated after significant schedule changes or compression, so you always know which activities currently control the finish date.
Is the critical path method on the PMP exam?
Yes. The critical path method is one of the most heavily tested scheduling topics on the PMP exam. You should expect both calculation questions — finding the critical path, project duration, or float from a network — and situational questions that reward focusing on critical-path activities rather than whichever task has slipped the most.
Is the critical path method on the CAPM exam?
Yes. The CAPM covers the critical path method, usually a little more directly than the PMP — often the definition of the critical path, the zero-float rule, or a straightforward calculation. Because the CAPM is scenario-based, you should still be ready to apply the concept in a short situation rather than only recall it.
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