Critical path method: How to use CPM for project management

Editor’s Note: This is an educational article about how to use the critical path method for project management. For specific Asana functionality please see our project management hub page.

Building out a project roadmap can help you visualize what needs to be done to reach your end goal. The critical path method helps you do exactly that. It’s a project management technique that involves mapping out key tasks or critical tasks necessary to complete a project. 

Leveraging this technique allows you to manage task dependencies and set realistic timeframes. Read on to find out how the critical path method works and how you can use it with your team to optimize project timelines. 

What is the critical path method (CPM)?

Also called critical path analysis (CPA), the critical path method (CPM) is a technique where you identify tasks that are necessary for project completion and determine scheduling flexibilities. A critical path in project management is the longest sequence of activities that must be finished on time in order for the entire project to be complete. Any delays in critical tasks will delay the whole project.

CPM revolves around discovering the most important tasks in the project timeline, identifying task dependencies, and calculating task durations.

CPM was developed in the late 1950s as a methodology to resolve the issue of increased costs due to inefficient scheduling. Since then, CPM has become popular for planning projects and prioritizing tasks. It helps you break down complex projects into individual tasks and gain a better understanding of the overall project flexibility.  

Why use critical path analysis?

CPA can provide valuable insight on how to plan projects, allocate resources, pace towards milestones, and schedule tasks. 

Here are some reasons why you should use the critical path method: 

• Improves future planning: CPM can be used to compare expectations with actual progress. The data used from current projects can inform future project plans. 

• Facilitates more effective resource management: CPM helps project managers prioritize tasks, giving them a better idea of how to avoid resource constraints. 

• Helps avoid bottlenecks: Bottlenecks in projects can result in lost valuable time. Plotting out project dependencies using a network diagram will give you a better idea of which activities can and can’t run in parallel, allowing you to schedule work accordingly.   

How to find a project's critical path

Finding the critical path involves identifying the longest path between the start and end of the project by comparing the duration of critical and non-critical tasks. Below is a breakdown of the steps, with examples. 

1. List activities

Use a work breakdown structure to list all the project activities or tasks required to produce the deliverables. The list of activities in the work breakdown structure serves as the foundation for the rest of the CPM.

For example, let’s say the marketing team is producing a new interactive blog post. Here are some tasks that might be in the work breakdown structure:

Once you have a high-level idea of everything that needs to be done, you can start identifying task dependencies for the whole project.

2. Identify dependencies

Based on your work breakdown structure, determine the tasks that are dependent on one another. This will also help you identify any work that can be done in parallel with other tasks.

Here are the task dependencies based on the example above:

• Task B is dependent on A

• Task C is dependent on B 

• Tasks C and D can run in parallel

• Task E is dependent on D

• Task F is dependent on C, D, and E

The list of dependent tasks is referred to as an activity sequence, which will be used to determine the critical path. 

3. Create a network diagram

The next step is to turn the work breakdown structure into a network diagram, which is a flowchart displaying the chronology of critical path activities. Create a box for each task and use arrows to depict task dependencies. 

You’ll add other time-bound components to the network diagram until you have the general project schedule figured out. 

4. Estimate task duration

To calculate the critical path, the longest sequence of tasks, you first need to estimate the duration of each activity. 

To estimate the duration, try: 

• Making educated guesses based on experience and knowledge

• Estimating based on previous project data

• Estimating based on industry standards

Alternatively, try using the forward pass and backward pass technique:

• Forward pass: This is used to calculate earliest start time (ES) and earliest finish time (EF) by using a previously specified start date. ES is the highest EF value from immediate predecessors, whereas EF is ES + duration. The calculation starts with 0 at the ES of the first activity and proceeds through the schedule. Determining ES and EF dates allows for early allocation of resources to the project. 

• Backward pass: This is used to calculate the latest start (LS) and latest finish (LF) dates. LS is LF - duration, whereas LF is the lowest LS value from immediate successors. The calculation starts with the last scheduled critical path activity and proceeds backward through the entire schedule.    

The early and late start and end dates can then be used to calculate float, or scheduling flexibility of each task. 

5. Calculate the critical path

Calculating the critical path can be done manually, but you can save time by using a critical path algorithm instead. 

Here are the steps to calculate the critical path manually:

Step 1: Write down the start and end time next to each sequence of activities to calculate the sequence's "duration."

• Find the start time of the first activity in the sequence

• Find the end time of the last activity in the sequence

• The duration is the end time of the last activity minus the start time of the first activity

Step 2: Determine the number of dependencies along each sequence.

Step 3: The sequence of activities with the longest duration (end of sequence date - beginning of sequence date) is the critical path. If multiple sequences of activities have the same duration, the sequence with the greater number of dependencies is the critical path.

Using the same example above, here’s what the critical path diagram might look like:

Once you have the critical path figured out, you can build the actual project schedule around it.

6. Calculate the float

Float, or slack, refers to the amount of flexibility of a given task. It indicates how much the task can be delayed without impacting subsequent tasks or the project end date. 

Finding the float is useful in gauging how much flexibility the project has. Float is a resource that should be used to cover project risks or unexpected issues that come up. 

Critical tasks have zero float, which means their dates are set. Tasks with positive float numbers belong in the non-critical path, meaning they may be delayed without affecting the project completion date. If you’re short on time or resources, non-critical tasks may be skipped. 

Calculating the float can be done with an algorithm or manually. Use the calculations from the section below to determine the total float and free float. 

Total float vs. free float

Here’s a breakdown of the two types of float:

• Total float: This is the amount of time that an activity can be delayed from the early start date without delaying the project finish date or violating a schedule constraint. Total float = LS - ES or LF - EF 

• Free float: This refers to how long an activity can be delayed without impacting the following activity. There can only be free float when two or more activities share a common successor. On a network diagram, this is where activities converge. Free float = ES (next task) - EF (current task)

There are a few good reasons why project managers benefit from having a good understanding of float:

• It keeps projects running on time: Monitoring a project’s total float allows you to determine whether a project is on track. The bigger the float, the more likely you’ll be able to finish early or on time.  

• It allows you to prioritize: By identifying activities with free float, you’ll have a better idea of which tasks should be prioritized and which ones have more flexibility to be postponed. 

It’s a useful resource: Float is extra time that can be used to cover project risks or unexpected issues that come up. Knowing how much float you have allows you to choose the most effective way to use it.

How to use the critical path method

Critical path methodology provides visibility into your project’s progress, allowing you to monitor tasks and their completion times. Below are some additional applications of CPM.

Compress schedules

Though not ideal, there are times when project deadlines may be pushed up. In those situations, there are two schedule compression techniques you can use: fast tracking and crashing.

• Fast tracking: Look at the critical path to determine activities that can be performed simultaneously. Running parallel processes will speed up the overall duration. 

• Crashing: This process involves allocating more resources to speed up activities. Before obtaining more resources, make sure that they are still within the project scope and let the stakeholders know of any changes.

Having the critical path plotted out can help you choose the appropriate strategy to meet updated deadlines.

Resolve resource shortages

Keep in mind that CPM doesn’t take resource availability into account. When there is a resource shortage, like an overbooked team member or a lack of equipment, you can use resource leveling techniques to solve the issue. 

These techniques aim to resolve resource overallocation issues and ensure that a project can be completed with the resources that are currently available. 

Resource leveling works by adjusting project start and end dates, so you may have to readjust the critical path or apply this technique to activities with float.

Compile data for future use

The schedule created from CPM is subject to change since you’re working with educated estimates for activity durations. You can compare the original critical path to the actual critical path as the project runs.

This data can be used as a reference to get more accurate task duration estimates for future projects. 

Critical path method vs. PERT

CPM and Program Evaluation and Review Technique (PERT) were both developed in the 1950s. PERT is used to estimate uncertainty around project activities by applying a weighted average of optimistic and pessimistic factors. It evaluates the amount of time needed to complete an activity. 

PERT uses three time estimates to find a range for the duration of an activity:

• Most likely estimate (M)

• Optimistic (O)

• Pessimistic (P) 

The calculation for PERT is: Estimated time = (O + 4M + P) / 6

The main difference between PERT and CPM is their level of certainty around activity durations—PERT is used to estimate the time required to complete activities, whereas CPM is used when the activity durations are already estimated. 

Let’s see how the two techniques compare:

• PERT manages uncertain project activities; CPM manages predictable project activities.

• PERT focuses on meeting or minimizing project duration; CPM focuses on time-cost-trade offs.

• PERT is a probabilistic model, and CPM is a deterministic model.

• PERT has three time estimates for each activity; CPM has just one.

Differences aside, both PERT and CPM analyze the following components:

• List of required tasks

• Estimated duration for each task

• Task dependencies

These two project management tools can be used in tandem to boost their effectiveness. You can use PERT to get more realistic estimates of task durations before proceeding to calculate the critical path and floats.  

Critical path method vs. Gantt chart

Gantt charts are horizontal bar charts that map out project activities, which can be tracked against a set timeline. Both CPM and Gantt charts show the dependencies between tasks. 

Let’s go over some differences between the two tools:

CPM

• Visualizes critical and non-critical paths and calculates project duration

• Displayed as network diagram with linked boxes

• Doesn’t show resources required

• Plots critical path activities on a network diagram without timescale

Gantt chart

• Visualizes how project activities are progressing

• Displayed as horizontal bar chart

• Shows resources required for each activity 

• Plots critical path activities on a timescale 

Gantt charts can be paired with CPM to track critical paths over time and keep your project running on schedule. 

Use your project’s critical path for better project management

CPM can be a useful asset in project management, particularly for planning tasks and managing resource constraints. With the help of project planning tools, you’ll be able to create schedules and track projects with ease. To further increase your work efficiency, check out these 12 tips to be more productive today.

FAQ: Critical path method

What is the critical path formula?

The critical path formula consists of two parts: the forward pass and the backward pass. The forward pass calculates the earliest start times (ES) and finish times (EF) for each activity, with EF determined by adding the activity's duration (t) to its ES.

• EF = ES + t (the duration of an activity)

The backward pass determines the latest start times (LS) and finish times (LF), setting the LF of the final activity equal to its EF and finding the LS by subtracting the activity's duration from its LF.

• LF – t (the duration of an activity)

How do you calculate the critical path in a project schedule?

To calculate the critical path in a project schedule, you first identify all possible paths through the project's activity network. Then, for each path, you sum the durations of all activities. The longest path is the critical path. This requires calculating the earliest and latest start and finish times for each activity.

What is a CPM schedule?

A CPM schedule identifies critical and non-critical tasks to prevent delays by focusing on the longest sequence of dependent tasks to calculate the shortest project duration.

When can you use the critical path method?

You can use CPM during the planning phase of a project to identify the longest sequence of dependent tasks and calculate the shortest possible completion time. CPM is particularly useful for optimizing schedules for complex projects with multiple activities, milestones, and task dependencies.