Slack Time Calculator (total float & free float, critical path)

Understanding the slack time calculator — concept and principle

This calculator works out the slack (float) of one project activity using the critical path method. From its earliest start, duration, latest finish and the earliest start of the next activity, it gives the total float, free float, early finish, late start and whether the activity is on the critical path. Project managers and students use it to see which tasks can slip and which cannot.

How long can this task slip? — with the default inputs: 4 days of float.

How it is calculated

Early finish EF = earliest start ES + duration (forward pass)
Late start LS = latest finish LF − duration (backward pass)
Total float = LS − ES = LF − EF
Free float = earliest start of the next activity − EF
Zero total float: the activity is on the critical path; negative float: the schedule already misses its deadline

Use day numbers from your schedule, with day 0 as the project start.

The principle

The critical path method calculates, for every activity, the earliest it can happen (forward pass) and the latest it can happen without delaying the project (backward pass). The gap between them is total float: how long the activity can slip before the project end date moves. Free float is narrower: how long it can slip without delaying any following activity. Free float is never larger than total float. Activities with zero total float form the critical path, the chain that sets the project duration.

Worked example

Defaults: ES day 4; duration 6 days; LF day 14; next activity's ES day 12.

EF = 4 + 6 = 10.
LS = 14 − 6 = 8.
Total float = 8 − 4 = 4 days.
Free float = 12 − 10 = 2 days.
Total float above zero: No — it can slip without delaying the project.

How to read the result

This activity can start up to 2 days late without affecting anyone else, and up to 4 days late without delaying the project end. Beyond 2 days, the next activity is pushed back and uses up its own float. Watch critical activities closely, and remember that float is shared along a path: if one activity uses it, later ones lose it.

Common mistakes

Confusing total float with free float.
Using calendar dates in one place and working-day numbers in another.
Forgetting that a dependency change recalculates all floats.
Treating float as spare time that belongs to one task.

Keep in mind: Float depends on the whole network of activities. Recalculate after any change in durations or dependencies.

Frequently asked questions

What is the difference between total float and free float?

Total float is slip before the project end moves; free float is slip before the next activity is delayed.

What does negative float mean?

The activity's late finish is earlier than its early finish, so the schedule cannot meet the deadline without changes.

Is slack the same as float?

Yes, the two terms are used interchangeably.

Sources: Open Exam Prep, CAPM study guide 6.4 "Critical Path and Schedule Calculations" (summary of the critical path method)

Checked 9 Nov 2026. Forward pass EF = ES + duration, backward pass LS = LF − duration, total float = LS − ES = LF − EF, free float as slip without delaying successors: Open Exam Prep, CAPM study guide 6.4 “Critical Path and Schedule Calculations” (secondary summary of the PMI critical path method); free float = ES(successor) − EF: standard formula.

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