How to Calculate Slack Time

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  • 0:04 What Is Slack Time?
  • 1:27 The Formula
  • 2:10 Using the Formula
  • 3:12 Example
  • 3:52 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

After completing this lesson, you'll understand what slack time is and how it's calculated. You'll also see why knowing your slack time is important when it comes to finishing a project on schedule.

What Is Slack Time?

You've probably heard the word slack being used. You might have even heard someone call someone else a slacker. What does that mean to you? The term slacking is usually connected with being lazy. If you are slacking at school, you aren't putting forth your best effort.

Well, slack time at work has a similar connotation. Slack time at work is connected with finishing a project on time. It's defined as the amount of time that activities related to a project can be delayed without having a negative impact on the project's overall completion. This might sound a bit weird; it might even sound like laziness. But in the business world, it's a bit different.

For example, picture a large computer company. In the new products department, a small group of people are gathered around a table, discussing the upcoming project of creating a brand-new computer to market. The project has a firm deadline. Each person is given a separate task to complete.

Some crucial tasks must be done by certain deadlines for the project to continue in a timely fashion. But other less time-consuming tasks don't have to be done until the more complex and involved ones are. This extra time for finishing simpler tasks is called slack. Even if these smaller tasks are started later, as long as they are completed by the deadline for larger tasks, the project will still be finished on time.

The primary tasks that have to be done in order for a project to continue on time usually have no slack time. These have to start promptly and finish on deadline.

The Formula

During the project planning period, there's a formula the project coordinator will use to calculate the slack time for each task at each stage of a project. It's a simple formula that you'll easily remember:

  • Slack time = Latest start time - Earliest start time

The latest start time is the latest that a task should begin to keep the project on time. The earliest start time is the soonest a certain task can begin. The earliest start time is often linked with the completion of previous tasks. With big projects, often times these smaller tasks are done early, and the slack time remains as the larger tasks progress toward completion. Knowing the slack time helps each employee know how fast to work to keep the project moving on time.

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