How To Calculate Free Slack In Project Management

The success of project management relies heavily on a comprehensive grasp of various elements, including the notion of slack. In this article, we will explore the procedure for determining Free Slack, an essential factor in effectively managing any project. However, prior to delving into the specifics, let us first define what slack means.

What is Slack?

In project management, slack, also known as float, refers to the amount of time that you can delay a task without delaying the entire project or a subsequent task. Essentially, it’s the flexibility that you have with the scheduling of tasks. This can be categorized into two: Total Slack and Free Slack.

What is Free Slack?

Free Slack is the amount of time that you can delay a task without delaying the next task. It’s a measure of the flexibility that doesn’t impact the scheduling of subsequent tasks. So, how do we calculate it?

Steps to Calculate Free Slack

Here are the steps to calculate Free Slack in your project schedule:

  1. Identify the earliest and latest that each activity can start and finish without extending the project’s completion time.
  2. Calculate the slack for each activity by subtracting the earliest start time from the latest start time (or earliest finish time from the latest finish time).
  3. The smallest amount of slack among all activities ending in a particular node will be the Free Slack for all activities leading into that node.
// pseudo code to calculate free slack
function calculateFreeSlack(EarliestStartTime, LatestStartTime) {
  let slack = LatestStartTime - EarliestStartTime;
  return slack;

In the above code, EarliestStartTime and LatestStartTime are the earliest and latest possible start times of an activity respectively. The function calculates the slack by subtracting the Earliest Start Time from the Latest Start Time.

Why is Free Slack Important?

Now, one might wonder why calculating Free Slack is important. In essence, it provides project managers with a degree of flexibility and wiggle room when dealing with potential delays or errors. Knowing how long a task can be delayed without causing disruption to the following tasks is critical for effective project management.

Moreover, understanding and properly utilizing Free Slack can lead to more streamlined project processes, improved resource allocation, and ultimately, successful project completion.


Effective project management is not just about getting tasks done, but also about understanding key project metrics like Free Slack to better manage and allocate resources. By knowing how to calculate Free Slack, project managers can gain more flexibility in their scheduling, making the project more efficient and effective.