In the critical path method, tasks with zero slack are on the critical path – any delay to them delays the whole project. Tasks with slack can be delayed by up to that many days, which gives flexibility for scheduling resources.
How to use the Slack Time Calculator
- Earliest start
- Latest start
- Task duration
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the earliest and latest start dates of the task and its duration.
Tips
- Track a typical week before planning – real figures beat estimates.
- Break big tasks into smaller pieces and plan buffer time for interruptions.
- Re-run the numbers when your inputs change so plans stay realistic.
- Use results as a guide for decisions, not as a precise prediction.
Common mistakes to avoid
- Counting calendar days when the deadline is in working days (or the reverse).
- Forgetting breaks, setup and handover time.
- Assuming every day is as productive as your best day.
Inputs explained
| Field | What to enter |
|---|---|
| Earliest start | A date |
| Latest start | A date |
| Task duration | In days |
What each result means
- Slack time = [Latest start] – [Earliest start]
- Slack in working days = workdays([Earliest start], [Latest start])
- Earliest finish = [Earliest start] + [Task duration]
- Latest finish = [Latest start] + [Task duration]
Example values
How the main result changes when Task duration changes and everything else stays at the example values above:
| Task duration (days) | Slack time (days) |
|---|---|
| 2.5 | 13 |
| 3.75 | 13 |
| 5 | 13 |
| 6.25 | 13 |
| 7.5 | 13 |
| 10 | 13 |
Formula
Slack = latest start − earliest start (= latest finish − earliest finish)Earliest finish = earliest start + duration; latest finish = latest start + durationWorked example
Inputs — Earliest start: 2026-10-25; Latest start: 2026-11-07; Task duration: 5 days.
Results — Slack time = 13 days; Slack in working days = 10; Earliest finish = Fri, October 30, 2026; Latest finish = Thu, November 12, 2026; Critical path? = No – the task can be delayed.
Frequently asked questions
What is slack time in project management?
The amount a task can slip without delaying the project end date.
What is the critical path?
The longest chain of dependent tasks; all tasks on it have zero slack.
What is the difference between free and total float?
Free float delays no following task; total float delays no project end date.
What formula does the Slack Time Calculator use?
Slack = latest start − earliest start (= latest finish − earliest finish).