How to Calculate and Show Slack Time on PERT Charts
When drawing the PERT activity chart, use an activity card that shows all the time details for an activity, like the one shown in the screenshot here. The time details that all activity cards must contain are:
- Duration: time required to complete an activity
- Early start date
- Early finish date – adding duration to the early start date
- Latest start date
- Latest finish date – adding duration to latest start date
- Slack time – difference between latest start date and early start date or between latest finish date and early finish date.
To start filling in the time details into the activity card, begin from the first activity and fill in the early start and early finish time for each activity. The early start time for the subsequent activities will be the early finish time of the preceding activity. In case there are too many preceding activities, use the finish time of the activity which ends after all the others.
The next thing needed to calculate slack time is to determine the latest start and finish times for each activity. For this, the calculations will begin backward, from end toward start, and the maximum time needed to complete the entire project should be used as the late finish time for all the final activities. The latest start time can be calculated by subtracting the duration for each activity from its latest finish time. Where there are two or more successor activities, use the smallest late start time from among all of the activities, as the latest finish time.
Finally, to calculate the slack time for each activity, either subtract late start from early start or subtract late finish from early finish. Put in the slack time in the space provided in the activity card. To find out the critical path for the PERT chart, just follow the path of the activities where the slack time is zero.
Screenshots taken by Sidharth Thakur