Critical Path Method (CPM): How to Find the Critical Path

The critical path method is a scheduling technique that identifies the longest sequence of dependent tasks in a project, and by doing so it tells you the minimum time your project can possibly take. Miss a task on that path, and the whole finish date slips; protect it, and you protect your deadline.

What is the critical path method (CPM)?

CPM is a scheduling algorithm that maps every task in a project, links them by dependency, calculates earliest and latest possible times for each task, and flags the chain of tasks that has zero scheduling flexibility as the critical path. That chain sets the project's minimum duration.

Origin: Engineers Morgan Walker at DuPont and James Kelley at Remington Rand developed CPM in 1957 to manage complex chemical plant shutdowns. Their goal was to reduce costly downtime, and CPM delivered -- cutting maintenance time significantly enough that the technique spread across industries within a decade.

Key insight: Not every task matters equally. Some tasks have wiggle room (called float or slack). Tasks on the critical path have none.

Key Facts

DuPont reported that CPM cut chemical plant maintenance shutdowns by an estimated 25% in the late 1950s (DuPont internal report, 1959).

PMI's PMBOK Guide treats CPM as a foundational schedule network analysis method, included in every edition from the first through the 7th (PMI, PMBOK Guide 7th Edition, 2021).

Gartner forecasts the global project management software market will exceed $9.8 billion by 2027, with schedule optimization features like CPM among the top drivers (Gartner, 2023).

Key terms

Before running the calculations, it helps to have these terms locked in.

Forward pass vs backward pass

CPM uses two sweeps through the network to calculate timing for every activity.

Forward pass (earliest times)

Start at the first activity and work toward the end:

• ES = the maximum EF of all immediate predecessors (or 0 if there are no predecessors)
• EF = ES + duration

This tells you the earliest possible finish date for the whole project.

Backward pass (latest times)

Start at the last activity and work back toward the beginning:

• LF = the minimum LS of all immediate successors (or the project end date for the final activity)
• LS = LF - duration

Float calculation

Once you have both passes:

• Float = LS - ES (equivalently, LF - EF)

Any activity where float equals zero sits on the critical path.

How to find the critical path in 6 steps

Step 1: List activities and durations

Write out every task required to complete the project and estimate how long each one takes.

• Break the work into discrete, measurable activities
• Assign a realistic duration to each (use historical data or expert estimates)
• Give every activity a unique identifier (A, B, C, or Task 1, Task 2, etc.)

Step 2: Identify dependencies

Determine which activities must finish before others can start.

• Ask "what has to be done before I can start this?" for every task
• Record predecessor-successor relationships in a table or dependency matrix
• Watch for finish-to-start, start-to-start, and finish-to-finish link types (finish-to-start is the default in CPM)

Step 3: Draw the network diagram

Visualize the project as a directed graph, with activities as nodes and dependencies as arrows.

• Place activities in left-to-right sequence based on dependencies
• Make sure every path from start to end is connected
• Use flowcharts or dedicated project planning tools to draw the diagram cleanly

Step 4: Run the forward pass

Sweep from left to right, calculating ES and EF for every activity.

• Start with ES = 0 for activities with no predecessors
• Apply ES = max(EF of predecessors) at every merge point
• Record EF = ES + duration for each activity

Step 5: Run the backward pass

Sweep from right to left, calculating LF and LS for every activity.

• Start with LF = project end date (the EF of the final activity) for activities with no successors
• Apply LF = min(LS of successors) at every burst point
• Record LS = LF - duration for each activity

Step 6: Identify zero-float activities (the critical path)

Calculate float for every activity and highlight the chain with float = 0.

• Float = LS - ES for each activity
• Activities where float = 0 form the critical path
• If two chains both have zero float, the project has multiple critical paths (see FAQ below)

Worked example: 7-task project

Consider a software release project with seven activities:

Critical path: A - C - E - F - G (total duration: 19 days, zero float throughout).

Activity B and D have 2 days of float each, meaning you could delay UI design by up to 2 days without pushing the launch date. But if backend build (E) slips even one day, every downstream activity slips too. That's where your attention belongs.

CPM vs PERT vs Gantt chart

These three tools are often confused, but they serve different purposes.

CPM and PERT complement each other well: run PERT first if durations are uncertain, then hand the resulting estimates to CPM to find the critical path. A what is a Gantt chart view is often the final communication layer on top of both.

When you're following a structured delivery approach, linking CPM to your waterfall methodology or Kanban board helps keep the schedule connected to actual work in progress.

Crashing and fast-tracking: how to compress the critical path

When the critical path is too long, you have two main options.

Crashing

Crashing means adding resources to critical-path activities to shorten their duration -- more developers on a build, an extra shift on a manufacturing run. It almost always increases cost, so the standard approach is to crash the activity that delivers the most time savings per dollar spent, then re-check the critical path (it may shift after the crash).

Risk: Cost overruns. You can only compress an activity so far before the law of diminishing returns kicks in.

Fast-tracking

Fast-tracking means running activities in parallel that were originally planned in sequence -- starting the backend build before the database schema is fully signed off, for example. It can save significant time at no extra budget.

Risk: Rework. If the predecessor activity changes after its successor has already started, you may need to redo work. Fast-tracking works best when the overlap is small and the dependency is loose.

Both techniques require you to re-run the forward and backward pass after making changes, because the critical path will shift.

Benefits and limitations of CPM

Benefits

• Reveals the project's true minimum duration before work begins
• Identifies where to focus management attention (zero-float activities)
• Creates a basis for trade-off decisions: cost vs. time, risk vs. speed
• Integrates naturally with resource leveling and business process management frameworks
• Supported in most project management competency frameworks including PMBOK and PRINCE2

Limitations

• Assumes task durations are known and fixed -- a shaky assumption on novel projects
• Network diagrams get unwieldy on large projects (hundreds or thousands of activities)
• Ignores resource constraints by default; a separate resource leveling step is needed
• Float calculations can create false confidence if estimates are inaccurate
• Doesn't capture uncertainty well -- use PERT alongside CPM when durations are estimates

Frequently asked questions

What is the difference between CPM and PERT?

CPM uses a single fixed duration estimate for each activity and produces a deterministic critical path. PERT uses three estimates -- optimistic, most likely, and pessimistic -- and produces a probability-weighted expected duration. CPM is better when you've done similar work before; PERT is better when durations are genuinely uncertain.

Can a project have more than one critical path?

Yes. If two or more chains of activities each have zero float and reach the same project end date, they are all critical paths. Multiple critical paths increase schedule risk because any slip on any of those chains delays the finish. When this happens, managers often prioritize resources to prevent both chains from slipping simultaneously.

Does the critical path change during a project?

It can, and often does. If a non-critical activity slips long enough to exhaust its float, it becomes critical. If a critical activity is completed ahead of schedule, another chain may become the new bottleneck. Best practice is to recalculate the critical path at every status update cycle, especially after scope changes or resource disruptions.

What is float and how do you calculate it?

Float (also called slack) is the amount of time an activity can be delayed without pushing out the project's end date. The formula is: Float = LS - ES (or equivalently, LF - EF). A float of zero means the activity is on the critical path and has no scheduling flexibility. A float of 3 days means you could start the activity up to 3 days late and still finish the project on time.

What software calculates the critical path?

Most project planning platforms include CPM calculations automatically once you enter tasks, durations, and dependencies. Common options include Microsoft Project, Primavera P6 (for large engineering projects), Smartsheet, TeamGantt, and Rework. Open-source tools like ProjectLibre also calculate float and highlight the critical path. Spreadsheet-based CPM is possible but tedious to maintain as the project grows.

Understanding the critical path is the first step; the next is building a system to monitor it continuously. When the RACI matrix defines who owns each critical-path activity, and your tracking tool recalculates float in real time, you shift from reactive firefighting to proactive schedule management -- which is where projects actually get delivered on time.