Determining Slack within your Microsoft Project Schedule

Most people using Microsoft Project pay strict attention to the critical path. Tasks which exist on the critical path have no slack, and determine the soonest possible time that the project can be completed. Where there’s a blowout in the duration of a critical task, the end date of the project will also necessarily blow out.

Slack, however, is the opposite. Where a task has slack, there’s a degree of allowable slippage before the task becomes critical and begins to affect the end date of the project.

Why is slack so important?

Also known as float or buffer, slack is particularly important when assessing an in-progress project. Where we’re looking at tasks being performed we need to know a couple of things. Firstly, if a task is close to overrunning its projected finish date, what impact is that going to have on other resources? Secondly, is that task overrun going to impact the critical path?

To determine this, we need tools to find out what slack exists within the schedule.

How to quantify slack

Have a look at the image below:

Critical task box

We first tick the Critical Tasks box to display the critical path, circled in black.  This shows us the critical path (shown in red), as well as the areas of slack, indicated by the green arrows.

This show us that Task X and Task Z both have available slack.

Next, we need to quantify how much slack exists in these areas.

One of the best ways to show slack is to use the Schedule table.

To do this:

  • Point to the empty cell above row header 1 (circled in black) and right click

A fly-out menu appears with a list of available tables

  • Select Schedule (circled in green)

Select Scedule

The Schedule table appears, as shown below.

Schedule Table

As you can see, the Free Slack and Total Slack columns appear, highlighted with green arrows.

Free Slack indicates how many working days this task can blow out before it impacts the task it’s driving.

Total Slack indicates how many days of slack this task can blow out before it impacts on the projects end milestone.

So, in the example above we see that Task X can blow out by 2 days before it impacts on the task its driving (Task Y) and can blow out by 6 days before it impacts the project’s end date.

By using the Schedule table you’ll get an excellent insight into how each non-critical task might be about to affect the start of other tasks or the project’s end date, should that task blow out.


Paul Silverman is a Director of ProjectTMA Pty. Ltd. As well as being a senior Microsoft Project training consultant he works with clients to help them incorporate Microsoft Project into their project management methodology. For more information call 1300 363 822 or email: paul.silverman@projecttma.com.au.

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