What is Slack Time in Project Management & How to Calculate It

Imagine you’ve been working hard on a major project which is due next week.

Everything is planned and working well until an unexpected delay happens. Panic sets in. Missing this deadline could mean losing a key client.

But what if you had a secret weapon with extra time to handle these surprises?

That’s where slack time comes in. It’s the extra time you have to complete a task without delaying the overall project schedule.

This article will show you how to address this complexity and also:

How to calculate slack time in four straightforward steps
The best practices for calculating slack time
An example of how to calculate slack time

Let’s dive in.

What is slack time in project management?

Slack time in project management is the total time a task can be delayed without putting other deliverables or the overall project deadline at risk.

Project managers calculate slack time during the planning phase by analyzing the flexibility of the project schedule and task timeline. They use different formulas for calculating slack time, which we’ll discuss in detail below.

Slack time is a project manager’s best friend as it offers a buffer against late starts, late finishes, and other project issues. It also helps them prioritize tasks, allocate resources efficiently, and plan projects around team capacity.

What are the different types of slack time?

There are two types of slack time:

Free slack time: The amount of time a task can be delayed without affecting any subsequent tasks.
Total slack time: The total amount of time a task can be delayed without impacting the project’s completion date.

The type of slack you want to calculate depends on what aspect of the project schedule you need to manage.

But before we go into calculations, let’s see the reasons why slack time matters.

Why is slack time important?

Slack time’s importance varies for different teams and managers, but here are some general reasons why slack time benefits project management:

Understanding and calculating slack time helps project managers accurately set realistic deadlines, plan projects better, and ensure smoother flow.
Slack time allows project managers to allocate resources strategically to prioritize tasks.
Projects with slack time are more flexible and less likely to be delayed.
Calculating slack in projects helps teams manage their workload to reduce overtime and stress.
Slack time buffers against risks and uncertainties to give project managers some breathing room when unexpected issues arise. Teams can shift focus between tasks based on available slack to increase efficiency.

Who needs to know slack time?

Project managers who understand slack time and how to calculate it are better equipped to optimize resources, mitigate delays, and keep projects flexible.

When they know how much slack time each task has, project managers can plan projects more realistically, identify potential delays, and adjust schedules dynamically.

Having this knowledge can help everyone involved in the project adapt to changes efficiently and keep the project on track.

To ensure everyone knows about slack time, project managers should communicate the project status constantly and create protocols for handling delays.

How can project managers use slack time?

Slack time gives project managers flexibility, control, and freedom to optimize project schedules.

By identifying slack in tasks, they can schedule and prioritize critical tasks with limited or no slack to ensure they get done. It also helps team leaders have more flexibility to manage overlapping resources and dependencies without impacting the project’s critical path.

This flexibility also helps them handle risks more efficiently when managing schedules. Knowing which tasks have slack prepares project managers to deal with uncertainties. They can allocate more resources to tasks with zero slack as a preventive measure against potential delays.

Also, understanding where slack exists means they can take calculated risks and potentially innovate their project scheduling approach.

Let’s say a project manager calculates slack time for two tasks: social media content creation and email campaign launch. They identify the first task—social media creation—has 4 days of slack, while the email campaign has none.

To keep the task with no slack on schedule, they make an informed resource allocation decision and move some of the people working on social media creation. It makes sense, as this task has 4 days of breathing room before it delays the whole project

The result?

Team members can work more productively and without pressure, which improves morale and reduces the risk of burnout.

How can you calculate slack time?

To calculate slack time, you need the following components:

Latest start time (LST): The latest point at which you can start a task without delaying the entire project. It’s typically determined by working backward from the project’s deadline.
Earliest start time (EST): The earliest point at which a task can begin if all prior tasks start and proceed without delay.

Slack time formula

Here’s an example of slack time calculation in project management using the formula:

For example, if the LST for a task is day 20 and the EST is day 15, the calculation would be:

Slack time = 20 – 15 = 5 days

Additionally, you can calculate slack time using finish dates with the following formula:

For example, if the EFT for a task is day 10 and the LFT is day 15, the calculation would be:

Slack time = 15 – 10 = 5 days

Here’s a step-by-step approach to calculating slack time in your next project:

Step 1: Determine the project’s EST

When calculating slack time, consider how complex your project is. For larger projects, it’s easier to break it into smaller chunks and calculate slack time for each project task.

Smaller projects don’t require such a detailed breakdown—just list all the tasks before you start calculating.

To find the EST for a task, analyze the project schedule from the start date, then look at the duration and dependencies of each preceding task.

For example, if a project starts on a Monday and the first task takes 3 days, the EST for the following task is the 4th day (Thursday.)

You can use Toggl Plan’s Project Timeline feature to easily set start and end dates at project and task levels.

To set up a Project timeline, you must first create a project. To do that:

Click on the ‘+’ sign on the sidebar next to the Projects section to create a new project.

In the Project settings window that appears, you can name your project, assign a client, add start and end dates, and enable the Timeline or Board view.

On the Project timeline, drag and drop tasks for easy planning, edit start and end dates, reassign them, and add milestones.

You can also import your tasks into Toggl Plan using a CSV file from a spreadsheet or tools like Asana or Excel to make project planning easier.

To do this, go to the 3-dot menu at the right top corner of Project views, and select ‘Import Tasks’.

Then, upload your .csv file and match your spreadsheet columns to data points that Toggl Plan supports.

Step 2: Determine the project’s LST

Next, find the LST of the project.
Determine the end of the project, and then work backward. Consider the project’s final deadline and how long each task takes, including any buffer times you’ve added for risk management.

Here’s an example:

Let’s say a creative agency has been hired to make an animation product video for a client. The deadline, which is the LST, is in 30 days. The final task—editing and rendering the video—will take 5 days to complete.

So, the LST for editing and rendering must start no later than day 25 if the agency wants to hit its deadline deliverable.

For any deliverables preceding the editing and rendering phase, the agency must subtract their duration from the final task’s LST. If the previous task (voiceovers) takes 3 days, its LST is day 22 (25 – 3 = 22).

This backward calculation continues until the agency reaches the start of the project timeline. At the end of the process, they will have determined the LST for each task in reverse order from the end of the project.

Step 3: Calculate the difference between the LST and EST

You now have the numbers to calculate the slack time for the whole project and its tasks.

For example, if the agency determines the scriptwriting for the project has an EST of day 8 and an LST of day 12, then:

Slack Time = 12 – 8 = 4 days

This means the agency can start scriptwriting any time between day 8 and day 12 without delaying the project.

Step 4: Cross-check your calculation

This last step helps you ensure the slack time you calculated from the start times is the same as the one from the completion times.

To do this cross-check, you need to:

Calculate the EFT and LFT for each task
Calculate slack time using finish dates
Compare the two slack times

If the results are equal, your slack time calculations are accurate. If they don’t match, you might need to review task durations, dependencies, and previous calculations.

For example, if the agency wants to cross-check their slack time calculation for the scriptwriting task, it can calculate slack time using finish dates.

First, the agency must calculate the EFT and LFT for this task:

EFT = EST + task duration = 8 + 5 = Day 13
LFT = LST + task duration = 12 + 5 = Day 17

Then, they would calculate slack with finish dates:

Slack time = LFT – EFT = 17-13 = 4 days.

And finally, the comparison, which in this case is equal:

Slack time from start dates = 4 days (LST – EST = 12 – 8 = 4 days)
Slack time from finish dates = 4 days (LFT – EFT = 17 – 13 = 4 days)

What are some best practices for calculating slack time?

Here are some best practices you can use to calculate slack time more efficiently:

Calculate slack for each task: Instead of calculating slack for the whole project, focus on each task individually. Identify slack time at the task level to pinpoint flexibility in your schedule and manage resources more efficiently.

Create highly detailed estimates for initial project phases: Detailed estimates for early project phases set a strong foundation for the project schedule. Early developments in the project might lead to changes, which will then allow you to refine estimates for later phases.
Don’t fall victim to the planning fallacy: The planning fallacy, where you underestimate the time needed to complete tasks, can severely affect slack time. Project managers should calculate slack time based on the most likely scenario rather than an overly optimistic one.

What is the relation between slack time and the critical path?

The critical path is the longest sequence of project tasks that must be completed to complete a project. It includes tasks without slack, so any delay in these tasks extends the project’s overall duration.

Slack time is closely related to this sequence as it helps identify which tasks are not on the critical path. Once identified, these tasks can be delayed without consequences over the project’s end date.

Here’s a quick rundown:

First, identify all of the project’s tasks to calculate slack on the non-critical path. You can do this using a work breakdown structure.
After you’ve broken down the project into smaller tasks, create a network diagram. This shows the sequence of tasks and their dependencies to help you identify the critical path and the slack for non-critical tasks.

Take a look at this diagram:

The critical path above includes tasks A, F, and G. The rest are non-critical tasks that can happen simultaneously; they are called parallel tasks.

Both slack time and critical path help you prioritize tasks and avoid project delays.

What is the difference between slack time and float?

The difference between slack time and float in project management is largely semantic. Both refer to the same concept and they are interchangeable in most project management contexts.

For example, project managers use slack in critical path methods and float when working with program evaluation and review technique analysis, PERT charts, agile methodologies, or work breakdown structures.

What is the difference between slack time and buffer time?

The difference between slack and buffer time is how they are allocated and used in project management.

Buffer time is a period of time during which there is no work to be done, which project managers add to the schedule.

Both slack time and buffer time protect against delays, but the difference is how they are added. While slack time arises naturally from the scheduling process, buffer time is strategically added to the project timeline.

Plan projects easily and more efficiently with Toggl

Calculating and using slack time to plan projects with realistic deadlines and smoother workflows is a winning strategy.

You’ve seen what slack time can do for project planning, so why not get Toggl Plan on board to:

Plan projects and tasks effortlessly with drag-and-drop timelines
Easily set and edit task start and end dates for each project
Schedule workflows more efficiently and improve project time management

The Toggl Team

Work tools to elevate your productivity – apps for incredibly simple time tracking and effective project planning.