Enter the latest possible start time and the earliest possible start time into the calculator to determine the slack time.

Slack Time Formula

The following formula is used to calculate a slack time.


  • Where ST is the slack time
  • LST is the latest possible start time
  • EST is the earliest possible start time

What is slack time?


A slack time is defined as the most amount of time a project can be delayed and still be completed within schedule.

Mathematically, slack time is equal to the latest possible start time minus the earliest possible start time, where any time after the lastest start time would result in project failure.

How to calclaute slack time?

Example Problem:

The following example outlines how to calculate slack time.

First, determine the earliest possible start date. For this example, the earliest date is November 10th.

Next, determine the latest possible start time. In this case, the latest start date is November 15th.

Finally, calculate the slack time by subtracting the earliest from the latest.


ST = 15 – 10

ST = 5 day slack time