Once the complete set of early and late start and finish times has been calculated for all activities, the total slack (TS) of each activity can be calculated. Activity total slack is defined as the maximum amount if time that an activity can be delayed without delaying project completion time. Because the "critical" activities are those in the sequence of the longest time path through the network, it follows that the activities on this path will have the minimum possible total slack. Also, because the target completion date (the EFT) of the final activity is taken as the LFT of the final activity, it can be seen that all critical activities will have total slack equal to zero. Likewise, if the project completion target date were taken as later than the EFT of the final activity, all activities on the critical path would have exactly this amount of slack and the slack of all noncritical activities would increase by an equal amount. For any activity, total slack can be calculated as :
TS = LST-EST = LFT-EFT