Enter your Earliest Start Time (EST) and Latest Start Time (LST) to calculate the slack time for any project task or milestone. The Slack Time Calculator applies the formula SL = LST − EST to tell you exactly how many days of buffer you have before a delay becomes critical. Use it to prioritize tasks, identify bottlenecks, and keep your project on track.
Slack Time
--
Task Status
--
Earliest Start Time
--
Latest Start Time
--
Slack time (also called float time) is the maximum amount of time a task can be delayed without causing a delay to the overall project or a dependent task. It represents the buffer between when a task can earliest start and when it must start at the latest. A slack time of zero means the task is on the critical path — any delay will push back the project deadline.
The slack time formula is: SL = LST − EST, where LST is the Latest Start Time and EST is the Earliest Start Time. Both values are typically expressed in days from the project start date. For example, if EST = 10 days and LST = 23 days, the slack time is 23 − 10 = 13 days.
First, identify the Earliest Start Time (EST) — the soonest a task can begin given preceding tasks. Second, determine the Latest Start Time (LST) — the latest it can begin without delaying the project. Finally, subtract EST from LST to get the slack time. Repeat this for every task in your project network.
A slack time of zero means the task is on the critical path. There is no room for delay — if this task runs late, the entire project will be delayed. Tasks on the critical path require the most attention and resource allocation from project managers.
Slack time gives project managers flexibility to reallocate resources, manage risks, and handle unexpected delays without missing the final deadline. Understanding which tasks have slack helps teams prioritize work, avoid bottlenecks, and make smarter scheduling decisions throughout the project lifecycle.
Yes. A negative slack time means the task is already behind schedule — the latest start time has passed the earliest feasible window. This signals that immediate action is needed to recover the timeline, such as fast-tracking tasks or adding resources to compress the schedule.
Total float (or total slack) is the delay a task can absorb without delaying the project finish date. Free float is the delay a task can absorb without delaying the next task in the sequence. The LST − EST formula computes total float. Free float requires knowledge of successor task timings as well.
Simply enter the Earliest Start Time (EST) and the Latest Start Time (LST) for the task you want to evaluate. The calculator will instantly compute the slack time using the formula SL = LST − EST and tell you whether the task is critical, has buffer time, or is already overdue.