What is a Very Large Project?
One of the challenges of managing a very large project in Microsoft Project is the amount of time it takes for the software to recalculate the project. What constitutes a very large project, you may ask? Generally speaking, a very large project meets one or more of the following criteria:
The project has thousands of tasks. The largest project I have ever seen personally had over 15,000 tasks.
The project has dozens or even hundreds of team members. One of the largest projects I have ever seen personally had 200 team members in the project.
The project has multiple tasks that have a large number of team members assigned to each task. One of the largest projects I have ever seen personally had multiple tasks with 160 team members assigned to each task.
When you perform any action that triggers a recalculation, Microsoft project automatically recalculates the data for every task, every resource, every assignment, and the entire project. For example, in the project shown in Figure 1, I changed the Duration value for the task ID #2 task from 3 days to 5 days. Microsoft Project automatically recalculated the entire project, which you can see from the blue cell background formatting (the Change Highlighting) in the Start and Finish columns for almost of the tasks.
Figure 1: Entire project recalculated after making a schedule change
In most projects, the recalculation takes less than a second to complete. But in a very large project, this recalculation procedure can take 5 to 10 minutes or more. This is why it can be so frustrating for Microsoft Project users to manage very large projects.
Disabling Automatic Recalculation
To make it easier to work with very large projects, for many years I have recommended that project managers of these types of projects should disable the automatic recalculation feature in Microsoft Project. Doing so allows the project managers to make multiple changes to a very large project, and then to manually recalculate the project at a time of their own choosing.
Warning: You do not need to disable automatic recalculation if you work with small, medium, or even large projects. My recommendation to disable automatic recalculation is only for project managers who manage very large projects, as defined by the criteria in the first topical section of this article.
To disable automatic recalculation in Microsoft Project, click the File tab and then click the Options tab in the Backstage. In the Project Options dialog, click the Schedule tab, and then scroll all the way down to the bottom of the page.
Near the bottom of the Schedule page, notice in Figure 2 that there is a section named Calculation. Notice that this section contains a single option named Calculate project after each edit. This option, when enabled, is what triggers the automatic recalculation of your projects.
Figure 2: Calculation option – currently enabled
To disable the automatic recalculation feature in your copy of Microsoft Project, simply select the Off option in the Calculation section of the Project Options dialog. Click the OK button when finished. Keep in mind that this option is not project-specific. Rather, it is an application option, which means that it applies to every project you open and edit.
Manually Recalculating the Project
After you disable automatic recalculation in your copy of Microsoft Project, you can make multiple changes to your project without the software recalculating your entire project for you automatically. For example, Figure 3 shows my project schedule after changing the Duration value of task ID #2 from 5 days back to 3 days. Notice that Microsoft Project applies the blue cell background formatting (the Change Highlighting) to the Duration and Finish columns for only this task, indicating that it did not recalculate the rest of the project.
Figure 3: Project schedule change after disabling automatic recalculation
When you are ready to manually recalculate your project schedule, there are three methods from which you can choose. My personal favorite method is to right-click on the Select All button (blank button immediately above the Task #0 row header) and to select the Recalculate Project item on the shortcut menu, such as shown in Figure 4.
Figure 4: Select the Recalculate Project item on the shortcut menu
The second method for manually recalculating a project is to press the F9 function key on your computer keyboard. The third method is to add the Calculate Now button to your Quick Access Toolbar and then to click this button each time you want to recalculate your project. Any of the three methods will work, as they allow you to force a recalculation of your entire project when you feel the time is right.