Slack time is a relevant concept in project management as it relates to proper task completion. This article will provide an overview of slack time. It will then continue by describing how we can calculate slack time. Different examples of slack time calculation and models to calculate slack time will also be included. The article will end with a comparison of slack time and total slack. When managing a project, it is your duty as a project manager to consider the time of task completion.
In the context of project management, slack time is defined as “the amount of time you can delay the task without interfering with another task or affecting the completion date of your project.” The phenomenon of slack time happens as a result of productivity imbalances among workers, and it is the responsibility of the project manager to ensure that project deadlines are met. There are two factors that must be taken into consideration in order to make the best use of slack time.
- First, you, as the project manager, must have a clear focus on your project goals to ensure that the deadlines will be met.
- Second, you must understand the importance of teamwork and allow your team to make suggestions on how to make the best use of the time.
Furthermore, this form of delegation will leave you more free time and will provide your workers with more autonomy.
Importance of Slack Time
Slack time is an important aspect of a project, so as an aspiring manager, you should have a clear understanding of its relevance. First, slack time indicates how much flexibility you, as the project manager, have in your project. This allows you to develop a more realistic timeline for activities. Furthermore, projects that include slack time tend to be higher in quality and stay within budget.
Slack also helps you, as a project manager, to understand the importance of particular tasks within a project. In addition to staff and equipment expenses, delays have a negative domino effect on other activities in the project. In fact, such missteps can have a critical impact on the project’s overall success. From the project manager’s viewpoint, Slack is an important prioritization tool that allows them to consider the best ways of allocating specific tasks to ensure greater productivity.
How to Calculate Slack?
The calculation of slack is rather straightforward, and it contains only a few simple steps.
Slack Time Formula
The following formula is used to calculate slack time:
ST = LST − EST
- ST (Slack Time): the amount of time an activity can be delayed without affecting subsequent tasks or the project completion date.
- LST (Latest Start Time): the latest time an activity can begin without delaying the project.
- EST (Earliest Start Time): the earliest time an activity can begin based on schedule constraints.
From a mathematical standpoint, slack time is equal to the latest possible start time minus the earliest possible start time, and any time after the latest start time would result in project failure.
Examples of Slack Time Calculation
As a project manager, you can easily use the abovementioned formula in order to perform a slack time calculation. Two different examples of slack time calculation will be provided in order to better clarify the process.
Example 1:
The following example outlines how to calculate slack time.
- First, determine the earliest possible start date. For this example, the earliest date is October 10th.
- Next, determine the latest possible start time. In this case, the latest start date is October 15th.
- Finally, calculate the slack time by subtracting the earliest from the latest.
ST = LST – EST
ST = 15 – 10
ST = 5-day slack time
Example 2:
Let’s say that you have to complete a task in a project by the 3rd of November. You realize that the earliest start time is the 15th of October and the latest start time is the 27th of October. In this case, your calculation should be as follows:
Slack Time= LST (27th of October)- EST (15th of October)
In this case, the slack time is 12, which means that you have 12 days to complete the task.
Slack Time in GANTT Chart and PERT Network
In the following part of the article, there will be two examples of slack time calculation in two different contexts which involve the GANTT chart and the PERT Network.
Example 1: PERT NETWORK
The calculation for slack time is conducted for every event in the following network as shown in the figure by recognizing the latest starting date and the earliest expected date. TL – TE = 0 for event 1.
Example 2: GANTT chart
In the image presented here, the light-green color shows the free slack available. As can be seen, there is a lot of room to extend the tasks without affecting the Critical Path.
Slack Time in the GANTT chart
Free Slack vs Total Slack
The free slack of an activity can be defined as “the time this activity can be delayed without impact on the following activity.” On the other hand, total slack is defined as “the amount of time a task can be delayed without impacting the delivery of the project.” So, while free slack is concerned with the forthcoming activity in a project, the impact of total slack tends to affect the entire project.
Free Slack formula:
Free Slack = ES (Of Successor) – EF (Of Current)
Total Slack formula:
Total Slack = LS – ES or LF – EF
Here, ‘LS’ indicates a late start date, while ‘ES’ indicates an early start date. Similarly, ‘LF’ indicates a late finish date, while ‘EF’ indicates an early finish date. It must be emphasized that free slack is also known as “free float,” while total slack is known as “total float.”
Conclusion
This article gave a clear, practical overview of slack time and why it matters for project managers. Used well, slack time increases scheduling flexibility, improves prioritization, and reduces the risk that delays cascade through a project. To get the most benefit, managers should measure and monitor slack (both free and total), communicate constraints and opportunities to their teams, and use slack proactively—reallocating resources, adjusting task sequences, or building contingency plans.
When incorporated into everyday planning and reviewed during status updates, slack becomes a simple but powerful tool for delivering higher-quality projects on time and within budget, enabling proactive risk mitigation, smoother resource reallocation, and improved stakeholder communication.
FAQs
How do you calculate slack time in project management?
The formula for calculating slack time (ST) is straightforward. Determine the latest start time (LST) and the earliest start time (EST). Subtract EST from LST to get the total slack time of the project.
Why is slack important in project management?
Slack provides schedule flexibility, enabling managers to absorb delays without harming milestone or project completion dates. It supports realistic planning, risk mitigation, resource optimization, and prioritization. By tracking slack, managers can reassign tasks, schedule buffers strategically, and reduce cascading delays across dependent activities.
What is positive slack?
Positive slack (or positive float) means an activity can be delayed for a finite number of time units without delaying the project finish date. It indicates non-critical tasks with a scheduling buffer. Positive slack allows flexibility for resource allocation, contingency planning, and minor timing adjustments without affecting delivery.
Suggested articles:
- Optimistic Time In Project Management
- What is Lag Time in Project Management? Explained with Examples
- Probabilistic Time Technique In Project Management
Shane Drumm, holding certifications in PMP®, PMI-ACP®, CSM, and LPM, is the author behind numerous articles featured here. Hailing from County Cork, Ireland, his expertise lies in implementing Agile methodologies with geographically dispersed teams for software development projects. In his leisure, he dedicates time to web development and Ironman triathlon training. Find out more about Shane on shanedrumm.com and please reach out and connect with Shane on LinkedIn.