Project scheduling involves organizing a sequence of tasks to ensure a successful outcome within a defined timeframe. To manage a project effectively, managers must determine the earliest possible moment each activity can begin. Identifying the earliest feasible start time (EST) prevents delays and helps establish a realistic project timeline. Calculating this metric is foundational for developing a reliable schedule that respects all task interdependencies.
What is the Earliest Start Time (EST)?
The Earliest Start Time (EST) represents the earliest point an activity can commence without violating any established constraints or sequence requirements. This metric is a fundamental concept in planning methodologies used to manage complex projects. Determining the EST for every activity helps project planners establish the shortest possible duration for completing the entire scope of work.
This scheduling analysis moves sequentially through the project timeline, from initiation to the final deliverable. Calculating the EST allows the project team to understand the soonest they can mobilize resources and begin work on any given task. This information is used to predict the earliest completion date for the entire project.
Data Preparation: Inputs Required for Calculation
Before any scheduling calculations can begin, three foundational pieces of data must be prepared and verified.
The first requirement is a comprehensive list of all activities necessary to complete the project scope. Each activity must be clearly defined and distinct from every other task in the sequence.
The second necessary input is an estimated duration for each activity. These durations are expressed in consistent units, such as days or weeks, and represent the expected time required to complete the work.
The final input is the set of precedence relationships, or dependencies, between tasks. These relationships specify which activities must be finished before their successor activities are allowed to start.
The Logic of the Forward Pass
Calculating the Earliest Start Time is accomplished through a systematic procedure known as the Forward Pass. This analysis moves chronologically through the project schedule, starting from the first activity and progressing toward the final milestone. The fundamental rule ensures that no activity begins before all its prerequisites are complete.
The core logic dictates that the EST for any activity is determined by the maximum of the Earliest Finish Times (EFTs) of all its direct preceding activities. Using the maximum value is required where multiple predecessor tasks converge on a single successor activity. This rule prevents schedule violations and maintains the integrity of the project sequence.
Step-by-Step Calculation Across the Project Network
Applying the Forward Pass logic begins with the initial activity of the project. For this starting activity, which has no predecessors, the Earliest Start Time is set to zero or the designated project start date. This establishes the chronological baseline from which all subsequent calculations flow forward.
For sequential activities that have only a single predecessor, the EST is straightforwardly determined. The activity can begin immediately upon the completion of its single preceding task. Therefore, its EST is equal to the Earliest Finish Time of that predecessor.
The complexity increases at merge activities, which have two or more direct predecessors. The maximum rule is applied rigorously here. The EST for the merge activity must be the largest of all the preceding activities’ EFTs, guaranteeing that the last required task is fully finished before the new work commences.
The Relationship Between EST and Earliest Finish Time (EFT)
The Earliest Start Time is intrinsically linked to its complementary metric, the Earliest Finish Time (EFT). The EFT represents the earliest moment an activity can be completed, assuming it starts at its EST. These two values define the earliest possible time window for any given activity.
The relationship between them is defined by a simple arithmetic formula: EFT equals the EST plus the activity’s estimated duration. Once the EST is calculated, adding the duration yields the EFT. This calculated EFT then becomes the necessary input for determining the EST of subsequent activities in the project sequence.
Practical Example: Calculating EST for a Simple Project
Consider a small project with four activities: Activity A (Duration 5), Activity B (Duration 3), Activity C (Duration 4), and Activity D (Duration 2). Activity A and Activity C must both be completed before Activity D can begin, while Activity B follows Activity A.
The process begins with Activity A, the project’s start. Since it has no predecessors, its EST is 0, and its EFT is 5 (0 + 5). Activity B follows Activity A, so B’s EST is 5 (A’s EFT), and its EFT is 8 (5 + 3).
Activity C is also a starting activity, so its EST is 0, and its EFT is 4 (0 + 4). Activity D is the merge point, dependent on both A and C. The EFTs of its predecessors are 5 (from A) and 4 (from C). Applying the maximum rule, the EST for Activity D must be 5, resulting in an EFT of 7 (5 + 2).