To find the critical path method, first list all project activities, then identify which activity initiations depend on the completion of others. Next, create a critical path diagram and calculate the duration and float times of all tasks. Finally, identify the critical path and those activities that can be completed in parallel to it.
Here are six steps to find the critical path.
1. List Project Activities
For the purposes of eliminating redundancy, if you have created a project management plan for your project, you likely created a work breakdown structure. Use this structure to source a list of all the activities you must complete for each milestone of your project.
If you have not created a work breakdown structure for your project, you should list all the activities you must complete to create your project’s final deliverable. For example, if your project aims to plan a wedding, the tasks that must be complete to produce a planned wedding may include meeting with the couple, writing a guest list, booking vendors and coordinating with the venue. List out everything you expect to do in order to prepare for the big day.
Now, estimate how much time it will take to complete each task. For example, it may take two days to gather the couple’s preferences, budget and wedding date and a week to select and book a venue. Continue adding a duration time to each task. Then, compile it all into a table with a row for a task ID―you can assign a letter of the alphabet to each one―the name of the task and a task duration in whatever time interval most appropriately fits your project.
| Task ID | Task | Task Duration (in Days) |
|---|---|---|
| A | Project start | 0 days |
| B | Gather couple's preferences and wedding date | 2 days |
| C | Book a venue | 7 days |
| D | Write a guest list | 7 days |
| E | Book vendors | 14 days |
| F | Design and send save-the-dates | 21 days |
| G | Select the wedding party | 3 days |
| H | Order the wedding attire | 21 days |
| I | Create a seating chart | 7 days |
| J | Help the couple write their vows | 7 days |
| K | Decorate the venue | 2 days |
| L | Plan the rehearsal dinner | 7 days |
| M | Wedding day | 1 day |
2. Identify Activity Dependencies
A dependency refers to an activity within a project that cannot be initiated until another activity has been completed. In our wedding planning project, for example, the planner must first book the venue, then write the guest list, given that the venue selection determines how many guests can attend the wedding comfortably. So, writing the guest list is dependent on first booking the venue.
To record dependencies, use the list from step one, then add an “immediate predecessors” column to the right side of the table. Next, for each activity, determine “which task needs to be completed before beginning this task?” Then, in the “immediate predecessors” column for each activity, list the Task ID of all activities that must be completed before the activity you are addressing can begin.
For example, in the table below, “writing the guest list” is dependent on booking the venue. So, we wrote C (the task ID for “book a venue”) in the “immediate predecessor” column for “write a guest list.” It is also important to know the couple’s preferences before writing the guest list in order to avoid uncomfortable confrontations during dinner. Therefore, we write the task ID for “gathering the couple’s preferences”—or B—in the immediate predecessor’s column for “write a guest list.”
Here’s an example of a dependencies table to serve as a guide for completing this step.
| Task ID | Task Description | Task Duration (in Days) | Immediate Predecessors |
|---|---|---|---|
| A | Project start | 0 days | |
| B | Gather couple's preferences and wedding date | 2 days | A |
| C | Book a venue | 7 days | B |
| D | Write a guest list | 7 days | B, C |
| E | Book vendors | 14 days | A, C, D |
| F | Design and send save-the-dates | 21 days | B, C |
| G | Select the wedding party | 3 days | B, C |
| H | Order the wedding attire | 21 days | B, G |
| I | Create a seating chart | 7 days | B, F, G |
| J | Help the couple write their vows | 7 days | B |
| K | Decorate the venue | 2 days | C, B |
| L | Plan the rehearsal dinner | 7 days | B, C, E, G, H, I, J |
| M | Wedding day | 1 day | A-M |
3. Create a Critical Path Diagram
To create a critical path diagram, draw a box to represent each task on the dependencies table you created in step two. Then, show dependencies by drawing arrows that connect each predecessor task to its dependent task(s). (You can see an example of this in the example section of this article.)
For example, since booking the venue (Task ID C) is dependent on gathering the couple’s preferences (Task ID B), create a box titled “gather the couple’s preferences – B” and a box labeled “book a venue – C.” Next, draw a line from the “gather the couple’s preferences” box to the “book a venue” box. Continue until all your tasks and their dependencies are recorded on your chart.
Pro tip: You can easily create a free critical diagram chart using Google Draw. Follow the prompts to choose a flowchart template, then adjust the template to your needs and add text to label each box. Finally, select the arrow tool to drag an arrow from one box to the next, thereby adding dependency arrows. Your diagram can then be saved to your Google Drive account.
4. Calculate the Duration of Each Activity
You have already figured out how long each activity will take. Now, it’s time to figure out the earliest start and end dates of each activity. This will help you to know when you must schedule each activity to ensure the entire project is finished on time with all tasks completed. Use the forward pass and backward pass techniques to calculate the earliest start and earliest finish dates of each activity.
Forward Pass
The forward pass helps you determine the earliest start time and earliest finish time you can employ for each activity. To do so, determine the early start time of each activity by recording the finish date of the previous activity. Put simply, the early start time is equal to the finish time of the previous activity. Then, add the duration of the activity to this recorded time.
Continue calculating the earliest start and finish day of each activity until you get to the last activity. The number you arrive at for the earliest finish day of last activity in the project is the earliest finish day of the project or the target completion day of the project.
Backward Pass
Your next calculation—the backward pass—will tell you the latest start date and latest finish date of each activity. Begin with the last activity and work backward in your project timeline to the very first activity. Use the following calculation to do so:
Late start (LS) = Late finish (LF) – activity duration
* Use the lowest late start (LS) value of the previous activity as the late finish value (LF) for this formula.
5. Estimate Float Time
In project management, figuring the float time helps you determine if an activity can take longer than expected without risking finishing the next task late or the overall project late. It can also tell you if you can start it late or early while still finishing all tasks in the project on time. Simply put, it tells you how much flexibility you have when scheduling and completing each task. Use the following formula to determine the float time for each activity:
Slack time = Late start (LS) – Early Start (ES)
6. Calculate the Critical Path
The critical path is the longest sequence of dependent activities. Some activities may run parallel to this path but they can be completed simultaneously around critical path activities without delaying the project’s finish date. To calculate the critical path, isolate the activities with a zero float time and put them in order. Alternatively, start with the first activity, add all dependent activities to it and then calculate the duration of the longest dependent sequence of activities.
The float times from each activity will now help you calculate the critical path. Isolate all activities with a float time of zero. Together, these make up your critical path. These activities cannot be started or ended early or late or the whole project may risk either not finishing on time or finishing with a lower quality. Other activities that have a float time of more than zero are activities that must be completed in parallel to the activities that make up your critical path.
Another way to calculate your critical path is by assigning a zero start time to the first activity. Add to that zero the duration of the first activity. From there, add subsequent activities that must be finished after your first activity and their duration times. Continue adding on dependent activities and their durations until you reach the last activity in the dependency sequence. The sequence of activities with the longest total duration makes up the critical path.
