Slack Time Calculator

Project Management & Critical Path Method (CPM) Float Mechanics

In project scheduling and the Critical Path Method (CPM), Slack Time represents the amount of schedule flexibility available for an activity.

Primary CPM Network Scheduling Equations

• Early Finish (EF): EF = ES + Duration

• Late Start (LS): LS = LF - Duration

• Total Float / Total Slack (TF):

TF = LS - ES = LF - EF

(Maximum delay permitted without delaying the final project completion milestone)

• Free Float / Free Slack (FF):

FF = ES_immediate successor - EF

(Maximum delay permitted without delaying the Early Start of any immediate successor activity)

• Interfering Float: Interfering Float = TF - FF

• Critical Path Condition: Critical Path TF = 0

How to Use This Calculator

Enter your Activity Duration (D — Days/Hours) (e.g. 5 in days), Early Start (ES — Schedule Day #) (e.g. 2 in day), Late Finish (LF — Schedule Day #) (e.g. 10 in day), and Immediate Successor Early Start (ES_succ) (e.g. 8 in day). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Formula Reference: Slack Time Calculator

Input variables:

activityDurationTime — Activity Duration (D — Days/Hours)

earlyStartEsTime — Early Start (ES — Schedule Day #)

lateFinishLfTime — Late Finish (LF — Schedule Day #)

successorEarlyStartEsTime — Immediate Successor Early Start (ES_succ)

Computed outputs:

totalSlackFloatTime — Total Slack / Float (TF)

freeSlackFloatTime — Free Slack / Float (FF)

earlyFinishEfTime — Early Finish (EF)

lateStartLsTime — Late Start (LS)

criticalPathStatus — Critical Path Activity?

Mathematical relationships extracted from calculation logic:

ef = es + d

criticalText = isCritical ? 'YES (Critical Path)' : 'NO (Non-Critical)'

Standard: NIST / ISO mathematical definitions

Worked Example

A construction framing task taking 5 days can start on Day 2 (finishing Day 7) but isn't required to finish until Day 10 (Late Finish). It has 3 days of Total Slack to absorb delays without jeopardizing the final handover date.

Understanding Your Result

Evaluate your result in the context of your specific use case and relevant industry standards or reference values. A single calculated result is a starting point — test two to three scenarios to understand how sensitive the outcome is to your key input assumptions. For high-stakes decisions, verify your result against a secondary source or subject-matter expert before acting.