How this calculator works
In critical path scheduling every task has an earliest start, set by when its predecessors can finish, and a latest start, set by when it must begin to keep the project end date intact. The gap between them is slack, or float. A task with zero slack is on the critical path: any delay moves the whole project. The calculation here works in calendar days between two dates, so a week of slack is seven days rather than five working days. Solve for either date once the slack is fixed, which is the usual way to test how far a task can slip before it becomes critical.
Formula
slack time = O(latest start) - O(earliest start), where O is the day number counted from 0001-01-01. Latest start = earliest start + slack days. Earliest start = latest start - slack days.
Worked example
An earliest start of 2026-11-02 and a latest start of 2026-11-09 is 7 days of slack. With 7 days of slack and an earliest start of 2026-11-02, the latest start is 2026-11-09, and with a latest start of 2026-11-09 the earliest start is 2026-11-02.
Advertisement
Frequently asked questions
Is slack time the same as free float?
No. Total float is the delay a task can take without moving the project end date, which is what this page measures. Free float is the delay that does not affect the earliest start of any successor task, and it is never larger than total float.
Should slack be counted in working days?
In most project plans yes, because weekend days are not available for work. This page counts calendar days, so convert a working day figure by removing the weekends inside the range.
What does zero slack mean?
The task is on the critical path. Its earliest and latest start dates coincide, so any slip moves the project finish date by the same amount.
Can the latest start fall before the earliest start?
Not in a valid schedule. The calculator rejects that input because negative slack means the plan is already infeasible and a task would need to start before its predecessors finish.