Comprehensive Calculator Guide
This calculator works out slack (or float): how many days a project task can slip without delaying the whole project, from its earliest start, its duration and its latest finish. A task that can start on day 5, takes 10 days and must finish by day 20 has 5 days of slack. Tasks with zero slack form the critical path: any delay to them delays the project.
The four terms
Earliest start (ES) is the first day the task can begin; earliest finish (EF) = ES + duration. Latest finish (LF) is the last day it can end without delaying the project; latest start (LS) = LF − duration.
Slack = LS − ES = LF − EF. In the critical path method, the longest chain of dependent tasks has zero slack, so it gets the closest watch.
How to Use
- Enter the task’s earliest start, counting the project start as day 0.
- Enter its duration and latest finish.
- Press “Calculate slack” to see the earliest finish, latest start and slack.
Formula Used
EF = ES + duration | LS = LF − duration | Slack = LS − ES
Practical Examples
- Example 1
- ES day 5, 10 days, LF day 20: EF day 15, LS day 10, slack 5 days.
- Example 2: critical task
- ES day 0, 12 days, LF day 12: zero slack.
Important Tips
- Work out slack for every task to find the critical path.
- Watch critical tasks daily.
- Don’t spend slack early.
Common Mistakes to Avoid
- Treating slack as guaranteed extra time for each task separately.
- Ignoring successor tasks when setting the latest finish.
- Mixing working days and calendar days.
Frequently Asked Questions
- What is slack in project management?
- The days a task can slip without delaying the project’s finish.
- What is the critical path?
- The longest chain of dependent tasks; all of them have zero slack.
- Can slack be negative?
- Yes, when the latest finish is before the earliest finish: the project is already late.