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Calculating Slack Time on a PERT Chart

written by: Sidharth Thakur • edited by: Jean Scheid • updated: 9/20/2012

Calculating slack time and presenting it on the project’s PERT chart is crucial to preventing delays in the project completion. Knowing how much float, or slack time, your team has on certain deadlines can help in better and more efficient allocation of resources.

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    Slack time, used in Program Evaluation and Review Technique (PERT), denotes how much an activity can be delayed beyond its earliest start date, without causing any problems in the completion of the project by its due date. Also known as float, slack time is applicable only to those activities which do not lie on the critical path of the PERT chart. For all activities that lie on the critical path the earliest start time will be the same as latest start time and the earliest finish time will be the same as latest finish time.

    On the other hand, all activities that lie along other paths of the PERT chart will have some difference between the earliest start time and the latest start time, and between the earliest finish time and the latest finish time. This difference in time is used to determine how much an activity can be delayed while still allowing the subsequent activities and the overall project to get completed as per schedule. The activities that lie on the critical path of the PERT chart are the most significant and must be started and completed strictly without any delay, as any delays in these activities will result in an overall delay in finishing the project.

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    Calculating Slack Time

    Calculating the slack time for an activity in a PERT chart is very easy and does not involve any complex calculations. There are two things that are needed to calculate the slack time:

    1. ES – the earliest time when an activity can be started
    2. LS – the latest time when an activity must be started.

    The slack time is calculated as:

    Slack Time = LS-ES

    Calculating Slack Time for Project Activities.bmp Alternatively, slack time can also be calculated as the difference between the latest finish time and the early finish time. Slack time must be calculated for all activities involved in the project. Calculating the slack time is another way of tracing the critical path on a PERT chart. All activities which return a ZERO as the slack time, are the critical activities.

    Screenshot taken by Sidharth Thakur

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    The Importance of Slack Time

    Slack time is a very integral part of the project process and it must be clearly shown on the PERT chart. Ignoring slack time can lead to consequences that may compromise the overall performance and efficiency of the project. Only when the slack times have been identified will it be possible to use them to the project’s advantage.

    Activities which have some slack time can be delayed if resources need to be diverted elsewhere for critical activities. Also at times, devising strategies to reduce slack time brings about an overall improvement in the productivity. The process of allocating resources to shorten the critical path and thus remove or reduce slack time from the PERT chart is termed as crashing. However, crashing a project should be done with caution as it is a cost intensive way of speeding up the process and involves heavy overhead costs, which could radically affect the total project budget.

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    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:Calculating Slack Time on Activity Cards.bmp 

    1. Duration: time required to complete an activity
    2. Early start date
    3. Early finish date – adding duration to the early start date
    4. Latest start date
    5. Latest finish date – adding duration to latest start date
    6. Slack time – difference between latest start date and early start date or between latest finish date and early finish date.

    Calculating early start and finsih time on PERT Chart.bmp 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.

    Calculating latest start and finsih time on PERT Chart.bmp 

    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.

    Calculating Slack Time on PERT Chart.bmp 

    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

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    You'll be glad you took the time to find out how much slack you have if you run into a scenario where you are falling behind schedule and more resources are needed on the critical path.


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