When you start using Microsoft Project to build project plans, you create and link tasks to show the project steps. You may have heard the term "critical path" in relation to the tasks, but what exactly does this mean? This article describes what the critical actually means, how to show the critical path in your Gantt charts and explains why it's so important.

A task is described as critical if changing its duration changes the duration of the entire project. So if you create and save a project file consisting of four tasks in series, then the overall project duration is the sum of the durations of the four tasks. If you change the duration of any of the tasks, then the entire project duration will change. Each of the four tasks are critical, and the path from start to finish through the four tasks is called the critical path. Changing any task duration on the critical path changes the project entire duration.

Suppose you've created and saved a different project plan, this time with two parallel sets of tasks which link to a final completion task. The upper of the parallel paths has longer task durations than the lower parallel path. Now when you work out the overall project duration, this depends on the longer of the two parallel paths. So all the tasks from start to finish through the longer path add up to make the overall project duration, and this is the critical path.

So basically the critical path is the flow of tasks from project start to finish, through the longest task duration options. If any task duration on the critical path is changed then the entire project duration changes.

The Gantt chart view can be formatted to show this critical path. We'll start with a four task project, all linked serially, so the links are all finish to start relationships, and each task duration is 2 days. So the total project duration is 8 days. You can turn on the Project Summary Task to show this - choose Tools, Options, View tab and tick "Show project summary task" in the lower left of the panel, and click OK to finish.

Now we'll format the Gantt chart to show the critical path. To do this click the "Gantt Chart Wizard" toolbar button. In the Gantt Chart Wizard click Next, then select critical path and click Next, accept resources and dates and click Next, accept yes for the line links, and click Next, then click the Format it button, and click Exit wizard to finish. You'll see that all four task are coloured red for critical. Try increasing one of the task durations and of course the entire project duration changes and all tasks stay red.

Now try the same exercise with the other example project with the two parallel paths. With the task displayed in the Gantt chart, again set Project to format the critical path as red using the Gantt chart wizard in the same way as we described earlier. When this has been formatted, you'll see that only the tasks from the start through the longer of the two parallel paths to the final task are shown in red as critical. So the critical path is the flow of tasks through the longer path, and changing the duration of one of these tasks affects the entire project duration.

Try changing the duration of one of the tasks in the shorter parallel path. Provided that the duration of the shorter path remains less than the longer path you'll see that this does not affect overall project duration and the shorter tasks remain non critical blue.

If you change one of more on the task durations in the shorter path until the duration of the shorter path becomes more than the duration of the longer path, the critical path will switch and the new longer parallel path becomes red and the other reverts back to blue.

Knowing the critical path tasks is essential in project management so resources can be assigned to tasks in the most efficient way. For example if a project duration must be reduced by a certain number of days, then we could assign more resources to a critical path so that it can be completed more quickly and hence reduce the overall project duration. We could also reduce some resources on non critical tasks, so that they take longer to complete without affecting the overall project duration and reassign them elsewhere on critical tasks.

You may be interested in learning more about Project critical paths and Microsoft Project in general. You'll find that there lots of courses available to help you increase your skills. The best ones offer clear explanations with lots of hands on exercises, so why not consider one. You'll not regret it.