How to Calculate and Use Slack Time

March 22, 2018 Heather Hurst

By Heather Hurst

Despite our best efforts, accurately predicting the right amount of time for tasks isn’t always possible. We also know that every project hits bumps in the road that can delay progress. But, what if you could plan not only for the amount of time each task will take but also for unexpected delays and still have your project completed on time?

This is called slack time, and it’s essential to build into your project every time. Slack time, sometimes also called “float,” is how many days you can delay a task without having to extend the project’s completion date.

Download our free guide "Enterprise Resource Management: The Complete Guide to Managing Talent and Time" for more on how you can manage project timelines successfully.

It’s a simple but powerful calculation that will give you insight into how tasks affect each other and where you might be underestimating how much time you need. Here’s how to calculate slack time in your projects and use it to keep your tasks and due dates running smoothly without threatening your overall timeline.

Finding the Earliest and Latest Start Dates

Before calculating slack time for a task, you need to know two things: the earliest and the latest you can start a task while keeping your overall project on time. To find these, you’ll build an earliest start schedule (ESS) and latest start schedule (LSS).

Simply put, an ESS is a schedule that shows the earliest date you can start each individual task. To build one, start with the first task and plot out the estimated days to completion. Then build each sequence of tasks that follow in the same way: pair each subsequent task with the number of days needed to finish it.

It might help to think of the ESS as a best-case scenario. It shows how the project would go if everything went exactly as planned and there were no delays.

On the other hand, an LSS is a schedule that’s built based on the latest date that you can start each task without compromising the critical path or pushing the project over schedule.

Build this by working backward, starting with the last task of the project. With the final task ending on the last day, and knowing how long it takes to complete, you can find the latest date you can start that final task. Then you can add the tasks that precede the final task, working backward until you have the latest start date for every task.

If the ESS is a best-case scenario, the LSS is a less-than-ideal scenario. It’s not quite a worst case, because it’s still based on finishing the project on time. But starting every task on the last day possible doesn’t give you much breathing room.

Now that you have these two schedules, you’ll use them to plug values into the slack equation and learn how much wiggle room you have for individual tasks.

Calculating Slack Time

Once you have all the information you need, calculating slack for an individual task is easy. The equation for slack is: Latest Start (LS) - Earliest Start (ES) = Slack Time.

You’ve already done the work of finding the earliest and latest days that you can start your task, and now you just plug them into the equation. Here’s how this might work:

You’re working on a 10-day project, and one of the upcoming tasks, Task C, is threatened because the assigned team member had a family emergency that took them out of the office for a day.

Based on your project sequence, you know that Task C will take one day, and that it can’t be started until Task A is finished. The earliest that Task A will be done is at the end of day five of the project. So, the earliest that you can start Task C is at the beginning of day six of the project. Your ES = 6.

You also know that Task D must wait for Task A (5 days), followed by two simultaneous tasks: Task B (3 days) and Task C (1 day). Thus, Task D can’t begin until day 9, which means the last day you can start Task C is on day 8 of the project. Your LS = 8.

LS (day 8) - ES (day 6) = Slack Time (2 days)

Your slack time is 2 days. In other words, you can delay starting Task C by two days without having to extend the finish date of the project. And since the assigned team member was only gone for one unscheduled day, the project should still be running as planned.

You don’t have to wait until a delay happens to calculate slack time—in fact, you can count on a delay in nearly all projects. Use slack time at the beginning of a project to be proactive and plan ahead for obstacles. It can also help you build a realistic timeline and start conversations with your team members about the tasks assigned to them.

Even the most experienced project managers can’t predict everything that will happen over the course of a project. In fact, in 2013, fewer than a third of projects were finished on time. But tools like slack time prepare you to handle small challenges and keep them from throwing the entire project off track.

See "How to Create a Workflow" for tips on working more efficiently so you can stick to your timeline and meet deadlines.

About the Author

Heather Hurst

Heather has enjoyed playing the game of marketing for the past 15 years, at the agency and corporate level, in both B2C and B2B companies. She's run PR campaigns that took her from the MTV Beach House to NASDAQ and many media outlets and content channels in between. She is currently the Corporate Marketing Director at Workfront.

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