What's the difference between total float and free float?

Total float is delay without affecting the project end date. Free float is delay without affecting any successor task. Free float ≤ Total float.

Why is zero slack critical?

Tasks with zero slack form the critical path — the longest chain through the project. Any delay to these tasks directly delays the project.

Can slack be negative?

Negative slack means the project is behind schedule. To finish on time, tasks with negative slack must be accelerated (crashed).

How do I calculate ES and LS?

Forward pass: ES = max(EF of all predecessors). EF = ES + Duration. Backward pass: LF = min(LS of all successors). LS = LF - Duration.

Is slack the same as buffer?

Slack is inherent in the schedule based on path analysis. Buffer is intentionally added time. Critical Chain Method adds explicit buffers; CPM reveals natural slack.

How does this help resource allocation?

Tasks with high slack can be delayed if their resources are needed on critical path tasks. This flexibility enables resource leveling.

Slack Time Calculator

Calculate slack (float) time in project schedules using task durations, dependencies, earliest/latest start and finish times.

Format: Name, Duration, ES, EF, LS, LF (one task per line)

Critical Path

A → C → D

Project Duration

11 days

Determined by critical path

Critical Path Tasks

A, C, D

Total Slack Available

4 days

Sum of all task total floats

Average Slack

0.8 days

Average total float per task

Maximum Float

2 days

Most flexible task

Schedule Type

Moderate

3 of 5 tasks are critical

Task Slack Analysis

Task	Dur	ES	EF	LS	LF	TF	Critical?
A	5	0	5	0	5	0	🔴 Yes
B	3	5	8	7	10	2	No
C	4	5	9	5	9	0	🔴 Yes
D	2	9	11	9	11	0	🔴 Yes
E	1	8	9	10	11	2	No

Float Distribution

0d float

2d float

0d float

2d float

Schedule Gantt (simplified)

Planning notes, formulas, and examples

About the Slack Time Calculator

The Slack Time Calculator determines how much schedule flexibility exists for tasks in a project. Slack time is the amount a task can be delayed without pushing back project completion. Tasks with zero slack are on the critical path, so any delay to them delays the entire project.

In project management, slack helps with resource allocation, risk management, and schedule planning. Total float shows how much a task can slip without affecting the project end date. Free float shows how much a task can slip without affecting any successor task.

This calculator lets you enter tasks with durations, earliest start (ES), earliest finish (EF), latest start (LS), and latest finish (LF) times. It computes total float, free float, and critical-path status for schedule analysis.

When This Page Helps

Use this calculator to see where a project schedule has flexibility and where it is tight. It helps with risk management, resource planning, and identifying tasks that need close attention.

How to Use the Inputs

Enter each task name and its duration (in days)
Enter the Earliest Start (ES) and Earliest Finish (EF) from the forward pass
Enter the Latest Start (LS) and Latest Finish (LF) from the backward pass
View calculated Total Float and Free Float for each task
Tasks with zero total float are on the critical path (highlighted)
Use presets to load example project schedules
Analyze the critical path and schedule buffer

Formula used

Total Float = LS - ES = LF - EF. Free Float = ES(successor) - EF(current). Critical Path: Tasks where Total Float = 0. Project Buffer = Minimum non-zero total float across near-critical tasks.

Example Calculation

Result: Task A: 0 float (critical), B: 2 days float, C: 0 float (critical)

Tasks A and C have zero float — they're on the critical path. Task B has 2 days of float (can slip 2 days without affecting the project end date).

Tips & Best Practices

Always run both forward and backward passes before calculating slack
Focus management attention on tasks with zero or negative float
Use free float to find tasks that can slip without affecting anything
Near-critical tasks (1-2 days float) deserve close monitoring too
Resource leveling often uses slack to shift task timing
In Agile, slack concept applies to sprint buffer and velocity variance

Critical Path Method (CPM) and Slack

The Critical Path Method identifies the longest sequence of dependent tasks. Tasks on this path have zero total float — they determine the minimum project duration. Non-critical tasks have positive float, providing scheduling flexibility. CPM was developed in the 1950s and remains the foundation of project scheduling.

Using Slack for Risk Management

Tasks with minimal slack are high-risk: any delay propagates. Project managers monitor near-critical paths (1-5 days of float) because they can become critical if delays occur. By tracking float trends over time, managers can detect schedule erosion early and take corrective action before delays become critical.

Slack in Different Project Methodologies

Waterfall projects use CPM slack extensively. Agile projects use sprint velocity variance as an analog. Critical Chain Method replaces individual task slack with shared project and feeding buffers. Lean construction uses the Last Planner System. Regardless of methodology, understanding where flexibility exists is always valuable.

Sources & Methodology

Last updated: June 1, 2026

Frequently Asked Questions

Total float is delay without affecting the project end date. Free float is delay without affecting any successor task. Free float ≤ Total float.