Slack Time Calculator

Enter a task's Early Start, Early Finish, Late Start, and Late Finish to calculate total slack (float) — how many days it can slip before it delays the project.

Quick Facts

Formula
Total Slack = Late Start − Early Start = Late Finish − Early Finish
Zero slack marks a critical-path task; any other value is scheduling flexibility. Use any consistent time unit (days, hours, weeks).

Your Results

Calculated
Total slack (float)
-
Late Start − Early Start
Cross-check slack
-
Late Finish − Early Finish (should match)
Task duration
-
Early Finish − Early Start
Critical path status
-
Zero slack = critical task

Ready

Enter Early Start, Early Finish, Late Start, and Late Finish, then press Calculate.

About the Slack Time Calculator

Slack time — also called float — is a core concept from the Critical Path Method (CPM), the standard technique project managers use to schedule tasks in a project network. Slack measures how much a task can be delayed without pushing back the project's overall finish date or breaking a dependency with another task. This calculator applies the standard CPM slack formula to the four dates that come out of a project schedule's forward and backward pass.

The formula

Every task in a CPM schedule has four key dates, normally produced by working forward through the network (earliest possible dates) and then backward from the deadline (latest allowable dates):

  • Early Start (ES) — the earliest the task can begin, given its predecessors.
  • Early Finish (EF) — the earliest the task can finish: EF = ES + duration.
  • Late Start (LS) — the latest the task can begin without delaying the project.
  • Late Finish (LF) — the latest the task can finish without delaying the project.

Total slack (float) is then:

Total Slack = Late Start − Early Start = Late Finish − Early Finish

Both expressions must give the same number, because a task's duration (EF − ES) is fixed and must equal LF − LS. If they disagree, one of the four dates was transcribed incorrectly from the schedule.

Reading the result

  • Slack = 0: the task sits on the critical path. It has zero scheduling flexibility — any slip delays the whole project unless the plan is changed.
  • Slack > 0: the task is non-critical. It can start or finish up to that many days later than its early dates without affecting the project finish date.
  • Slack < 0: the schedule is infeasible as planned — the required (late) dates fall before the achievable (early) dates. This happens when a hard deadline is tighter than the network allows, and it signals a need to re-baseline, add resources, or compress the schedule (crashing or fast-tracking).

Total slack vs. free slack

This calculator computes total slack, the amount a task can slip without delaying the entire project. A related but different figure, free slack, is the amount a task can slip without delaying the early start of its very next successor task — it requires knowing that successor's early start and is always less than or equal to total slack. Total slack is the figure most commonly meant by "slack time" and the one used to identify the critical path.

Frequently Asked Questions

What is slack time (float) in project scheduling?
Slack time, also called float, is the amount of time a task can be delayed without delaying the project finish date or violating a dependency. It is computed with the Critical Path Method formula: Total Slack = Late Start − Early Start = Late Finish − Early Finish. A task with zero slack is on the critical path.
What does zero slack mean?
Zero slack means the task is on the critical path: its early and late dates are identical, so it has no scheduling flexibility. Any delay to a zero-slack task delays the entire project's finish date, unless the schedule is re-planned.
Can slack time be negative?
Yes. Negative slack means the late dates fall before the early dates, which happens when a hard deadline is earlier than the schedule can achieve. It signals the plan is already behind the required finish date and needs re-baselining, added resources, or schedule compression (crashing or fast-tracking).
Why do Late Start minus Early Start and Late Finish minus Early Finish need to match?
Because a task's duration is fixed, Early Finish − Early Start must equal Late Finish − Late Start. If those two duration figures do not match, one of the four dates was entered incorrectly relative to the others.

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