Slack Time Calculator


About Slack Time Calculator (Formula)

A Slack Time Calculator is a project management tool used to determine the amount of time that a particular task within a project can be delayed without affecting the overall project timeline. In project scheduling, tasks are often interconnected, and delays in one task can impact subsequent tasks and the project’s completion date. Slack time, also known as float or slack, allows project managers to identify tasks that have flexibility in their deadlines and those that are critical to the project’s success.

The formula for calculating slack time is relatively straightforward:

Slack Time (S) = Late Start (LS)Early Start (ES)


  • Slack Time (S) represents the amount of time a task can be delayed without affecting the project’s completion, typically measured in days or hours.
  • Late Start (LS) is the latest allowable start time for a task based on the project’s completion date.
  • Early Start (ES) is the earliest start time for the task based on its dependencies and the project’s schedule.

The critical path method (CPM) and the program evaluation and review technique (PERT) are common project management techniques that use slack time calculations to identify critical tasks and prioritize resource allocation.

Here’s how slack time is calculated:

  1. Determine the project’s critical path: The critical path is the sequence of tasks that, when delayed, would extend the project’s completion time.
  2. Calculate early start (ES) and early finish (EF) times: These are determined by analyzing task dependencies, durations, and their sequencing. Early start is the earliest time a task can begin, while early finish is the earliest time it can be completed.
  3. Calculate late finish (LF) and late start (LS) times: Late finish is the latest time a task can finish without delaying the project, while late start is the latest time it can start.
  4. Calculate slack time (S): Subtract the early start time from the late start time to find the slack time for each task.

The Slack Time Calculator is a valuable tool for project managers and teams to assess project timelines, allocate resources effectively, and identify tasks that require special attention to avoid project delays. By understanding slack time, project managers can make informed decisions about task priorities, resource allocation, and potential schedule adjustments to ensure that the project stays on track and meets its deadlines. It is an essential component of effective project planning and management.