Forward Pass vs Backward Pass in CPM: Key Differences and How to Calculate Them

Staying on schedule is one of the biggest challenges in project management. That’s where the Critical Path Method (CPM) helps. It highlights the most critical tasks in your project—those that must be completed on time to prevent delays.

At the heart of CPM are the Forward Pass and Backward Pass. The Forward Pass helps you figure out the earliest start and finish times for each task. The Backward Pass tells you the latest times tasks can start without pushing back the project.

In this blog, we’ll explain how both passes work, how they fit together, and how you can use them to create better project schedules. We’ll also cover how to calculate float (or slack time) to give you some flexibility. Whether you're new to project management or want to improve your scheduling, this guide will help.

What is Forward Pass?

The Forward Pass is a key step in the Critical Path Method (CPM) used to calculate the earliest start and earliest finish times for each task in a project.

Here’s how it works:

• Start with the first task and set its earliest start time to 0 (or the project’s start time).
• Add the task duration to get the earliest finish time.
• For tasks that depend on others, use the earliest finish time of the previous task as the earliest start time for the next one.

The Forward Pass helps you map out the earliest possible timeline for your project, showing when each task can start and finish. It’s the first step in determining how long your project will take.

(For a deeper dive into the Forward Pass and its role in project management, check out our complete guide on the Forward Pass Project Management.)

What is Backward Pass?

The Backward Pass is the process of calculating the latest start (LS) and latest finish (LF) times for each task in a project. It helps determine the latest possible times that activities can start and finish without delaying the overall project.

Its primary purpose is to identify how much flexibility (or float) exists in the schedule by calculating the latest times tasks can begin and end while still meeting the project deadline.

Key Components

• Late Start (LS): The latest time an activity can start without affecting the project’s end date.
• Late Finish (LF): The latest time an activity can finish without causing delays to the project’s timeline.

Steps to Perform a Backward Pass:

1. Start with the last activity: Set the Late Finish (LF) of the final task to its Earliest Finish (EF).
2. Calculate Late Start (LS): Subtract the task’s duration from the Late Finish (LF) to find the Late Start.

(LS): LS = LF - Duration

3. Move backward through the tasks: For each preceding activity, update the Late Finish (LF) by using the Late Start (LS) of the subsequent task.
4. Repeat until all activities are updated.

Example

Using the same project from the forward pass example:

• Activity A: Duration = 3 days, no predecessors.
• Activity B: Duration = 4 days, depends on A.
• Activity C: Duration = 2 days, depends on A.

Forward Pass Results:

• EF of A = 3 days
• EF of B = 7 days
• EF of C = 5 days
• Project Completion = 7 days

→ Forward pass results: EF of A = 3, EF of B = 7, EF of C = 5, project completion = 7 days.

Backward Pass Calculation:

• For B: LF = 7 (project EF), LS = 7 - 4 = 3 days.
• For C: LF = 7 (project EF), LS = 7 - 2 = 5 days.
• For A: LF = min(LS of B, LS of C) = min(3, 5) = 3, LS = 3 - 3 = 0 days.

Key Differences Between Forward Pass and Backward Pass

The forward pass and backward pass are essential calculations in the Critical Path Method (CPM) for project planning, each serving distinct purposes in determining project timelines and flexibility. Below are the key differences between the two:

1. Direction of Calculation

• Forward Pass: Moves from the start of the project to the end, progressing through the project network in chronological order.
• Backward Pass: Moves from the end of the project to the start, working backward through the network.

2. Focus

• Forward Pass: Determines the earliest possible start (ES) and finish (EF) times for each activity, establishing the shortest project duration.
• Backward Pass: Calculates the latest allowable start (LS) and finish (LF) times for each activity to meet the project deadline without delays.

3. Outputs

• Forward Pass: Produces Early Start (ES) and Early Finish (EF) for each activity, identifying the earliest project completion time.
• Backward Pass: Yields Late Start (LS) and Late Finish (LF) for each activity, indicating the latest times tasks can occur without impacting the project.

4. Role in CPM

• Forward Pass: Establishes the project’s critical path and minimum duration by calculating the earliest completion times.
• Backward Pass: Identifies float/slack (scheduling flexibility) for activities and confirms the critical path by determining which tasks have no leeway.

Summary Table

How Forward Pass and Backward Pass Work Together

The forward pass and backward pass in the Critical Path Method (CPM) work together to create a clear, practical project schedule.

• Forward Pass: Calculates the earliest start (ES) and early finish (EF) times for each task, setting the shortest project timeline and identifying the critical path.
• Backward Pass: Determines the latest start (LS) and late finish (LF) times, revealing float (flexibility) for non-critical tasks to avoid delays.
• Collaboration: The forward pass establishes the fastest timeline, while the backward pass ensures tasks stay within allowable limits, optimizing scheduling and resource use.

Example: In a 7-day project, the Forward Pass establishes the critical path, and the Backward Pass identifies available float for non-critical tasks, allowing for better resource allocation and efficient scheduling.

Calculating Float/Slack Using Forward and Backward Pass

Float (or slack) is the amount of time a task can be delayed without affecting the project’s overall completion date. It helps project managers identify which tasks are flexible and which ones are critical to the timeline.

(For more details on Float/Slack, check out our comprehensive guide on Float in project management)

How to Calculate Float:

1. Key Terms:

   • Earliest Start (ES): The earliest time a task can begin, calculated during the Forward Pass.
   • Earliest Finish (EF): The earliest time a task can finish, calculated during the Forward Pass.
   • Latest Start (LS): The latest time a task can start without delaying the project, calculated during the Backward Pass.
   • Latest Finish (LF): The latest time a task can finish without delaying the project, calculated during the Backward Pass.

2. Formula for Float/Slack:

Float = LS - ES or Float = LF - EF

→ This calculation reveals how much time a task can be delayed before affecting the project deadline.

Example Calculation:

Consider the following project tasks:

Float Calculation for Each Task:

1. For Activity A:

   • Float = LS - ES = 0 - 0 = 0 days
   • Float = 0: No flexibility, Activity A must start on time.

2. For Activity B:

   • Float = LS - ES = 3 - 3 = 0 days
   • Float = 0: No flexibility, Activity B must start on time.

3. For Activity C:

   • Float = LS - ES = 3 - 3 = 0 days
   • Float = 0: No flexibility, Activity C must start on time.

In this example, all tasks have 0 float, meaning any delay will push the project completion date. There’s no room for flexibility, and every task is critical.

Practical Tips for Accurate CPM Calculations

To ensure precise forward and backward pass calculations in the Critical Path Method (CPM), follow these practical tips:

1. Accurate Task Durations: Estimate task durations realistically using historical data or expert input. Break down large tasks for better accuracy.
2. Define Clear Dependencies: Clearly identify task relationships (Finish-to-Start, Start-to-Start, etc.) to ensure correct calculations of ES and LS.

1. Regularly Update the Schedule: Continuously update the schedule as the project progresses. Recalculate the critical path when changes occur.
2. Use Project Management Tools: Leverage software tools like TaskFord for automatic calculations and visualization of tasks.
3. Account for Resource Constraints: Consider resource availability when estimating durations. Balance workloads to avoid delays.
4. Review Float/Slack Time: Calculate float for non-critical tasks to identify flexibility. Use slack time strategically to adjust the schedule.

These tips will help ensure your CPM calculations are accurate and that the project stays on track.

Conclusion

The Forward Pass and Backward Pass are crucial for creating an accurate project schedule. The Forward Pass sets the earliest start and finish times, while the Backward Pass ensures tasks are completed on time by calculating the latest start and finish times.

By calculating float and slack, you can identify flexible tasks and prevent delays. Regular updates and collaboration with stakeholders ensures your project stays on track and on schedule.

Continue Reading

• Gantt Chart Critical Path: Key Differences, Use Cases & How to Find It.
• Critical Path Method in Project Management: A Path to Success
• Gantt Chart Dependencies: 4 Common Types & When to Use Them